Glucocorticoid manipulations in free-living animals: considerations of dose delivery, life-history context, and reproductive state
Crossin, G.T., Love, O.P., Cooke, S.J. and T.D. Williams
30 : 116-125. PDFAbstract:
1. Experimental glucocorticoid (GC) manipulations can be useful for identifying the mechanisms that drive life history and fitness variation in free-living animals, but predicting the effects of GC treatment can be complicated. Much of the uncertainty about the effects of GC manipulations stems from their multi-faceted role in organismal metabolism, and their variable influence with respect to life-history stage, ecological context, age, sex, and individual variation.
2. Glucocorticoid hormones have been implicated in the regulation of parental care in many vertebrate taxa but in two seemingly contradictory ways, which sets up a potential corticosterone-induced “reproductive conflict”. GCs mediate adaptive physiological and behavioural responses to stressful events, and elevated levels can lead to trade-offs between reproductive effort and survival (e.g. the current reproduction versus survival hypothesis). The majority of studies examining the fitness effects of GC manipulations extend from this hypothesis. However, when animals are not stressed (likely most of the time) baseline GCs act as key metabolic regulators of daily energy balance, homeostasis, osmoregulation, and food acquisition, with pleiotropic effects on locomotor activity or foraging behaviour. Slight increases in circulating baseline levels can then have positive effects on reproductive effort (e.g. the corticosterone fitness/adaptation hypotheses), but comparatively few GC manipulation studies have targeted these small, non-stress induced increases.
3. We review studies of GC manipulations and examine the specific hypotheses used to predict the effects of manipulations in breeding wildlife. We argue that given the dichotomous function of GCs the current ‘reproduction versus survival’ paradigm is unnecessarily restrictive and predicts only deleterious GC effects on fitness. Therefore, a broader set of hypotheses should be considered when testing the fitness effects of GC manipulations.
4. When framing experimental manipulation studies, we urge researchers to consider three key points: life-history context (e.g. long- vs. short-lived, semelparous vs. iteroparous, etc), ecological context, and dose delivery.
Large-scale oceanographic fluctuations drive Antarctic petrel survival and reproduction
Descamps, S., Tarroux, A., Lorentsen, S-H., Love, O.P., Varpe, O. and N.G. Yoccoz
39 : 116-125. PDFAbstract:
Polar Regions are experiencing environmental changes at unprecedented rates. These changes can spread throughout entire food webs from lower trophic levels to apex predators. As many top predators forage over large areas, these indirect effects may be associated with large-scale patterns of climate variability. Using global climate indices that are known to impact the Southern Ocean ecosystem (the El Niño Southern Oscillation and Antarctic Oscillation Indices) we assessed their efficacy to predict variation in the demographic parameters of Antarctic seabirds. First, we used a long-term dataset on adult survival (estimated from capture–mark–recapture data) and reproduction of Antarctic petrel Thalassoica antarctica, from the largest known breeding colony (Svarthamaren, Dronning Maud Land) and examined whether large-scale oceanographic fluctuations impact survival and reproduction. Second, we conducted an exhaustive literature review to determine whether the effects of large-scale environmental variability on Antarctic seabirds have a coherent fingerprint across the Antarctic continent and nearby islands. We found that most of the variation in both reproductive success, timing of hatching, and survival of Antarctic petrels can be accurately modeled using the two modes of large-scale climate variability in Antarctica. The literature review, combined with the results from our field study, suggests that while the anticipated trends in the global patterns of climatic variability will generally have detrimental effects on populations of top predators in the Southwest Atlantic, these conclusions cannot be extrapolated to all seabird populations in Antarctica without additional data.
Success stories and emerging themes in conservation physiology
The potential benefits of physiology for conservation are well-established and include greater specificity of management techniques, determination of cause-effect relationships, increased sensitivity of health and disturbance monitoring, and greater capacity for predicting future change. While descriptions of the specific avenues in which conservation and physiology can be integrated are readily available and important to the continuing expansion of the discipline of “conservation physiology”, to date there has been no assessment of how the field has specifically contributed to conservation success. However, the goal of conservation physiology is to foster conservation solutions and it is therefore important to assess whether physiological approaches contribute to downstream conservation outcomes and management decisions. Here we present eight areas of conservation concern, ranging from chemical contamination to invasive species to ecotourism, where physiological approaches have led to beneficial changes in human behaviour, management, or policy. We also discuss the shared characteristics of these successes, identifying emerging themes in the discipline. Specifically, we conclude that conservation physiology: i) goes beyond documenting change to provide solutions; ii) offers a diversity of physiological metrics beyond glucocorticoids (stress hormones); iii) includes approaches that are transferable among species, locations, and times; iv) simultaneously allows for human use and benefits to wildlife; v) is characterized by successes that can be difficult to find in the primary literature. Overall, we submit that the field of conservation physiology has a strong foundation of achievements characterized by a diversity of conservation issues, taxa, physiological traits, ecosystem types, and spatial scales. We hope that these concrete successes will encourage the continued evolution and use of physiological tools within conservation-based research and management plans.
Baseline glucocorticoids are drivers of body mass gain in a diving seabird
Hennin, H.L., Berlin, A. and O.P. Love
Ecology and Evolution
6 : 1702–1711. PDFAbstract:
Spring and fall migration phenology of an Arctic-breeding passerine
McKinnon, E.A., Macdonald, C.A., Gilchrist, H.G. and O.P. Love
Journal of Ornithology
Do baseline glucocorticoids simultaneously represent fitness and environmental quality in a declining aerial insectivore?
Madliger, C.L. and O.P. Love
Unpredictable perturbation reduces breeding propensity regardless of pre-laying reproductive readiness in a partial capital breeder
Legagneux, P., Hennin, H.L., Gilchrist, H.G., Williams, T.D. Love, O.P. and J. Bêty
Journal of Avian Biology
Energetic physiology mediates individual optimization of breeding phenology in a migratory Arctic seabird
Hennin, H.L., Legagneux, P., Bêty, J., Gilchrist, H.G., Williams, T.D., and O.P. Love
Conservation implications of a lack of relationship between baseline glucocorticoids and fitness in a wild passerine
Madliger, C.L. and O.P. Love
The application of physiological measures to conservation monitoring has been gaining momentum and, while a suite of physiological traits are available to ascertain disturbance and condition in wildlife populations, glucocorticoids (i.e., cortisol and corticosterone) are the most heavily employed. The interpretation of glucocorticoid (GC) levels as biomarkers of disturbance is based on the assumption of an underlying predictive relationship between GC levels and fitness. More specifically, the interpretation of GC levels as sensitive, early-warning indicators of population change necessitates that GCs and metrics of population persistence are linked. However, the relationship between GCs and fitness may be highly context-dependent, changing direction, or significance, depending on the GC measure, fitness metric, life history stage, or other intrinsic and extrinsic contexts considered. We examined the relationship between baseline plasma GC levels measured at two different periods of the breeding season and three metrics of fitness (offspring quality, reproductive output, and adult survival) in wild female tree swallows (Tachycineta bicolor). Specifically, we investigated whether: i) a relationship between baseline GC metrics and fitness exists in our population; ii) whether the inclusion of energetic contexts such as food availability, reproductive investment, or body mass could alter or improve the strength of the relationship between GCs and fitness; iii) whether energetic contexts could better predict fitness compared to GC metrics. Importantly, we investigated these relationships in both natural conditions and under an experimental manipulation of foraging profitability (feather clipping) to determine the influence of an environmental constraint on GC-fitness relationships. We found a lack of relationship between baseline GCs and both short- and long-term metrics of fitness in control and clipped birds. In contrast, loss in body mass over the reproductive season positively predicted reproductive output (number of chicks successfully leaving the nest) in control birds; however, the relationship was characterized by a low R2 (5%), limiting the predictive capacity, and therefore the application potential, of such a measure in a conservation setting. Our results indicate that baseline GCs will not be easily employed as conservation biomarkers across some species and life history stages, and stresses the importance of ground-truthing GC-fitness relationships.
Linking pre-laying energy allocation and timing of breeding in a migratory arctic raptor
Lamarre, V., Legagneux, P., Franke, A., Love, O.P. & J. Bêty
Implications of mercury and lead concentrations on breeding physiology and phenology in an Arctic bird
Provencher, J.F., Forbes, M., Hennin, H.L., Love, O.P., Braune, B.M., Mallory, M. & H.G. Gilchrist
Employing individual measures of baseline glucocorticoids as population-level conservation biomarkers: considering within-individual variation in a breeding passerine
Madliger, C.L. & O.P. Love
Costs of reproduction and carryover effects in breeding albatrosses
Crossin, G., Phillips, R., Lattin, C., Romero, L.M., Bordeleau, X., Harris, C.M., Love, O.P. and T.D. Williams
Temporal overlap and repeatability of feather corticosterone levels: practical considerations for use as a biomarker
Harris, C.M., Madliger, C.L. and O.P. Love
Pre-natal stress exposure generates higher early survival and smaller size without impacting developmental rate in a Pacific salmon
Capelle, P., Semeniuk, C.A.D., Sopinka, N., Heath, J.W. and O.P. Love
Journal of Experimental Zoology Part A: Ecological Genetics and Physiology
Integrative biologists have long appreciated that the effective acquisition and management of energy prior to breeding should strongly influence fitness-related reproductive decisions (timing of breeding and reproductive investment). However, because of the difficulty in capturing pre-breeding individuals, and the tendency towards abandonment of reproduction after capture, we know little about the underlying mechanisms of these life-history decisions. Over 10 years, we captured free-living, Arctic-breeding common eiders (Somateria mollissima) up to three weeks before investment in reproduction. We examined and characterized physiological parameters predicted to influence energetic management by sampling baseline plasma Glucocorticoids (i.e., corticosterone), very-low density lipoprotein (VLDL), and vitellogenin (VTG) for their respective roles in mediating energetic balance, rate of condition gain (physiological fattening rate) and reproductive investment. Baseline corticosterone increased significantly from arrival to the initiation of reproductive investment (period of rapid follicular growth – RFG), and showed a positive relationship with body mass, indicating that this hormone may stimulate foraging behaviour to facilitate both lipid deposition and investment in egg production. In support of this, we found that VLDL increased throughout the pre-breeding period, peaking as predicted during RFG. Female eiders exhibited unprecedentedly high levels of VTG well before their theoretical RFG period, a potential strategy for pre-emptively depositing available protein stores into follicles while females are simultaneously fattening. This study provides some of the first data examining the temporal dynamics and interaction of the energetic mechanisms thought to be at the heart of individual variation in reproductive decisions and success in many vertebrate species.
Mid-winter temperatures, not spring temperatures, predict breeding phenology in the European starling Sturnus vulgaris
Williams, T.D., Bourgeon, S. Cornell, A., Ferguson, L., Fowler, M., Fronstin, R., O.P. Love
Royal Society Open Science
Feather corticosterone reveals effect of moulting conditions in the autumn on subsequent reproductive output and survival in an Arctic migratory bird
Harms, N.J., Legagneux, P., Gilchrist, H.G., Bêty, J., Love, O.P., Forbes, M.R., Bortolotti, G.R., and Soos, C.
Proceedings of the Royal Society of London B
The power of physiology in changing landscapes: considerations for the continued integration of conservation and physiology
Madliger, C.L. and Love, O.P.
Integrative and Comparative Biology
Sources of diel variation in energetic physiology in an Arctic-breeding, diving seaduck
Steenweg, R.J., Hennin, H.L., Bêty, J., Gilchrist, H.G., Williams, T.D., Crossin, G.T. and O.P. Love
General and Comparative Endocrinology
Cold tolerance, and not earlier arrival on breeding grounds, explains why males winter further north in an Arctic-breeding songbird
Macdonald, C.A., McKinnon, E.A., Gilchrist, H.G. and O.P. Love
Journal of Avian Biology
The oxidative cost of acoustic signals: examining steroid versus aerobic-activity hypotheses in a wild bird
Vertebrate vocalizations are widespread secondary sexual signals used for mate attraction and territory defense, and variation in signal quality is often condition-dependent and impacts reproductive outcomes. Although vocal signal performance is known to reflect various aspects of male quality, few studies have examined the underlying mechanisms mediating its costs and hence its honesty. Using a population of Arctic-breeding snow buntings (Plectrophenax nivalis), we compared the “Oxidative Handicap Hypothesis”, which predicts that testosterone-induced increases in oxidative stress provide a direct mechanistic basis for ensuring the honesty of many secondary sexual signals, to the “Aerobic Activity Hypothesis”, which predicts that it is the aerobic activity involved with signal production (i.e., vocal performance or defending a large territory) and not testosterone directly that links signal quality and oxidative stress. Males singing at faster rates had higher levels of both reactive oxygen metabolites and non-enzymatic antioxidant capacity in the plasma (i.e., without an increase in overall oxidative stress), enabling certain males to produce high quality signals while also mitigating the costs of an associated increase in oxidative stress. However, these results were completely independent of plasma testosterone levels, supporting the role of aerobic performance in directly affecting oxidative stress. Although song performance was not linked to reproductive parameters in our dataset, our research is the first to test these competing hypotheses in a behavioural trait and results suggest that oxidative stress may be an underlying physiological cost preventing low quality individuals from producing high quality signals.
Assessing baseline stress physiology as an integrator of environmental quality in a wild avian population: implications for use as a conservation biomarker
Stress hormones (i.e., glucocorticoids such as corticosterone and cortisol) have been widely proposed as biomarkers of habitat quality and disturbance. However, there is growing evidence that baseline glucocorticoid (GC) levels are highly context-dependent, potentially confounding their utility for inferring population-level disturbance depending on the life history stage and the duration, severity, and type of environmental change being measured. Determining which aspects of an organism’s environment are consistently reflected by baseline GC levels is therefore of paramount importance to establishing how they may be best suited to conservation monitoring goals. We investigated the relationship between baseline GC levels and three extrinsic (food availability, inter-specific nest competition, intra-specific competition) and two intrinsic (reproductive investment, body condition) environmental contexts in breeding female tree swallows (Tachycineta bicolor) at two reproductive stages. We combined this with a manipulation of energetic demand (i.e., a decrease in foraging profitability) to determine whether baseline GCs reflect the extrinsic or intrinsic environment when females are faced with an unexpected disturbance. Baseline GC levels were not reflective of any environmental component in control females, regardless of reproductive stage. However, levels increased and were reflective of a decrease in body mass when females were challenged during the offspring provisioning period. Our findings suggest that baseline GCs may not always be indicative of the environmental contexts we associate with variation in habitat quality, particularly when individuals are operating within their expected energetic demand. In a conservation sense, baseline GCs may be more valuable in reflecting unexpected perturbations, which could limit their applicability as sensitive, predictive biomarkers across a diversity of systems.
Snow buntings sing individually distinctive songs and show inter-annual variation in song structure.
Baldo, S., Mennill, D.J., Guindre-Parker, S., Gilchrist, H.G. & Love, O.P.
Wilson Journal of Ornithology
126 : 333-338. PDFAbstract:
Revisiting the condition-dependence of melanin-based plumage.
Guindre-Parker, S. & Love, O.P.
Journal of Avian Biology
45 : 29-33 PDFAbstract:
Support against the condition-dependence of melanin plumage signals has relied on data from species exhibiting both melanin- and carotenoid-based plumage ornaments. As the mechanisms leading to variation in carotenoid- and melaninbased plumage differ fundamentally, these systems may not be ideal to assess the condition-dependence of melanin signals. Instead, we hypothesized that melanin-plumage is more likely to signal condition in purely achromatic species. We performed a meta-analysis reviewing evidence for condition-dependent melanic plumage: we compared the net effect size for the relationship between melanin traits and condition in species that are achromatic versus species that also display a carotenoid-based trait. Our results indicate that melanin plumage is condition-dependent in species of both plumage types. Contrarily to our prediction, this finding suggests that melanin ornament condition-dependence is not conditional on the context of other ornaments within a species. Instead, melanin ornaments should be viewed as potential conditiondependent signals in all species.
Evidence for baseline glucocorticoids as mediators of reproductive investment in a wild bird.
Love, O.P., Bourgeon, S., Madliger, C.L., Semeniuk, C.A.D. & Williams, T.D.
General & Comparative Endocrinology
199 : 65-69 PDFAbstract:
Determining the mechanisms that mediate investment decisions between current and future reproductive attempts is still a key goal of life-history studies. Since baseline levels of stress hormones (glucocorticoids – GCs) act as predictive and labile regulators of daily energetic balance in vertebrates they remain excellent candidates for mediating investment decisions both within and across reproductive attempts. Using free-living female European starlings (Sturnus vulgaris) we experimentally reduced investment in current reproduction (number of offspring raised in the first brood) to examine whether baseline corticosterone (CORT) acted as a hormonal mediator preparing individuals for a predictable increase in future investment (number of offspring raised in the second brood). Although treatment and control birds raised the same total amount of offspring across two broods, the experimental birds increased reproductive investment in second broods to compensate for the reduced investment in the first brood. Data on both mean and intra-individual changes in baseline CORT support the idea that an increase in baseline CORT between the incubation stages in treatment birds strongly predicted this increase in investment. Importantly, we measured the increase in baseline CORT during late incubation prior to the increase in energetic demand associated with increased reproductive investment in offspring, indicating that flexible within-individual changes in baseline GCs can act as a labile mechanism preparing individuals for predictable increases in reproductive investment. As such, our experimental results indicate that elevated baseline GCs can prepare individuals for investment in energetically expensive life-history stages, rather than simply being elevated as a consequence of increased effort or demand. This suggests that short-term preparative increases in baseline GCs benefit individuals by successfully allowing them to maximize fitness under varying environmental conditions.
Evaluating gonadosomatic index as an estimator of reproductive condition in the invasive round goby, Neogobius melanostomus.
Zeyl, J.N., Love, O.P. & Higgs, D.M.
Journal of Great Lakes Research
40 : 164–171 PDFAbstract:
Using gonadosomatic index cut-off scores has become a standard protocol for selecting reproductive fish in studies on the reproductive biology of round goby, Neogobius melanostomus, a significant invader of the Laurentian Great Lakes, but the validity of this practice has not been validated with histological staging. The goal of the current study was to evaluate the effectiveness of using gonadosomatic index (GSI) cut-off scores to classify reproductive status in male and female round goby by documenting associations between GSI, sex steroids, and gonad development. Gonadal stage was determined in both sexes using hematoxylin and eosin histology. Plasma 11- ketotestosterone and testosterone were measured in males, and testosterone and 17β-estradiol were measured in females. Gonadosomatic index cut-off scores were effective in selecting spawning capable individuals at higher GSI values, but GSI values were limited in the ability to make further distinctions of gonadal stage and missed many spawning capable females. In females, testosterone levels were highest during vitellogenic growth and declined prior to ovulation. 17β-estradiol displayed a similar, but non-statistically significant pattern. Males with developing testes had higher levels of 11-ketotestosterone —but not testosterone —than reproductively immature males, although levels of these androgens were overall positively correlated in males. The findings indicate that the conventional GSI cut-off scores (1% in males, 8% in females) accurately assign spawning capable condition in both sexes; however, they may also exclude some spawning capable females with lower GSIs.
No selection on immunological markers in response to a highly virulent pathogen in an Arctic breeding bird.
Legagneux, P., Berzins, L.L., Forbes, M., Harms, N.J., Hennin, H., Bourgeon, S., Gilchrist, H.G., Bêty, J., Soos, C., Love, O.P., Descamps, S., Foster, J. and G. Burness
In natural populations, epidemics provide opportunities to look for intense natural selection on genes coding for life history and immune or other physiological traits. If the populations being considered are of management or conservation concern, then identifying the traits under selection (or ‘markers’) might provide insights into possible intervention strategies during epidemics. We assessed potential for selection on multiple immune and life history traits of Arctic breeding common eiders (Somateria mollissima) during annual avian cholera outbreaks (summers of 2006, 2007 & 2008). We measured pre-laying body condition, immune traits, and subsequent reproductive investment (i.e., clutch size) and survival of female common eiders and whether they were infected with Pasteurella multocida, the causative agent of avian cholera. We found no clear and consistent evidence of directional selection on immune traits; however, infected birds had higher levels of haptoglobin than uninfected birds. Also, females that laid larger clutches had slightly lower immune responses during the pre-laying period reflecting possible down-regulation of the immune system to support higher costs of reproduction. This supports a recent study indicating that birds investing in larger clutches were more likely to die from avian cholera and points to a possible management option to maximize female survival during outbreaks.
Variation in plasma corticosterone in migratory songbirds: a test of the migration-modulation hypothesis and use as an environmental monitoring tool
Wagner, D.N., Green, D.J., Cooper, J.M., Love, O.P. and T.D. Williams
Physiological and Biochemical Zoology
87 : 695-703. PDFAbstract:
Physiological mechanisms underlying migration remain poorly understood, but recent attention has focused on the role of the glucocorticoid hormone corticosterone (CORT) as a key endocrine regulator of migration. The “Migration-Modulation Hypothesis” (MMH) proposes that baseline CORT levels are elevated in migratory birds to facilitate hyperphagia and lipogenesis, and that further elevation of CORT in response to acute stress is suppressed. Consequently, CORT levels may be a poor indicator of individual condition or environmental variation in migratory birds. Here we tested the MMH by measuring baseline and stress-induced CORT levels in Common Yellowthroats (Geothlypis trichas) during fall migration over two consecutive years. Birds had low baseline plasma CORT levels at initial capture (<5 ng/ml), and then showed a robust stress response with plasma CORT levels increasing to c.50 ng/ml within 10-20 minutes. Our data therefore clearly do not support the MMH. Baseline CORT did not vary with body mass, time of capture, Julian date, or year. Individual variation in the rate of increase in CORT was correlated with Julian day, being higher later in the migration period. Our data suggest that plasma CORT can theoretically be a useful metric in migration studies, but may not be the most appropriate tool for general monitoring purposes due to the large sampling effort required.
Multigenerational outbreeding effects in Chinook salmon (Oncorhynchus tshawytscha)
Outbreeding, mating between genetically divergent individuals, may result in negative fitness consequences for offspring via outbreeding depression. Outbreeding effects are of notable concern in salmonid research as out-breeding can have major implications for salmon aquaculture and conservation management. We therefore quantified outbreeding effects in two generations (F1 hybrids and F2 backcrossed hybrids) of Chinook salmon (Oncorhynchus tshawytscha) derived from captively-reared purebred lines that had been selectively bred for differential performance based on disease resistance and growth rate. Parental lines were crossed in 2009 to create purebred and reciprocal hybrid crosses (n = 53 families), and in 2010 parental and hybrid crosses were crossed to create purebred and back-crossed hybrid crosses (n = 66 families). Although we found significant genetic divergence between the parental lines (FST= 0.130), reciprocal F1 hybrids showed no evidence of outbreeding depression (hybrid breakdown) or favorable heterosis for weight, length, condition or survival. The F2 backcrossed hybrids showed no outbreeding depression for a suite of fitness related traits measured from egg to sexually mature adult life stages. Our study contributes to the current knowledge of outbreeding effects in salmonids and supports the need for more research to better comprehend the mechanisms driving outbreeding depression.
Ecological and medical researchers are investing great effort to determine the role of Maternally-Derived Stress (MDS) as an inducer of phenotypic plasticity in offspring. Many researchers have interpreted phenotypic responses as unavoidable negative outcomes (e.g., small birth weight, high anxiety); however, a biased underestimate of the adaptive potential of MDS-induced effects is possible if they are not viewed within an ecologically relevant or a life-history optimization framework. We review the ecological and environmental drivers of MDS, how MDS signals are transferred to offspring, and what responses MDS induces. Results from four free-living vertebrate systems reveals that although MDS induces seemingly negative investment trade-offs in offspring, these phenotypic adjustments can be adaptive if they better match the offspring to future environments; however, responses can prove maladaptive if they unreliably predict (i.e., are mismatched to) future environments. Furthermore, MDS-induced adjustments that may prove maladaptive for individual offspring can still prove adaptive to mothers by reducing current reproductive investment, and benefitting lifetime reproductive success. We suggest that to properly determine the adaptive potential of MDS, researchers must take a broader integrated life-history perspective, appreciate both the immediate and longer term environmental context, and examine lifetime offspring and maternal fitness.
Maternal adversity and ecological stressors in natural populations: the role of stress axis programming in individuals, with implications for populations and communities.
Biomedical researchers have long appreciated that maternal stressors can induce preparative and adaptive programming in offspring via exposure to maternal Glucocorticoids (GCs). However, few ecologists are aware of the capacity for maternal GC exposure to translate ecological and environmental stressors into preparative and adaptive programmed offspring responses in free-living systems. We review a growing body of experimental work indicating that circulating maternal GCs link ecological stressors with adaptive programming of the stress axis. Throughout, we emphasise that natural and human-induced ecological stressors play a fundamental role in programming the capacity of individuals, populations and communities to respond to both predictable and unpredictable ecological change via translating maternal adversity into responsive programming of the vertebrate stress axis.
To encourage rigorous testing of this paradigm in a broad range of ecological systems, we introduce the principal extrinsic stressors with a recognised potential to alter maternal circulating GC levels. We then review from the biomedical literature regarding the underlying physiological and epigenetic mechanisms of stress-induced programming of individual phenotypes to predict how variation in ecological stressors can produce individual variation in stress axis management.
To appreciate the potential evolutionary inertia (i.e. adaptive value) of maternally programmed individual variation, we review key recent studies in free-living systems that test its adaptive function, and then discuss how variation in stress-axis programming may scale up to influence populations and ecological communities.
Given the huge potential of this field, it is encouraging that ecologists are beginning to examine how and why maternal GCs translate ecological and environmental stressors into preparative stress axis programming in free-living systems.
Complex signaling with simple plumage: multiple ornaments in an achromatic species.
We investigated whether achromatic plumage traits can act as multiple ornaments in an Arctic-breeding passerine, the snow bunting (Plectrophenax nivalis). Specifically, we examine whether multiple ornaments are providing multiple differing messages, are redundant, are unreliable signals of male quality, or are aimed at different receivers. We measured plumage reflectance and pigmentation patterns made conspicuous during male inter- and intrasexual displays that advertise different plumage regions. Our results indicate that although several aspects of male plumage may have redundant messages, different body regions appear aimed at different receivers. The wings of males—displayed primarily toward females during courtship—appear to indicate a pair’s future reproductive performance. Conversely, melanin-based plumage reflectance displayed during intrasexual threat displays provides information on territory features and a male’s capacity to defend it (i.e., territory size, territory quality, testosterone levels). Taken together, we suggest that snow buntings have multiple ornaments that provide information of differential importance in inter- versus intrasexual communication. This study demonstrates that achromatic plumage traits can serve in complex communication. Key words: achromatic plumage, individual quality, multiple ornaments, multiple receivers, Plectrophenax nivalis.
Alula size signals male condition and predicts reproductive performance in an Arctic-breeding passerine.
Guindre-Parker, S., Gilchrist, H.G., Baldo, S. & Love, O.P
Journal of Avian Biology
44 : 209-215 PDFAbstract:
While studies of achromatic plumage signaling are scarce relative to chromatic ornaments, achromatic ornaments have the potential to act as an effi cient form of visual communication due to the highly conspicuous contrast between black and white body regions. Recently, achromatic plumage refl ectance has been shown to indicate condition, yet the conditiondependence of achromatic patch size remains unstudied. Here we show the fi rst evidence that alula size, an achromatic plumage patch, has the potential to signal a male ’ s condition and predict reproductive performance. In Arctic-breeding snow buntings Plectrophenax nivalis , the size of the alula simultaneously predicted pre-breeding physiological health and the number of off spring produced, through an intermediate variable (lay date). Snow buntings appear to pair assortatively; males and females arriving earlier pair together, and changes in body condition over the breeding season are positively related within pairs. We suggest that simple achromatic plumage patches, like alula size, have the potential to act as condition-dependent signals. Consequently, females may benefi t from assessing these signals to reliably evaluate a male ’ s condition and reproductive potential as a means of maximizing their reproductive success.
Baseline corticosterone in wintering marine birds: methodological considerations and ecological patterns.
Palm. E., Esler, D., Anderson, E., Williams, T.D., Love, O.P. & M. Wilson.
Physiological and Biochemical Zoology
86 : 346-353 PDFAbstract:
Previous studies have related levels of plasma corticosterone (CORT) of seabirds to variation in foraging conditions during the breeding period, but it is unclear whether similar relationships between foraging conditions and baseline CORT exist during other life stages. We validated methods for identifying baseline CORT of lethally sampled birds and assessed variation in baseline CORT relative to winter habitat conditions. We collected free-living white-winged scoters (Melanitta fusca) at four wintering sites during December and February. We found increasing CORT values beyond 3 min after time since flush (the duration between initial flush and death), presumably reflecting acute stress responses. Our results demonstrate that it is possible to obtain baseline CORT from lethally sampled birds if the time from initial flush until death is measured. Our study sites varied appreciably in exposure to wind and waves, predation danger, diving depths, and the fraction of preferred foods in scoter diets. Despite these habitat differences, baseline CORT did not vary across sites or winter periods. We interpret this lack of variation as evidence that birds select wintering areas where they can successfully manage site-specific costs and maintain physiological homeostasis.
Condition-dependent auditory processing in the round goby (Neogobius melanostomus): links to sex, reproductive condition, and female estrogen levels.
Neural responses to sensory stimuli often differ between sexes, vary seasonally, and can be regulated by endocrine activity, but the ecological and physiological mechanisms driving such patterns are not well understood. The current study examined how auditory function in the round goby (Neogobius melanostomus), a vocal teleost, co-varied with sex, reproductive condition and female plasma 17β-estradiol level. Auditory evoked potentials were collected in response to tone pips (100–600 Hz) and a natural round goby pulse vocalization. Additionally, saccule hair cell densities were compared across reproductive groups. Auditory threshold was evaluated in terms of pressure and particle acceleration, and response amplitude and onset latency were measured at 10 dB above threshold. Relative to males, females displayed lower auditory thresholds in response to the natural vocalization and to tones at 300–600 Hz, and had a higher density of saccule hair cells. The 17β-estradiol level was positively associated with amplitude and latency for the pulse stimulus and with both threshold and amplitude for tones at 100–200 Hz in females. Relative to non-reproductive males, reproductive males exhibited longer response latencies at 100–200 Hz. The results demonstrate sexual dimorphism in auditory function in a teleost fish as well as intra-sexual variation, partially based on hormone levels. The current research further identifies links between auditory function and reproductive behaviors in fishes and provides a finer-scaled analysis of how this behavior is reflected at the level of the sensory systems facilitating signal reception.
The oxidative costs of territory quality and offspring provisioning
The costs of reproduction are an important constraint that shapes the evolution of life histories, yet our understanding of the proximate mechanism(s) leading to such life-history trade-offs is not well understood. Oxidative stress is a strong candidate measure thought to mediate the costs of reproduction, yet empirical evidence supporting that increased reproductive investment leads to oxidative stress is equivocal. We investigated whether territory quality and offspring provisioning increase oxidative
Determining how animal populations are linked in space and time is important for identifying factors influencing population dynamics and for effective conservation and management. Arctic-breeding migratory passerines are declining and at risk due to forecasted climate change, but are a challenge to monitor due to their inaccessible breeding locations, long-distance migration routes and small body size. For the first time, we combine sub-gram geolocator technology and stable-isotope analysis with mark-recapture (banding) and citizen science data to determine patterns of migratory connectivity for multiple populations of a declining North American Arctic-breeding passerine, snow bunting (Plectrophenax nivalis). We show strong evidence for an east-west parallel migratory system, with Hudson Bay acting as a migratory divide. While band recoveries suggest strong migratory connectivity among eastern wintering populations (more than 95% of band recoveries reveal connections between western Greenland and eastern North America), novel application of geolocators and stable-hydrogen isotope analysis to a Canadian breeding population revealed a high degree of migratory connectivity within western North American wintering populations. Our results also show distinct differences in migratory distance between eastern and western populations, and illustrate how applying multiple techniques can effectively be used to track migration patterns of remote populations. Differences in annual distribution and migratory distance suggest that separate consideration of eastern and western wintering populations may improve future conservation and management efforts for this species.
First Report of a Snow Bunting x Lapland Longspur Hybrid.
Macdonald, C.A., Martin, T., Ludkin, R., Hussell, D., Lamble, D. and Love, O.P.
65 : 344-348 PDFAbstract:
In late April 2011, photographs of an apparent male snow bunting (Plectrophenax nivalis) × Lapland longspur (Calcarius lapponicus) hybrid were taken at St. Lewis Inlet, Newfoundland and Labrador, Canada, while the bird was foraging in a mixed flock of both species along a previously documented spring migratory route. As far as we are aware, this is the first hybridization of these species documented anywhere in the world. The bird was identified as a male on the basis of longspur nape coloration, and it appears to have the head, beak, and back coloration and patterning of a Lapland longspur, but the chin, chest and throat, and overall appearance of a snow bunting. Although our research team has banded more than 50 000 birds of both species over the past 30 years across the latitudinal range of both species, we have never observed such a hybrid. While these Arctic-breeding species overlap spatially and temporally during wintering, migration, and breeding, longspurs and buntings have distinct sexual characters and breed in different ecological niches, which may account for the reproductive isolation or low rates of hybridization of these species. While we were unable to conduct detailed morphological or genetic comparisons on this particular individual for phylogenic interpretation, this report highlights the importance of reporting field observations that may indicate ecological changes affecting the hybridization rates of these inaccessible Arctic species.
Primary and secondary sexual characters in alternative reproductive tactics of Chinook salmon: Associations with androgens and the maturation-inducing steroid.
Butts, I.A.E., Love, O.P., Farwell, M. and T.E. Pitcher.
General and Comparative Endocrinology
175 : 449-456 PDFAbstract:
The proximate mechanisms that underlie the evolution of within-sex variation in mating behavior, sexual characters and reproductive investment patterns are still poorly understood. Species exhibiting alternative reproductive tactics (ARTs) are ideal model systems to examine these mechanisms. Chinook salmon (Oncorhynchus tshawytscha) exhibits two distinct ARTs: hooknoses, which are large males that establish spawning dominance hierarchies via intense male–male competition and jacks, which are smaller precocious sneaking males that steal fertilizations via sperm competition. In this study, we examine plasma testosterone (T), 11-ketotestosterone (11-KT) and maturation-inducing steroid (MIS; 17a,20bdihydroxy- 4-pregnen-3-one) profiles of spawning hooknoses and jacks. Furthermore, we examine relationships between androgens and primary (gonad mass, gonadosomatic index and sperm traits) and secondary (total mass, body size, hump depth and kype length) sexual characters. Relationships between MIS and sperm traits are also examined. We found that hooknoses and jacks did not significantly differ in terms of plasma T, 11-KT or MIS concentrations. Moreover, we found significant positive relationships between levels of both androgens within each ART. There were no significant relationships between androgens, MIS and sperm traits. T and 11-KT concentrations co-varied positively with gonad investment and kype length in jacks. In hooknoses, 11-KT concentration was positively related to total mass, hump depth and condition factor. Overall, these findings suggest that there are differential androgen effects for each of the ARTs in Chinook salmon.
Manipulation of developmental stress reveals sex-specific effects of egg size on offspring phenotype.
Love, O.P. and Williams, T.D.
Journal of Evolutionary Biology
24 : 1497-1504 PDFAbstract:
The general lack of experimental evidence for strong, positive effects of egg size on offspring phenotype has led to suggestions that avian egg size is a neutral trait. To better understand the functional significance of intra-specific variation in egg size as a determinant of offspring fitness within a life-history (sex-specific life-history strategies) and an environmental (poor rearing conditions) context, we experimentally increased developmental stress (via maternal feather-clipping) in the sexually size-dimorphic European starling (Sturnus vulgaris) and measured phenotypic traits in offspring across multiple biological scales. As predicted by life-history theory, sons and daughters had different responses when faced with developmental stress and variation in egg size. In response to developmental stress, small egg size in normally fastergrowing sons was associated with catch-up growth prior to attaining larger adult size, resulting in a reduction in developmental stability. Daughters apparently avoided this developmental instability by reducing growth rate and eventual adult body mass and size. Interestingly, large egg size provided offspring with greater developmental flexibility under poor growth conditions. Large-egg sons and daughters avoided the reduction in developmental stability, and daughters also showed enhanced escape performance during flight trials. Furthermore, large egg size resulted in elevated immune responses for both sexes under developmental stress. These findings show that there can be significant, but complex, context-specific effects of egg size on offspring phenotype at least up to fledging, but these can only be demonstrated by appreciating variation in the quality of the offspring environment and life histories. Results are therefore consistent with egg size playing a significant role in shaping the phenotypic outcome of offspring in species that show even greater intra-specific variation in egg size than starlings.
Individual optimization of reproduction in a long-lived migratory bird: a test of the condition-dependent model of laying date and clutch size.
Descamps, S., Gilchrist, H.G., Love, O.P. and Bêty, J.
25 : 671-681 PDFAbstract:
Optimality theory predicts that both timing of arrival and arrival state on the breeding area will determine reproductive timing and investment in migratory organisms. We tested this idea using a condition-dependent individual optimization model (Ardea 68, 1980, 225 and The American Naturalist 143, 1994, 698) in common eider ducks through descriptive data, path analyses and experimental manipulation.
Our results support the causal pathways drawn from the optimization model indicating that individuals adjust their reproductive decisions as a function of their arrival date and body condition at arrival.
Independent of body condition, early-arriving females had a longer pre-laying period, but still initiated their nests earlier, and produced larger clutches than late-arriving birds. Independent of arrival date, females in good condition laid earlier than those in poor condition. Manipulation of pre-laying female body condition confirmed that the relationship between condition and laying date was causal.
Female common eiders appear to optimize reproductive decisions in response to both their external (i.e. environmental conditions affecting the egg-value) and internal (i.e. body condition) states. These adjustments seem to minimize the fitness costs of reproduction, in which higher clutch size is not associated with an apparent lower survival or future breeding probability.
Our study emphasizes the importance of (i) simultaneously considering the timing of migration, the state of individuals and the seasonal change in egg-value to understand clearly birds’ breeding decisions and (ii) appreciating the potential proximate and ultimate factors explaining why some individuals delay breeding and ⁄ or produce small clutches.
Avian cholera, post-hatching survival and selection on hatch characteristics in a long-lived bird.
Descamps, S., Forbes, M.R., Gilchrist, H.G., Love, O.P. and Bêty, J.
Journal of Avian Biology
42 : 39-48 PDFAbstract:
Infectious diseases can have dramatic impacts on animal population dynamics, but how they influence vital rates remains understudied. We took advantage of the appearance of an avian cholera epizootic in an arctic colony of common eiders Somateria mollissima to study variation in juvenile survival and selection on hatch characteristics in relation to this highly infectious disease. Avian cholera is one of the most important infectious diseases affecting wild birds and is thought to primarily affect adult survival. Here, we show that avian cholera was associated with a 90% decline in duckling survival, leading to almost zero recruitment. Before the cholera outbreak, there was significant stabilizing selection on hatching date and significant positive directional selection on hatching mass. During cholera outbreaks, selection on hatch characteristics was no longer significant. These results were based on a low sample of surviving ducklings in cholera years, but suggested that date and mass at hatching did no longer affect duckling survival in the presence of cholera. These effects of avian cholera on post-hatching survival were likely not only the consequence of the disease per se, but also a consequence of an increase in predation rates that followed the emergence of avian cholera. Our results emphasize the dramatic direct and indirect impacts that infectious disease can have on vital rates, and thus population dynamics.
Pre-laying climatic cues can time reproduction to optimally match offspring hatching and ice conditions in an Arctic marine bird.
Love, O.P., Gilchrist, H.G., Descamps, S., Semeniuk, C.A.D. and Bêty, J.
164 : 277-286 PDFAbstract:
Individuals breeding in seasonal environments are under strong selection to time reproduction to match offspring demand and the quality of the post-natal environment. Timing requires both the ability to accurately interpret the appropriate environmental cues, and the flexibility to respond to inter-annual variation in these cues. Determining which cues are linked to reproductive timing, what these cues are predicting and understanding the fitness consequences of variation in timing, is therefore of paramount interest to evolutionary and applied ecologists, especially in the face of global climate change. We investigated inter-annual relationships between climatic variation and the timing of reproduction in Canada’s largest breeding population of Arctic common eiders (Somateria mollissima) in East Bay, Nunavut. Warmer spring temperatures predicted both earlier mean annual laying dates and the earlier ice-free conditions required by ducklings for post-natal growth. Warmer springs had higher variation in this temperature cue, and the population laying distribution became increasingly positively-skewed in warmer summers, potentially indicating that more lowquality females had the opportunity to commence laying in warmer years. Females that timed laying to match duckling hatching just prior to fully ice-free conditions obtained the highest duckling survival probability. Inter-annual data on repeated breeding attempts revealed that the individuals examined show a similar degree of laying flexibility in response to climatic variation; however, there was significant individual variation in the absolute timing of laying within an average year. This work sheds light on how reproductive timing is related to and influenced by variation in local climate and provides vital information on how climate-related variation in reproductive timing influence a fitness measure in an Arctic species. Results are especially relevant to future work in polar environments given that global climatic changes are predicted to be most intense at high latitudes.
Using life-histories to predict and interpret variability in yolk hormones.
Love, O.P., Gilchrist, H.G., Bêty, J., Wynne-Edwards, K.E., Berzins, L. and Williams, T.D.
General and Comparative Endocrinology
163 : 169-174 PDFAbstract:
Variation in yolk hormones is assumed to provide the plasticity necessary for mothers to individually optimize reproductive decisions via changes in offspring phenotype, the benefit being to maximise fitness. However, rather than routinely expecting adaptive variation within all species, the pattern and magnitude of yolk hormone deposition should theoretically relate to variation in life-histories. Here we present data on intra-clutch variation in yolk corticosterone in three species along a developmental continuum (European starling (Sturnus vulgaris): fully altricial; black guillemot (Cepphus grylle): semi-precocial; common eider (Somateria mollissima): fully precocial) to examine how and why variation in life-histories might relate to the evolution of variation in yolk steroids. Starlings and guillemots showed a significant increase in yolk corticosterone across the laying sequence; however, we found no pattern within eider clutches. Moreover, starlings showed the largest difference (94.6%) in yolk corticosterone between first- and last-laid eggs, whereas guillemots showed a moderate difference (58.9%). Despite these general species-specific patterns, individuals showed marked variation in the intra-clutch patterns of yolk corticosterone within each species indicating potential differences in intra-clutch flexibility among females. It is well documented that exposure to elevated yolk glucocorticoids reduces offspring quality at birth/hatching in many taxa and it has therefore been proposed that elevated yolk levels may modulate offspring competition and/or facilitate brood reduction under harsh conditions in birds. Our data suggests that intra-clutch variation in yolk corticosterone has the potential to act as an adaptive maternal effect in species where modulation of competition between nest-bound offspring would benefit mothers (starlings and guillemots). However, in precocial species where mothers would not benefit from a modulation of offspring quality, intra-clutch variation in yolk hormones may play little or no adaptive role. While future phylogenetically-controlled studies will be helpful in examining questions of adaptive mechanisms once more data on yolk corticosterone becomes available, our results nonetheless suggest that research on the evolutionary role of yolk hormones can benefit by a priori incorporating species-specific life-history-driven hypotheses.
Yolk corticosterone enhances flight performance and muscle metabolic capacity in juvenile European starlings.
Chin, E.H., Love, O.P., Verspoor, J.J., Williams, T.D., Rowley, K. and G. Burness.
Proceedings of the Royal Society of London B
276 : 499-505 PDFAbstract:
Exposure to maternally derived glucocorticoids during embryonic development impacts offspring phenotype. Although many of these effects appear to be transiently ‘negative’, embryonic exposure to maternally derived stress hormones is hypothesized to induce preparative responses that increase survival prospects for offspring in low-quality environments; however, little is known about how maternal stress influences longer-term survival-related performance traits in free-living individuals. Using an experimental elevation of yolk corticosterone (embryonic signal of low maternal quality), we examined potential impacts of embryonic exposure tomaternally derived stress on flight performance, wing loading, muscle morphology and muscle physiology in juvenile European starlings (Sturnus vulgaris). Here we report that fledglings exposed to experimentally increased corticosterone in ovo performed better during flight performance trials than control fledglings. Consistent with differences in performance, individuals exposed to elevated embryonic corticosterone fledged with lower wing loading and had heavier and more functionally mature flight muscles compared with control fledglings. Our results indicate that the positive effects on a survivalrelated trait in response to embryonic exposure to maternally derived stress hormones may balance some of the associated negative developmental costs that have recently been reported. Moreover, if embryonic experience is a good predictor of the quality or risk of future environments, a preparative phenotype associated with exposure to apparently negative stimuli during development may be adaptive.
Shifts in metabolic demands in growing altricial nestlings illustrate context-specific relationships between BMR and body composition.
Vézina, F., Love, O.P., Lessard, M. and Williams, T.D.
Physiological and Biochemical Zoology
82 : 248–257 PDFAbstract:
Basal metabolic rate (BMR) in animals is interpreted as reflecting the size and metabolic intensity of energy-consuming tissues. However, studies investigating relationships between the mass of specific organs and inter-individual variation in BMR have produced inconsistent patterns with regard to which organs have the largest impact on BMR variation. Because of the known flexibility in organ mass and metabolic intensity within individual organs, relationships between BMR and body composition variables are bound to be context specific. Altricial nestlings are excellent models to illustrate this phenomenon because of the extreme variation in body composition occurring during growth. Using European starlings at three age classes, we studied changes in body composition together with its effect on variation in resting metabolic rate (RMR) in order to highlight the context-specific nature of these relationships. Our data suggest a transition in metabolic costs during growth in starling nestlings. During the linear phase of growth, energy is mainly consumed by tissue-synthesis processes, with fast-growing organs having a large influence on RMR. In the plateau phase of growth, the energy expenditure is transferred to functional costs, with high-intensity organs having a predominant effect on RMR variation. Our data illustrates the context-specific nature of organ mass–metabolic rate correlations, which complicates inter- and intra-specific comparisons of BMR. In the future, such comparisons must be done while taking the physiological state of the study animal into account.
The adaptive value of stress-induced phenotypes in the wild: effects of maternally-derived corticosterone on sex allocation, cost of reproduction and maternal fitness.
Love, O.P. and Williams, T.D.
172 : E135-E149 PDFAbstract:
The question of why maternal stress influences offspring phenotype is of significant interest to evolutionary physiologists. Although embryonic exposure to maternally derived glucocorticoids (i.e., corticosterone) generally reduces offspring quality, effects may adaptively match maternal quality with offspring demand.We present results from an interannual field experiment in European starlings (Sturnus vulgaris) designed explicitly to examine the fitness consequences of exposing offspring to maternally derived stress hormones. We combined a manipulation of yolk corticosterone (yolk injections) with a manipulation of maternal chick-rearing ability (feather clipping of mothers) to quantify the adaptive value of corticosteroneinduced offspring phenotypes in relation to maternal quality. We then examined how corticosterone-induced “matching” within this current reproductive attempt affected future fecundity and maternal survival. First, our results provide support that low-quality mothers transferring elevated corticosterone to eggs invest in daughters as predicted by sex allocation theory. Second, corticosterone-mediated sex-biased investment resulted in rapid male-biased mortality resulting in brood reduction, which provided a better match between maternal quality and brood demand. Third, corticosterone-mediated matching reduced investment in current reproduction for low-quality mothers, resulting in fitness gains through increased survival and future fecundity. Results indicate that the transfer of stress hormones to eggs by low-quality mothers can be adaptive since corticosteronemediated sex-biased investment matches the quality of a mother to offspring demand, ultimately increasing maternal fitness. Our results also indicate that the branding of the proximate effects of maternal glucocorticoids on offspring as negative ignores the possibility that short-term phenotypic changes may actually increase maternal fitness.
Sex-specific variability of the immune system across life-history stages.
Love, O.P., Salvante, K.G., Dale, J. and Williams, T.D.
172 : E99–E112 PDFAbstract:
Organisms theoretically manage their immune systems optimally across their life spans to maximize fitness. However, we lack information on (1) how the immune system is managed across life-history stages, (2) whether the sexes manage immunity differentially, and (3) whether immunity is repeatable within an individual. We present a within-individual, repeated-measures experiment examining life-history stage variation in the inflammatory immune response in the zebra finch (Taeniopygia guttata). In juveniles, agedependent variation in immune response differed in a sex- and context-specific manner, resulting in no repeatability across stages. In adults, females displayed little stage-dependent variation in immune response when laying while receiving a high-quality (HQ) diet; however, laying while receiving a low-quality (LQ) diet significantly reduced both immune responses and reproductive outputs in amanner consistent with a facultative (resource-driven) effect of reproduction on immunity. Moreover, a reduced immune response in females who were raising offspring while receiving an HQ diet suggests a residual effect of the energetic costs of reproduction. Conversely, adult males displayed no variation in immune responses across stages, with high repeatability from the nonbreeding stage to the egg-laying stage, regardless of diet quality (HQ diet, rp0.51; LQ diet, rp0.42). Females displayed high repeatability when laying while receiving the HQ diet (rp0.53); however, repeatability disappeared when individuals received the LQ diet. High-response females receiving the HQ diet had greater immune flexibility than did low-response females who were laying while receiving the LQ diet. Data are consistent with immunity being a highly plastic trait that is sex- specifically modulated in a context-dependent manner and suggest that immunity at one stage may provide limited information about immunity at future stages.
Plasticity in the adrenocortical response of a free-living vertebrate: the role of pre- and post-natal developmental stress.
Love, O.P. and Williams, T.D.
Hormones and Behavior
54 : 496-505 PDFAbstract:
Sex differences in DHEA and estradiol during development in a wild songbird: jugular versus brachial plasma
Chin, E.H., Shah, A.M., Schmidt, K.L., Sheldon, L.D., Love, O.P. and Soma, K.
Hormones and Behavior
54 : 194-202 PDFAbstract:
Sexual differentiation of the brain has traditionally been thought to be driven by gonadal hormones, particularly testosterone (T). Recent studies in songbirds and other species have indicated that non-gonadal sex steroids may also be important. For example, dehydroepiandrosterone (DHEA) – a sex steroid precursor that can be synthesized in the adrenal glands and/or brain – can be converted into active sex steroids, such as 17β-estradiol (E2), within the brain. Here, we examine plasma DHEA and E2 levels in wild developing European starlings (Sturnus vulgaris), from hatch (P0) to fledging (P20). Blood samples were collected from either the brachial vein (n=143) or the jugular vein (n=129). In songbirds, jugular plasma is enriched with neurally-synthesized steroids and, therefore, jugular plasma is an indirect measure of the neural steroidal milieu. Interestingly, brachial DHEA levels were higher in males than females at P4. In contrast, jugular DHEA levels were higher in females than males at P0 and P10. Brachial E2 levels were higher in males than females at P6. Surprisingly, jugular E2 levels were not high and showed no sex differences. Also, we calculated the difference between brachial and jugular steroid levels. At several ages, jugular steroid levels were lower than brachial levels, particularly in males, suggesting greater neural metabolism of circulating DHEA and E2 in males than females. At a few ages, jugular steroid levels were higher than brachial levels, suggesting neural secretion of DHEA or E2 into the general circulation. Taken together, these data suggest that DHEA may play a role in brain sexual differentiation in songbirds.
Sex-specific development of avian flight performance under experimentally altered rearing conditions.
Verspoor, J.J., Love, O.P., Rowland, E., Chin, E.H. and Williams, T.D.
18 : 967-973 PDFAbstract:
Numerous studies have examined predation risk resulting from the costs of impaired flight performance associated with many key life-history stages such as reproduction and migration. Interestingly, although avian nestlings experience multiple resource-based physiological trade-offs and undergo considerable morphological and physiological changes during postnatal development, there is no data available on how nestlings manage the competing demands of growth and the development of flight ability at this critical life-history stage. We examined numerous morphological traits to determine which are responsible for variation in flight performance in juvenile European starlings (Sturnus vulgaris), a sexually size-dimorphic passerine. We then manipulated maternal quality during chick rearing (via feather clipping) to examine sex-specific sensitivity of fledgling flight performance to the quality of the rearing environment. Results suggest that the mechanics underlying variation in juvenile flight performance are relatively simple, being principally determined by the ratio of pectoral muscle mass to body mass (BM) and the surface area of the wings. Interestingly, although the maternal quality manipulation decreased BM and structural size in daughters, only the flight performance of sons was negatively affected. Our results suggest that a survival-related trait can be significantly affected in the larger sex when raised under stressful conditions. Furthermore, measuring only BM and structural size may not be sufficient in understanding how the sexes are affected by stressful rearing conditions in sexually size-dimorphic species. Key words: development, escape performance, european starling, flight ability, predation risk, sexual size dimorphism. [Behav Ecol 18:967–973 (2007)]
Manipulating rearing conditions reveals developmental sensitivity of the smaller sex in a passerine bird, the European starling
Rowland, E., Love, O.P., Verspoor, J.J., Sheldon, L. and Williams, T. D.
Sturnus vulgaris. Journal of Avian Biology
38 : 612-618 PDFAbstract:
Traditionally, studies of sexually size-dimorphic birds and mammals report that the larger sex is more sensitive to adverse environmental conditions during ontogeny. However, recent studies in avian species that exhibit moderate size-dimorphism indicate that the smaller sex may be more sensitive to poor rearing conditions. To better understand sex-specific sensitivity in a passerine exhibiting moderate size-dimorphism, we examined growth, cell-mediated immunity (CMI) and survival of European starling Sturnus vulgaris nestlings following an experimental reduction of maternal rearing ability (via a feather-clipping manipulation). Contrary to conventional theory, daughters showed reduced growth in both body mass and measures of structural size in response to the maternal treatment. In contrast, sons showed no reductions in any of these traits in relation to the treatment. No sex-specific differences in nestling CMI were found for either group, although CMI of nestlings raised by manipulated mothers were higher than those of control nestlings. Finally, fledging sex ratios did not change from those at hatching indicating that neither sex appeared differentially sensitive to the maternal treatment in terms of mortality. These results reveal that variation in the quality of the rearing environment can have significant effects on the smaller sex of a passerine exhibiting moderate dimorphism and as such support recent studies of species with small-moderate sexual size-dimorphism. Combined results suggest that sex-specific effects of environmental variation on nestling development may be both context- (i.e., brood size, resource level, hatching order) and temporally- (when during development they occur) specific. Furthermore, more studies are needed that examine multiple traits at several developmental stages and then follow the sexes over the longerterm to examine potential effects on fitness.
Plasma binding globulins and the acute stress response.
Breuner, C.W., Lynn, S.E., Julian, G.E., Cornelius, J.M., Heidinger, B.J., Love, O.P., Sprague, R.S., Wada, H. and Whitman, B.A.
Hormones and Metabolic Research
38 : 260-268 PDFAbstract:
Within studies of acute stress physiology an increase in glucocorticoid secretion is thought to be the primary mediator of tissue response to stress. Corticosteroid-binding globulin may regulate tissue availability of steroids, but has not been considered a dynamic component of the acute stress response. Here, we examined CBG level over the common 60-minute time frame in an acute capture and handling protocol to investigate whether CBG capacity is dynamic or static over short stressors. Using a comparative approach, we measured CBG response to capture and handling stress in nine species of birds, representing five orders and nine families. CBG capacity significantly declined within 30–60 minutes of capture in five of the nine species examined. This decline may serve to significantly increase the level of corticosterone reaching tissues during acute stress.
Stress hormones: a link between maternal condition and sex-biased reproductive investment.
Love, O.P., Chin, E.H., Wynne-Edwards, K.E. and Williams, T.D.
166 : 751-766 PDFAbstract:
In species where offspring fitness is sex-specifically influenced by maternal reproductive condition, sex allocation theory predicts that poor-quality mothers should invest in the evolutionarily less expensive sex. Despite an accumulation of evidence that mothers can sex-specifically modulate investment in offspring in relation to maternal quality, few mechanisms have been proposed as to how this is achieved. We explored a hormonal mechanism for sex-biased maternal investment by measuring and experimentally manipulating baseline levels of the stress hormone corticosterone in laying wild female European starlings (Sturnus vulgaris) and examining effects on sex ratio and sex-specific offspring phenotype adjustment. Here we show that baseline plasma corticosterone is negatively correlated with energetic body condition in laying starlings, and subsequent experimental elevation of maternal baseline plasma corticosterone increased yolk corticosterone without altering maternal condition or egg quality per se. Hormonal elevation resulted in the following: female-biased hatching sex ratios (caused by elevated male embryonic mortality), lighter male offspring at hatching (which subsequently grew more slowly during postnatal development), and lower cellmediated immune (phytohemagglutinin) responses in males compared with control-born males; female offspring were unaffected by the manipulation in both years of the study. Elevated maternal corticosterone therefore resulted in a sex-biased adjustment of offspring quality favorable to female offspring via both a sex ratio bias and a modulation of male phenotype at hatching. In birds, deposition of yolk corticosterone may benefit mothers by acting as a bet-hedging strategy in stochastic environments where the correlation between environmental cues at laying (and therefore potentially maternal condition) and conditions during chick-rearing might be low and unpredictable. Together with recent studies in other vertebrate taxa, these results suggest that maternal stress hormones provide a mechanistic link between maternal quality and sex-biased maternal investment in offspring.
Brood size and environmental conditions sex-specifically affect nestling immune response in the European starling
Chin, E.H., Love, O.P., Clark, A.M. and Williams, T.D.
Sturnus vulgaris. Journal of Avian Biology
36 : 549-554 PDFAbstract:
In sexually size-dimorphic species, the larger sex can be more sensitive to stressful environmental conditions, often resulting in reduced growth and elevated mortality rates. Development of the immune system is regarded as highly resource dependent, and recent data suggest that nestling passerines experience a possible resource-based trade-off between growth and immunity. Given the hypothesized importance of maximizing growth for the larger sex, the corresponding immune system may also exhibit similar sensitivity to limited resources. To better understand how natural variation in brood size and resources might differentially affect growth and immune function in nestlings of a sexually size-dimorphic species, we examined the relationship between brood size and inter-sexual differences in cell-mediated immunity (CMI) and survival in European starling Sturnus vulgaris nestlings where males are larger in both mass and structural size.We hypothesized that male CMI response should be negatively impacted by increasing sibling competition (brood size), especially during periods of low resource availability. In a year of reduced parental provisioning rates and reduced chick growth rates, male offspring exhibited the predicted negative relationship, whereas female CMI response was unaffected. However, in a year of improved provisioning rates and chick growth, neither sex exhibited a negative relationship between immune response and brood size. Thus, natural variation in brood size can affect sex-specific immunity differently in offspring of a sexually size-dimorphic passerine. However, this relationship appears resource-dependent, suggesting that the hypothesized resource-based trade-off may be compensated for in years of adequate resource abundance.
Mediation of a corticosterone-induced reproductive conflict.
Love, O.P., Breuner, C.W., Vézina, F. and Williams, T.D.
Hormones and Behavior
46 : 59-65 PDFAbstract:
Current research in birds suggests that a conflict should exist during reproduction for the role of the glucocorticoid corticosterone (CORT). While elevated levels have been correlated with the increased energetic demand of raising offspring, elevated CORT levels have traditionally been implicated in reproductive abandonment. We examined the relationship between CORT and nest desertion in breeding wild female European starlings (Sturnus vulgaris) incorporating analyses of both total circulating levels and ‘free’, unbound CORT through analysis of corticosteroid-binding globulin (CBG). Free baseline CORT levels of nest-abandoning birds were significantly higher than nonabandoning birds within each stage, with chick-rearing birds exhibiting the highest free baseline CORT levels, while concurrently remaining the most resistant stage to nest desertion. Elevated free baseline CORT levels in chick-rearing birds were not due to increased total CORT secretion, but rather to a decrease in CBG levels. Overall, our results suggest that CORT and CBG interact to play a role in mediating the increased energetic demand of offspring, while minimizing the chances of nest desertion, thereby alleviating any potential behavioral conflict for CORT during reproduction. Furthermore, these results demonstrate that the traditional view of the role of CORT during reproduction is much more complex than previously appreciated. Together with mounting evidence, we suggest that elevated corticosteroid levels are an inherent and necessary part of reproduction in nonmammalian tetrapods.
What causes the decrease in haematocrit during egg production?
Wiliams, T. D., Challenger, W.O., Christians, J.K., Evanson, M., Love, O., and F. Vézina
18 : 480-488 PDFAbstract:
Anaemia has been reported in wild animals, typically associated with traumatic events or ill health. However, female birds routinely become ‘anaemic’ during egglaying; we sought to determine the causes of this reduction in haematocrit.
Haematocrit in female European Starlings ( Sturnus vulgaris Linnaeus) decreased between pre-breeding and egg-laying in 3 out of 4 years (the decrease was marginally non-significant in the fourth year). This was independent of changes in ambient temperature altering the metabolic requirements for thermoregulation.
There was a positive relationship between haematocrit and plasma levels of the yolk precursor vitellogenin among egg-laying birds, supporting the hypothesis that the initial reduction in haematocrit is caused by increased blood volume associated with osmoregulatory adjustments to elevated levels of yolk precursors.
However, haematocrit did not always recover upon cessation of egg production, remaining low at clutch completion (2 of 4 years), incubation (1 of 2 years) and chick rearing (1 of 4 years), suggesting an additional cause of the prolonged reduction in haematocrit.
Given the magnitude and prolonged nature of the changes in haematocrit we report, and the interannual variation in haematocrit even during chick-rearing (47–54%), we suggest that ‘anaemia’ associated with egg production might have implications for aerobic performance during later stages of breeding.
Corticosterone levels during post-natal development in captive American kestrels (Falco sparverius).
Love, O.P., Bird, D.M. and Shutt, L.J.
General and Comparative Endocrinology
130 : 135-141 PDFAbstract:
We investigated post-natal development of the adrenocortical stress-response system in captive American kestrels (Falco sparverius) by measurements of baseline and stress-induced levels of corticosterone at ages 10, 16, 22, and 28 days post-hatching. Baseline levels of corticosterone increased significantly during post-natal development and although chicks aged 10- and 16-days old exhibited comparable baseline corticosterone levels, those of 22-day-old chicks were significantly higher and those of 28-day-old chicks close to fledging were higher than all younger groups. Chicks in this study exhibited low levels of stress-induced corticosterone early in development and did not exhibit adult-type stress-induced levels of corticosterone until 22 days of age post-hatching. Finally, although baseline and stress-induced levels of 28-day-old birds were significantly higher than one-year-old adults, there was no relationship between baseline corticosterone concentrations and time to nest departure. The fact that baseline levels of corticosterone are low during early development and then increase during later development may be an adaptation to the negative effects of chronically elevated corticosterone levels and as previously noted in other studies may minimize these negative effects on rapid growth and development in young birds, potentially maximizing normal growth. The ability of even young kestrel chicks to elevate corticosterone levels in response to stress suggests that they may be able to physiologically cope with food shortages associated with unpredictable food resources which wild kestrels often face.
Repeated restraint and sampling results in reduced corticosterone levels in developing and adult captive American kestrels (Falco sparverius).
Love, O.P., Shutt, L.J., Silfies, J.S. and Bird, D.M
Physiological and Biochemical Zoology
76 : 753-761 PDFAbstract:
The study of intraspecific variation in adrenocortical function in birds through measures of glucocorticoids is an emerging and potentially useful tool in the field of behavioral and evolutionary endocrinology. However, the effects of handling of study animals on endocrine-mediated phenotypic development and endocrine phenotypic variation have rarely been considered (Dufty et al. 2002), despite the fact that handling may produce physiological effects that can bias subsequent physiological and behavioral interpretations (Clinchy et al. 2001). These potential effects increase in importance during long-term studies such as those examining endocrine-mediated life-history trade-offs (Zera and Harshman 2001) and endocrine system disruption by human-caused environmental perturbations (Dufty et al. 2002; Love et al. 2003c). Studies of adrenocortical function in birds through measures of the stress hormone corticosterone (B) are one such area routinely involving the recapture, rehandling, and resampling of animals, potentially affecting variation in corticosterone itself.
Effects of dietary PCB exposure on adrenocorticol function of captive American Kestrels (Falco sparverius).
We experimentally examinedthe effects of dietary exposure to polychlorinatedbiphenyls (PCBs) on adrenocortical function in American kestrels (Falco sparverius). Nine captive male American kestrels previously exposedto a PCB mixture (AroclorTM1248 : 1254 : 1260; 1 : 1 : 1) in their diet were subjected to a standardized capture, handling and restraint protocol designed to produce an increase in circulating corticosterone. A similar protocol has been appliedto a wide range of avian species andwas usedhere to evaluate the response of PCB- exposed and control kestrels to a defined physical stressor. Both baseline and stress-induced corticosterone levels were significantly lower in PCB-exposed birds when compared with control birds of the same age. PCB-exposed birds exhibited significantly lower corticosterone levels during the corticosterone response when compared with control birds, independent of body condition. Furthermore, baseline corticosterone concentrations exhibited a hormetic response characterizedby an invertedU-shaped dose response in relation to total PCB liver burden. These results support several recent studies which report decreased levels of circulating corticosterone in PCB- exposed wild birds. The results presented here provide the first evidence that exposure to an environmentally relevant level of PCBs (approximately 10 mg/kg body weight) can impair the corticosterone stress response in kestrels, potentially increasing the susceptibility of birds to environmental stressors such as severe weather and predatory and human disturbance.