Frances Bonier

Do baseline glucocorticoids predict fitness

Baseline glucocorticoid (cort) levels are increasingly employed as physiological indices of the relative condition or health of individuals and populations. Often, high cort levels are assumed to indicate an individual or population in poor condition and with low relative fitness (the Cort-Fitness Hypothesis). We review empirical support for this assumption, and find that variation in levels of baseline cort is positively, negatively, or non-significantly related to estimates of fitness. These relationships between levels of baseline cort and fitness can vary within populations and can even shift within individuals at different times in their life history. Overall, baseline cort can predict the relative fitness of individuals and populations, but the relationship is not always consistent or present.

The relationship between fitness and baseline glucocorticoids in a passerine bird

Glucocorticoid (cort) hormones are increasingly applied in studies of free-ranging animals, with elevated baseline cort levels generally assumed to indicate individuals or populations in worse condition and with lower fitness (the Cort-Fitness Hypothesis). The relationship between cort and fitness is rarely validated and studies investigating the cort-fitness relationship often find results inconsistent with the Cort-Fitness Hypothesis. The inconsistency of these studies may result in part from variation in the cort-fitness relationship across life history stages. Here we address the following questions in a two-year study in free-ranging tree swallows (Tachycineta bicolor): (1) Do baseline cort levels correlate with fitness within a life history stage? (2) Does the cort-fitness relationship vary across different life history stages? (3) Does the cort-fitness relationship vary across life history stages within an individual? (4) Does reproductive effort influence cort levels, and do cort levels influence reproductive effort? We measured baseline cort and fitness components in female birds of known breeding stages. We find correlations between baseline cort levels and fitness within some life history stages, but the relationship shifts from negative during early breeding to positive during late breeding, even within the same individuals. A positive relationship between baseline cort and fitness components during the nestling period suggests that reproductive investment may elicit higher cort levels that feedback to reallocate more effort to reproduction during critical periods of nestling provisioning. Our findings provide reason to question the Cort-Fitness Hypothesis, and have implications for the application of cort measures in monitoring the condition of populations of conservation concern.

Urban birds have broader environmental tolerance

Urbanization dramatically changes the composition and diversity of biotic communities. The characteristics distinguishing species that persist in urban environments, however, are poorly understood. Here we test the hypothesis that broadly adapted organisms are better able to tolerate urbanization, using a phylogenetically controlled, global comparison of birds. We compared elevational and latitudinal distributions of 217 urban birds found in 73 of the worlds largest cities with distributions of 247 rural congeners to test the hypothesis that urban birds possess broader environmental tolerance. Urban birds had markedly broader environmental tolerance than rural congeners, as estimated by elevational and latitudinal distributions. Our results suggest that broad environmental tolerance may predispose some birds to thrive in urban habitats. The mechanisms mediating such environmental tolerance warrant further investigation, but probably include greater behavioural, physiological and ecological flexibility.

Within seasons and among years: When are corticosterone levels repeatable?

Hormones play a central role in integrating internal and external cues to help mediate life-history decisions as well as changes in behavior and physiology of individuals. Describing the consistency of endocrine traits within and among individuals is an important step for understanding whether hormonal traits are dependable predictors of phenotypes that selection could act upon. However, few long-term field studies have investigated the individual consistency of hormonal traits. Glucocorticoid hormones mediate homeostatic responses to environmental variation as well as stress responses to acute, unpredictable disturbances. We characterized the repeatability of plasma corticosterone concentrations in two species of free-living passerines across multiple years. We found repeatability in baseline corticosterone concentrations in both sexes of great tits (Parus major) and in female tree swallows (Tachycineta bicolor) within the breeding season but no repeatability of this trait among seasons or across years. Stress-induced levels of corticosterone were only assessed in great tits and were not repeatable in either sex. Our data suggest that in line with their function in mediating responses of individuals to longer-term and acute demands, both baseline and stress-induced plasma corticosterone concentrations are rather plastic traits. However, individuals may differ in their degree of trait plasticity and hence in behavioral and physiological responses to a variety of organismal challenges.

The stress of parenthood? Increased glucocorticoids in birds with experimentally enlarged broods

Variation in baseline glucocorticoid (cort) levels can be attributed, at least in part, to differences in energetic demands confronting individuals. Elevated baseline cort levels are routinely interpreted as indicating individuals in poor condition, with low relative fitness. However, when greater reproductive effort increases energetic demands, individuals with high cort might paradoxically be those with the highest fitness. Here, we experimentally test the hypothesis that increased reproductive demand causes increases in baseline cort (the Cort-Adaptation hypothesis). We measured maternal baseline cort before and after experimentally enlarging and reducing brood sizes in tree swallows (Tachycineta bicolor). Females with experimentally enlarged broods had greater increases in baseline cort and fledged more offspring than females with reduced broods. Additionally, females with greater increases in baseline cort had higher offspring-provisioning rates than females with lower changes in cort. These findings demonstrate that increased reproductive demand can cause increased baseline cort. As yet, we do not know if these increases in cort cause increased allocation of resources towards reproduction, but the positive relationship between parental behaviour and cort suggests that increased cort does not always interfere with reproductive investment, and might instead facilitate it.

Constraints, concerns and considerations about the necessity of estimating free glucocorticoid concentrations for field endocrine studies

Summary 1. We evaluate the utility of measuring corticosteroid-binding globulins (CBG) and estimating free glucocorticoid (GC) concentrations for field endocrine studies. We assert that for three general reasons, measurement of free GCs might not be more useful than total GCs for many studies. 2. First, estimates of so-called ‘free’ GCs are likely inaccurate, in part because of the following: (i) other factors in the blood also bind GCs, (ii) CBG binds plasma steroids other than GCs and (iii) CBG binding affinity can vary with local conditions, such as enzymatic activity and tissue temperature. 3. Second, evidence suggests an active role for CBG-bound GCs, CBG or both, in the vertebrate stress response, calling into question the validity and generality of the free hormone hypothesis. 4. Third, free and total GCs function over different time frames. Free GCs are likely important in the seconds-to-minutes time-scale of interaction with tissues, but total GCs could function at minutes-to-hours time-scales by serving as the reservoir to continue supplying GCs to tissues. As transcription regulators, most GC effects manifest in hours; thus, total GCs would be the most appropriate measure for estimating total biological impact. 5. Our understanding of the biochemistry and the biological actions of both GCs and CBG indicates that total GC concentrations are currently less prone to error and more biologically interpretable than estimates of free hormone. Although further work is necessary, total GC titres currently remain the most accurate and informative estimates of stress hormone levels to address biological questions in nature.

Spatiotemporal Patterns in Nest Box Occupancy by Tree Swallows Across North America

Data from the North American Breeding Bird Survey (BBS) suggest that populations of aerial insectivorous birds are declining, particularly in northeastern regions of the continent, and particularly since the mid-1980s. Species that use nest boxes, such as Tree Swallows (Tachycineta bicolor), may provide researchers with large data sets that better reveal finer-scale geographical patterns in population trends. We analyzed trends in occupancy rates for ca. 40,000 Tree Swallow nest-box-years from 16 sites across North America. The earliest site has been studied intensively since 1969 and the latest site since 2004. Nest box occupancy rates declined significantly at five of six (83%) sites east of -78° W longitude, whereas occupancy rates increased significantly at four of ten sites (40%) west of -78° W longitude. Decreasing box occupancy trends from the northeast were broadly consistent with aspects of a previous analysis of BBS data for Tree Swallows, but our finding of instances of increases in other parts of the continent are novel. Several questions remain, particularly with respect to causes of these broadscale geographic changes in population densities of Tree Swallows. The broad geographic patterns are consistent with a hypothesis of widespread changes in climate on wintering, migratory, or breeding areas that in turn may differentially affect populations of aerial insects, but other explanations are possible. It is also unclear whether these changes in occupancy rates reflect an increase or decrease in overall populations of Tree Swallows. Regardless, important conservation steps will be to unravel causes of changing populations of aerial insectivores in North America. RESUME. Les donnees provenant du Releve des oiseaux nicheurs (BBS) de l’Amerique du Nord semblent indiquer que les populations d’insectivores aeriens sont en declin, particulierement dans les regions du nord-est du continent et depuis le milieu des annees 1980. Grâce aux especes qui utilisent les nichoirs, comme l’Hirondelle bicolore (Tachycineta bicolor), les chercheurs ont acces a une grande quantite de donnees qui revelent davantage les tendances des populations a l’echelle fine. Nous avons analyse la tendance du taux d’occupation par l’Hirondelle bicolore a partir de 40 000 nichoirs-annees provenant de 16 sites repartis en Amerique du Nord. Le site le plus ancien est suivi intensivement depuis 1969 et le plus recent, depuis 2004. Le taux d’occupation des nichoirs a diminue significativement dans 5 des 6 sites (83 %) situes a l’est du 78° de longitude ouest, tandis qu’il a augmente significativement dans 4 des 10 sites (40 %) a l’ouest du 78° de longitude ouest. La tendance a la baisse de l’occupation observee dans le nord-est concorde dans les grandes lignes avec les resultats d’une analyse anterieure des donnees du BBS pour cette espece, mais les hausses que nous avons observees dans d’autres regions de l’Amerique du Nord s’averent nouvelles. Plusieurs interrogations demeurent, notamment en ce qui a trait aux causes de ces changements dans la densite des populations d’Hirondelles bicolores a grande echelle. Ces vastes tendances geographiques concordent avec l’hypothese de changements climatiques generalises qui toucheraient les aires d’hivernage, de migration ou de nidification, lesquels pourraient affecter differemment les populations d’insectes aeriens, mais d’autres explications sont aussi possibles. En outre, nous ne savons pas si ces changements du taux d’occupation refletent une hausse ou une baisse dans l’ensemble des populations d’Hirondelles bicolores. Neanmoins, pour assurer leur conservation, il sera important d’elucider les causes des changements de populations chez les insectivores aeriens en Amerique du Nord.

Experimental Food Restriction Reveals Individual Differences in Corticosterone Reaction Norms with No Oxidative Costs

Highly plastic endocrine traits are thought to play a central role in allowing organisms to respond rapidly to environmental change. Yet, not all individuals display the same degree of plasticity in these traits, and the costs of this individual variation in plasticity are unknown. We studied individual differences in corticosterone levels under varying conditions to test whether there are consistent individual differences in (1) baseline corticosterone levels; (2) plasticity in the hormonal response to an ecologically relevant stressor (food restriction); and (3) whether individual differences in plasticity are related to fitness costs, as estimated by oxidative stress levels. We took 25 wild-caught house sparrows into captivity and assigned them to repeated food restricted and control treatments (60% and 110% of their daily food intake), such that each individual experienced both food restricted and control diets twice. We found significant individual variation in baseline corticosterone levels and stress responsiveness, even after controlling for changes in body mass. However, these individual differences in hormonal responsiveness were not related to measures of oxidative stress. These results have implications for how corticosterone levels may evolve in natural populations and raise questions about what we can conclude from phenotypic correlations between hormone levels and fitness measures.

PRE-MIGRATORY LIFE HISTORY STAGES OF JUVENILE ARCTIC BIRDS: COSTS, CONSTRAINTS, AND TRADE-OFFS

Many young birds on the Arctic tundra are confronted by a challenging task: they must molt their feathers and accumulate fat stores for the autumn migration before climatic conditions deteriorate. Our understanding of the costs and constraints associated with these stages is extremely limited. We investigated post-juvenal molt and premigratory fattening in free-ranging juvenile White-crowned Sparrows (Zonotrichia leucophrys gambelii) on the Arctic tundra. We found evidence for trade-offs between premigratory fat accumulation and molt: heavily molting birds had significantly less fat. Birds increased the rate of fat accumulation as the season progressed, but we found no evidence of a similar increase in rate of molt. Using a controlled captive study to isolate the energetic costs of body feather replacement, we found no difference in fat or size-corrected mass of birds actively growing body feathers as compared to controls. Molting birds, however, consumed 17% more food than controls, suggesting a significant cost of body feather growth. Our results provide evidence of significant costs, constraints, and trade-offs associated with post-juvenal molt and premigratory fat accumulation in young Arctic birds.

Revisiting Jablonski (1993): cladogenesis and range expansion explain latitudinal variation in taxonomic richness.

Abstract The increase in diversity towards the equator arises from latitudinal variation in rates of cladogenesis, extinction, immigration and/or emigration of taxa. We tested the relative contribution of all four processes to the latitudinal gradient in 26 marine invertebrate orders with extensive fossil records, examined previously by David Jablonski. Coupling Jablonskis estimates of latitudinal variation in cladogenesis with new data on patterns of extinction and current distributions, we show that the present‐day gradient in diversity is caused by higher rates of cladogenesis and subsequent range expansion (immigration) at lower latitudes. In contrast, extinction and emigration were not important in the creation of the latitudinal gradient in ordinal richness. This work represents one of the first simultaneous tests of the role of all four processes in the creation of the latitudinal gradient in taxonomic richness, and suggests that low tropical extinction rates are not essential to the creation of latitudinal diversity gradients.