Stress and reproductive hormones reflect inter-specific social and nutritional conditions mediated by resource availability in a bear-salmon system.

H M Bryan,J E G Smits,K E Wynne-Edwards,P C Paquet,C T Darimont

doi:10.1093/conphys/cou010

Abstract

Food availability can influence the nutritional and social dynamics within and among species. Our investigation focused on grizzly and black bears in coastal British Columbia, Canada, where recent and dramatic declines in their primary prey (salmon) raise concerns about potentially negative effects on bear physiology. We examined how salmon availability relates to stress and reproductive hormones in coastal grizzly (n = 69) and black bears (n = 68) using cortisol and testosterone. In hair samples from genotyped individuals, we quantified salmon consumption using stable isotope analysis and hormone levels by enzyme immunoassay. To estimate the salmon biomass available to each bear, we developed a spatially explicit approach based on typical bear home-range sizes. Next, we compared the relative importance of salmon consumption and salmon availability on hormone levels in male bears using an information theoretical approach. Cortisol in grizzly bears was higher in individuals that consumed less salmon, possibly reflecting nutritional stress. In black bears, cortisol was better predicted by salmon availability than salmon consumption; specifically, individuals in areas and years with low salmon availability showed higher cortisol levels. This indicates that cortisol in black bears is more strongly influenced by the socially competitive environment mediated by salmon availability than by nutritional requirements. In both species, testosterone generally decreased with increasing salmon availability, possibly reflecting a less competitive environment when salmon were abundant. Differences between species could relate to different nutritional requirements, social densities and competitive behaviour and/or habitat use. We present a conceptual model to inform further investigations in this and other systems. Our approach, which combines data on multiple hormones with dietary and spatial information corresponding to the year of hair growth, provides a promising tool for evaluating the responses of a broad spectrum of wildlife to changes in food availability or other environmental conditions.

Highlights

Resource availability can affect animal physiology via complex interactions among nutritional, ecological and social conditions
We collected hair samples from black and grizzly bears on the central coast of British Columbia in May and June of 2009–2011
Our model assumes that testosterone will generally reflect social conditions, whereas cortisol will reflect social and nutritional conditions experienced by bears

Summary

Introduction

Resource availability can affect animal physiology via complex interactions among nutritional, ecological and social conditions. The androgen, testosterone, which may reflect male reproductive investment and activity during the breeding season (Kempenaers et al, 2008), can be modulated by social challenges, such as those that occur over access to mates or fitness-enhancing resources (Wingfield et al, 2001; Oliveira, 2004) Both hormones can be measured in hair, which is thought to primarily reflect chronic hormone levels integrated over the period of hair growth (i.e. several months to years, depending on population-specific patterns of hair growth; Koren et al, 2002; Macbeth et al, 2010; Meyer and Novak, 2012). A number of recent studies have demonstrated that hormonal measurements of hair reflect biologically meaningful endocrine activity in wildlife (reviewed by Meyer and Novak, 2012), including bears (Ursus spp.; Macbeth et al, 2010, 2012; Bourbonnais et al, 2013; Bryan et al, 2013b; Malcolm et al, 2013)

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Conclusion