Single aggressive interactions increase urinary glucocorticoid levels in wild male chimpanzees.

Roman M Wittig,Klaus Zuberbühler,Anja Weltring,Catherine Crockford,Tobias Deschner,Allan Siegel

doi:10.1371/journal.pone.0118695

Abstract

A basic premise in behavioural ecology is the cost-benefit arithmetic, which determines both behavioural decisions and evolutionary processes. Aggressive interactions can be costly on an energetic level, demanding increased energy or causing injuries, and on a psychological level, in the form of increased anxiety and damaged relationships between opponents. Here we used urinary glucocorticoid (uGC) levels to assess the costs of aggression in wild chimpanzees of Budongo Forest, Uganda. We collected 169 urine samples from nine adult male chimpanzees following 14 aggressive interactions (test condition) and 10 resting events (control condition). Subjects showed significantly higher uGC levels after single aggressive interactions compared to control conditions, likely for aggressors as well as victims. Higher ranking males had greater increases of uGC levels after aggression than lower ranking males. In contrast, uGC levels showed no significant change in relation to aggression length or intensity, indicating that psychological factors might have played a larger role than mere energetic expenditure. We concluded that aggressive behaviour is costly for both aggressors and victims and that costs seem poorly explained by energetic demands of the interaction. Our findings are relevant for studies of post-conflict interactions, since we provide evidence that both aggressors and victims experience a stress response to conflict.

Highlights

A central principle in behavioural ecology is that the function and evolution of behaviour is analysed in terms of economic logic as costs and benefits, which determines an individual’s fitness [1, 2]
We investigated the effect of duration and intensity of aggression on urinary glucocorticoid (uGC) levels, since fighting duration is correlated with energy use, and contact aggression can more lead to injuries than non-contact aggression
Results of the General Linear Mixed Models (GLMM) which included all data showed that the target behaviour significantly influenced the relative uGC levels (Table 2A; likelihood ratio test: χ2 = 5.35, df = 1, p = 0.021)

Summary

Introduction

A central principle in behavioural ecology is that the function and evolution of behaviour is analysed in terms of economic logic as costs and benefits, which determines an individual’s fitness [1, 2]. Cost-benefit analyses are regularly used to predict how animals should behave to maximize their net fitness gains, including during fights [3, 4]. It has been surprisingly difficult to quantify the true costs and benefits of a behaviour. Uk/], in providing core funding for Budongo Conservation Field Station. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript

Methods

Results

Conclusion