RESEARCH ARTICLE Single Aggressive Interactions Increase Urinary Glucocorticoid Levels in Wild Male Chimpanzees Roman M. Wittig 1,2,3☯ *, Catherine Crockford 1,2,3☯ *, Anja Weltring 1 , Tobias Deschner 1 , Klaus Zuberbühler 2,3,4 1 Max Planck Institute for Evolutionary Anthropology, Department of Primatology, Leipzig, Germany, 2 University of St. Andrews, School of Psychology and Neurobiology, St. Andrews, United Kingdom, 3 Budongo Conservation Field Station, Masindi, Uganda, 4 University of Neuchâtel, Cognitive Science Centre, Neuchâtel, Switzerland ☯ These authors contributed equally to this work. * wittig@eva.mpg.de (RMW); crockfor@eva.mpg.de (CC) 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 psy- chological level, in the form of increased anxiety and damaged relationships between oppo- nents. 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 sin- gle 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 ag- gression 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. 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 fit- ness [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]. Despite its importance, it has been surprisingly difficult to quantify the true costs and benefits of a behaviour. One approach PLOS ONE | DOI:10.1371/journal.pone.0118695 February 25, 2015 1 / 13 OPEN ACCESS Citation: Wittig RM, Crockford C, Weltring A, Deschner T, Zuberbühler K (2015) Single Aggressive Interactions Increase Urinary Glucocorticoid Levels in Wild Male Chimpanzees. PLoS ONE 10(2): e0118695. doi:10.1371/journal.pone.0118695 Academic Editor: Allan Siegel, University of Medicine & Dentistry of NJ - New Jersey Medical School, UNITED STATES Received: August 28, 2014 Accepted: January 23, 2015 Published: February 25, 2015 Copyright: © 2015 Wittig et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability Statement: Table 1 shows the data set that provides the underlying numerical values behind any figures, tables, graphs, and statistical analyses that are necessary to replicate the findings in the study. For further questions contact Roman Wittig at wittig@eva.mpg.de. Funding: Financial support was provided by British Academy [http://www.britac.ac.uk/] (CC), Leakey Foundation [http://www.leakeyfoundation.org/] (CC, RMW, TD), Leverhulm Trust [http://www.leverhulme. ac.uk/] (KZ), Max Planck Society [http://www.eva. mpg.de/] (AW, CC, RMW, TD) and the Royal