Single Aggressive Interactions Increase Urinary Glucocorticoid Levels in Wild Male Chimpanzees

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.

Social support reduces stress hormone levels in wild chimpanzees across stressful events and everyday affiliations

“…26). Urine was either pipetted from plastic bags, after the bags were tied over a forked stick and held in the urine stream when subjects were sitting in a tree, or from leaf matter when urination occurred on the ground after subjects had moved away.…”

“…In contrast to previous correlational studies, where GC measures were typically averaged over extended time periods, which included a multitude of stressful events9101123, we used an event sampling approach measuring uGC levels that corresponded to a single event26. Applying a fixed latency for clearance of GCs into urine2627, we created relative uGC levels by dividing uGC levels relating to the event by uGC levels relating to before the event.…”

“…Applying a fixed latency for clearance of GCs into urine2627, we created relative uGC levels by dividing uGC levels relating to the event by uGC levels relating to before the event. Events were stressors (intergroup encounters), everyday affiliations (grooming) or control periods (resting without social interaction).…”

Sex Differences in the Development of Aggressive Behavior in Rhesus Macaques (Macaca mulatta)

“…The occurrence of aggression enhances stress levels, and having to deal with aggression might have crucial consequences on individuals' fitness and survival (Crockford et al 2008;Honess and Marin 2006;Wittig et al 2015). Further, intragroup aggression often involves several individuals, so that physiological stress may not be limited to the initial recipient of the aggression and can therefore have negative consequences on the fitness and survival of all group members (Ha et al 2011;Wittig et al 2015).…”

“…Further, intragroup aggression often involves several individuals, so that physiological stress may not be limited to the initial recipient of the aggression and can therefore have negative consequences on the fitness and survival of all group members (Ha et al 2011;Wittig et al 2015). Therefore, aggression is usually categorized as socio-negative interactions that have a mostly negative impact on social relationships (Crockford et al 2012;Fraser et al 2008;Nakamichi 2001;Worlein et al 1988).…”