Abstract
Socioecological theories predict that, in mammals, feeding and mating competitions affect male and female energetic conditions differently but energetic studies investigating both sexes simultaneously are rare. We investigated the effect of socioecological factors on the energetic conditions of male and female western chimpanzees, a long-lived species with high degrees of male-male competition. We used behavioural data collected on one chimpanzee community in the Taï National Park over 12 months, phenological data and urinary c-peptide (UCP) measures, a marker of energy balance. We found a positive effect of food availability on UCP levels in both sexes. Dominance rank also affected chimpanzee UCP levels. High-ranking females had higher UCP levels than low-ranking ones but only in periods when no oestrus females were present in the community. In contrast, high-ranking males had higher UCP levels than low-ranking males in the presence of oestrus females but lower UCP levels in their absence. Our results suggest that oestrus female presence lessened the competitive advantages of high-ranking females in feeding competition and that low-ranking males bore higher energetic costs related to mating competition than high-ranking ones. Yet caution should apply in interpreting these results since the statistical model was only close to significance. High-ranking male and female chimpanzees spent significantly less energy. Furthermore, all chimpanzees significantly spent less time feeding and spent more energy when food availability was high. Finally, our behavioural measure of energy intake and expenditure did not correlate with UCP levels highlighting the value of non-invasive hormonal markers for field studies.Significance statementGeneral socioecological theories hypothesize that the social grouping dynamic and energetics of females are highly influenced by food competition, whereas in males, competition for sexual partners is more influential for these factors. Recent studies in the non-invasive physiological assessment of energy balance in primates have begun to test the implied relationship between chimpanzee socioecology and individual energetic condition, with inconsistent results. However, only a few studies have investigated this relationship concurrently for both sexes. Here, using non-invasive measures of energy balance in wild western chimpanzees, we found that the energetics of both males and females are related to ecological factors, such as food availability. However, female energy balance appears also to be related to increased male mating competition, as this can result in increased aggression directed from males to females, with apparent energetic costs for females.
Highlights
Energetic management, i.e. balancing energy intake and expenditure to maintain a positive energy balance, is a key determinant of animal reproductive success and survival (Hobbs 1989; Bosaeus et al 2002; Stubbs and Tolkamp 2006)
We studied one community of chimpanzees over 12 months in Taï National Park using a combination of phenological measures to assess food availability, behavioural observations to assess energy intake and energy spent on activities and urinary c-peptide (UCP) levels as a marker of energy balance
In our first model testing the effect of socioecological factors on chimpanzee energy balance (UCP levels), we found that the full model was not significantly different from the null model (LRT: χ2 = 16.81, df = 11, P = 0.052)
Summary
I.e. balancing energy intake and expenditure to maintain a positive energy balance, is a key determinant of animal reproductive success and survival (Hobbs 1989; Bosaeus et al 2002; Stubbs and Tolkamp 2006). Primates exhibit a wide variety of social organization ranging from solitary to large cohesive multi-male multi-female groups (Mitani et al 2012) To better understand this variety and the social dynamics within groups, the socioecological model, linking food distribution and abundance to primate social dynamics, was developed in the 1980s (Wrangham 1980; van Schaik 1989). Females are limited in their energetic acquisition by withingroup feeding competition (van Schaik 1989; Barton and Whiten 1993; Saito 1996; Wittig and Boesch 2003; Murray et al 2006) This effect is expected to be unequal across conspecifics living in the same community since the high-ranking ones can monopolize access to food resources (Greengrass 2005; Pusey et al 2005; Kahlenberg et al 2008; Nurmi et al 2018). The effect of dominance rank on energy intake in males is expected to be lower than in females, and rank effects on male energy balance are expected mainly in periods of high mating competition where high-ranking males compete to access fertile females (Muller and Wrangham 2004)
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