The energetics of male-male endurance rivalry in free-ranging rhesus macaques, Macaca mulatta

In many animal species, males engage in endurance rivalry, competing for mating opportunites over extended durations. In the rhesus macaque, a seasonally breeding primate species, energetically costly mating behaviours such as consortship lead to greater reproductive success, and the ability of males to improve their body condition before the mating season may be crucial in determining their ability to use such strategies. Here, we explore relationships between male dominance rank, behaviour, body mass index (BMI) and energetics (assessed through urinary C-peptide of insulin levels, UCPs) in free-ranging rhesus macaques during a 6-month birth season and subsequent 6-month mating season. We analysed measures of general activity, mating behaviours and restlessness, here defined as the rate of change between behaviours. During the birth season, high-ranking males spent more time feeding on high-energy food, were less restless and spent less time travelling than low-ranking males. Restlessness and travelling time were both negatively correlated with male BMI and UCPs. Males in good condition during the birth season were in good condition at the onset of the mating season. During the mating season, high-ranking males participated in more consortships, which were positively correlated with copulatory activity. Copulation was negatively correlated with UCP levels, demonstrating the cost of such mating activities. As a consequence, by the end of the mating season, high-ranking males were in the worst condition. Our results suggest that male rhesus macaques engage in competitive endurance rivalry, and that male condition built during the previous birth season is an integral part of mating success.