Within- and between-group feeding competition in Siberut macaques (Macaca siberu) and Assamese macaques (Macaca assamensis)

Abstract

Feeding competition remains of research interest as it affects growth, survival and reproductive success of individuals. It also limits group size and shapes the social structure of animal groups. Feeding competition is thought to be determined by food resource characteristics, but other factors like predation risk can influence feeding competition as well. The socioecological models have been developed to explain the observed variation in social relationships, by linking resource characteristics to the type and strength of feeding competition, which ultimately influences social relationships. These models mainly focused on females, as female reproductive success is usually more limited by food than that of males. My thesis addresses two major gaps related to the feeding competition aspects of the socioecological models. Firstly, it addresses the lack of empirical data examining feeding competition under low predation risk to test some model predictions, i.e. that groups become less cohesive and feed in small feeding groups or even alone if predation risk is low, resulting in reduced contest competition. Secondly, it addresses the often neglected role of males within socioecological models, as there is some evidence that males can also actively defend food resources and thus can play an important role in between-group feeding competition. To address the first research gap, I studied within-group feeding competition in Siberut macaques (Macaca siberu), a species endemic to Siberut island (W-Sumatra, Indonesia), where predation risk is low as carnivore predators have been absent for at least 0.5 m years. In accordance with model predictions, I found that average feeding group size was small, individuals (including juveniles) were frequently feeding alone in food patches, and individuals spread out continuously without forming sub-groups. The lack of felids thus influences the foraging and feeding behavior and reduces contest competition, reflected by a very low aggression rate in food patches. I addressed the second gap of research using Assamese macaques (Macaca assamensis) at Phu Khieo Wildlife Sanctuary, Thailand. I evaluated the role and consequences of male group size on between-group competition by investigating the influence of male group size on home range size and the effect on female reproductive success. I found (indirect) evidence that male food resource defense is an important part of between-group competition in that species, and that females benefit from more males in the group through an increased home range size which increased female reproductive success. A literature review revealed that there is direct or indirect evidence for male resource defense in 17 primate species, suggesting that male resource defense may be more common than previously assumed. In sum, my thesis fills two major gaps related to feeding competition aspects of socioecological models. It contributes to the debate about the merit and validity of socioecological models by confirming model predictions for a species living under low predation risk, and shows that it is important to measure food resource characteristics on a scale relevant to the study animals. A comparison of Siberut macaques and Sulawesi macaques shows that their competitive regimes differ largely, although they all live on oceanic islands under low predation risk and feed mainly on fruits. This shows the importance of detailed measurements of behavior, food resource characteristics and other ecological conditions, and prompts us to be careful with generalizations based on coarse dietary categorizations. In addition, various factors have been previously proposed to be added to the socioecological models, to improve their explanatory power. If future studies can demonstrate that male food resource defense influences between-group feeding competition in many species, we should consider including males as an explanatory factor in future socioecological models.