Cracking Capuchins : Investigating stone tool use by white-faced capuchin monkeys using camera traps

Abstract

The use of tools enables human and non-human animals alike to achieve goals that would otherwise be unattainable. Whether it is a human using a needle to sew clothing, bees placing feces on hive entrances to deter predators, or a chimpanzee fishing for termites with a thin stick — tool use in the animal kingdom manifests in diverse ways, reflecting the ecological challenges and cognitive adaptations of each species. Yet, despite occurring in many different taxa — from insects to primates — tool-using animals make up only a small fraction of all known animal species, suggesting that tool use has evolved several times independently. Understanding the factors that drive the emergence, persistence, and spread of tool use is of great interest, not only to better grasp the role tool use played in hominin evolution but also because of the remarkable variation observed both within and between species. The evolution of tool use has been linked to various ecological, social, and cognitive factors, which often overlap and can be difficult to distinguish.

In this thesis, I use a unique study system to investigate what drives tool use to arise, persist, and spread in groups of animals. Since 2017, motion-triggered camera traps have been used to study the white-faced capuchin monkeys (Cebus capucinus imitator) living on the islands of Coiba and Jicarón in Panama, which have a tradition of stone tool use unique to the Cebus genus. These capuchins are uniquely suited to study the drivers of tool-use evolution, because tool-using and non-tool-using groups coexist in the same habitat, allowing for comparisons that exclude many ecological factors.

To better understand the variation in tool use between groups, it is necessary to examine how it varies within groups, as behavior spreads through individual dispersal. In chapter 1, I consider the striking male bias in tool use on Jicarón island. I show that females have the physical ability and opportunity to use tools, but do not engage in this behavior. This absence is not due to sampling bias, and has important implications for the cultural transmission of tool use. In chapter 2, I examine the development of tool use by comparing tool use proficiency between age classes. Using detailed frame-by-frame coding of one year of tool use events at two different experimental anvil sites, I show that the development of tool use proficiency is a slow process in which social attention and social tolerance play important roles. In chapter 3, I explore a potential ecological driver of the tool use behavior on Jicarón, namely the tidal cycles. Using a novel statistical approach, I compare the timing of coastal activity in relation to the tidal cycles between tool-using and non-tool-using capuchins. I find that tool-using capuchins show a stronger correlation of coastal activity with tidal cycles, suggesting that tool use facilitates more efficient exploitation of intertidal resources.

In chapter 4, I describe a previously undocumented social tradition of interspecies abduction. On Jicarón, the same capuchins that use tools have also begun to abduct and carry infant howler monkeys. This behavior suggests that the environmental and social conditions that favor innovation of tool use might also favor innovation more generally. Importantly, this chapter highlights that innovations can emerge and spread via social learning even in the absence of a clear functional advantage. In chapter 5, I examine one possible consequence of tool use by comparing social cohesion between a tool-using group and a non-tool-using group. Because stone tool use relies on hammerstones and anvils, which can be monopolized by individuals, it is likely to increase within-group competition. I found that, in order to overcome this competition, the tool-using group may have become less socially cohesive. Notably, this shift in social structure affects not only tool-using males but the entire group, with cascading effects on within- and between-group dynamics.

Taken together, these chapters highlight that tool use is likely to require a combination of many factors to develop. My findings suggest that conditions conducive to innovation of tool use may also facilitate innovation more broadly and that islands may provide unique environments in which these factors co-occur. Furthermore, I propose that once localized tool-use traditions emerge, they may drive secondary behavioral differences, such as changes in social cohesion, that further differentiate tool-using and non-tool-using individuals. Finally, I demonstrate the power of camera traps as a non-invasive method for studying the behavior of unhabituated, wild primates in great detail, providing insights into everything from spatial and temporal activity patterns to the emergence and spread of a novel cultural tradition.