Anthropology has a long history of examining the important roles of kinship in human societies, and an important tool in that process has been the use of the comparative method. White-faced capuchin monkeys (Cebus capucinus) are an ideal model organism for comparison to human studies, because they share important similarities such as the presence of full siblings and multiple generations of kin within groups. Primate societies are highly structured – factors such as kinship, age, and dominance rank play crucial roles in determining how and at what frequencies individuals interact. This variation in turn has an impact on the strength of social bonds that develop between different individuals. Research on the mechanisms underlying inbreeding avoidance in capuchins can contribute substantially to our broader understanding of kin recognition and how its limits result in a wide diversity of social structures even within species. In this thesis, I draw upon behavioral, genetic, and hormonal data to study mechanisms of kin recognition, particularly in the context of inbreeding avoidance, in a wild population of capuchins living in or near the Lomas Barbudal Biological Reserve in Costa Rica. First, I show that capuchin infants have multiple potential cues available that can be used to detect close kinship and relatedness to group members: these being spatial proximity, age proximity, and male alpha status. Second, I show that inbreeding avoidance in capuchins is strongest between social partners related at the levels of father-daughter. Third, I provide evidence that early spatial proximity to group members is one of the mechanisms involved in inbreeding avoidance in capuchins.

Answers to many of the most important questions about the evolution of primate social strategies still elude us, due to the difficulties in extracting information about primates’ decision-making processes and the fitness consequences of their behavioral choices. These sorts of questions cannot be answered experimentally: the answers must be inferred from the behavioral patterns of wild populations in long-term studies. It is only now that sufficiently sophisticated methods are being developed to allow us to make precise inferences from these messy observational data sets. In this thesis I will ask questions about how wild primates make decisions about allocations of favors to social partners, and about the fitness consequences of their behavioral choices.

In the following chapters, I examined: (1) reciprocity in capuchin monkeys’ social relationships; (2) the relationship between adult female capuchins’ sociality and longevity; and (3) the decision rules that capuchins use when soliciting an ally in a coalitionary fight. I used behavioral, genetic, and demographic data of wild white-faced capuchin monkeys (Cebus capucinus). The data came from 11 capuchin social groups and were collected over a 19-year period at Lomas Barbudal Biological Reserve and surrounding private lands in Guanacaste, Costa Rica. To answer the questions above, I used innovative statistical techniques currently unrepresented in the primatological literature. To analyze reciprocity and the fitness benefits of dyadic relationships, I used the Social Relations Model (Kenny 1994). To model capuchin monkey choices of allies in coalitionary aggression, I employed conditional logistic regression. Both of these modeling techniques allow me to take into account the particularities of a dataset collected in the wild.

My findings show that capuchin monkeys reciprocate in grooming and coalitionary support exchanges. When choosing allies in coalitionary aggression, capuchins use both rank and relationship quality of the potential partner to make a decision. Finally, sociality is associated with fitness outcomes in female capuchins: More social females tend to live longer than less social ones. Together, these chapters will provide insights into capuchin monkeys’ social relationships, decision making and social relationships’ effects on fitness.

Laughter is a universal human behavior that likely evolved from a play vocalization in the common ancestor of great apes. Comparative research on play signals, their associated emotions, and their cognitive effects can shed light on laughter’s origins and on human sociality more broadly. This dissertation investigates the evolutionary roots of play and laughter through theoretical and empirical studies in humans and our primate relatives. Chapter 1 provides a general introduction to the evolution of play and laughter and their significance for our understanding of human emotion, cognition, and social bonding. I highlight the importance of social bonds in research I conducted on the development of sex differences in play in white-faced capuchins and on playful teasing behaviors in apes. Chapter 2 is a literature review synthesizing research on laughter and play vocalizations across the animal kingdom, adding support to the theory that human laughter likely evolved from a pant-like play signal. I present a comprehensive review of species that vocalize during play, along with acoustic descriptions of the calls. I also discuss potentially unique aspects of human laughter and implications for the evolution of language. Chapters 3 and 4 describe a set of cognitive bias experiments I conducted with humans and one of our closest ape relatives, the bonobos. I tested whether hearing laughter leads to optimism bias in judgements of ambiguous stimuli, which would indicate a positive affective response. The study in chapter 3 found that bonobos tended to approach ambiguous stimuli more often after hearing unfamiliar conspecific laughter. However, the study in chapter 4 did not find evidence that humans show greater optimism bias or more positive self-reported feelings after hearing infant laughter. In chapter 5, I conclude with a general discussion of these findings and future directions. While only apes produce human-like laughter, many animals have play vocalizations that appear to involve positive emotions. Laughter perception and contagion likely include ancient emotional mechanisms, and perhaps some derived ones. Further comparative research can illuminate the extent to which emotional and behavioral contagion underpin play vocalizations across species and the unique psychosocial mechanisms associated with human laughter. Overall, this dissertation grounds the study of laughter in evolutionary theory and research on animal play, using an interdisciplinary approach to tackle both proximate and ultimate explanations for this ubiquitous behavior.