Date of Thesis



Although play is seen in a variety of vertebrate species, its evolutionary function is still largely unknown. Hypotheses regarding the evolutionary function of play suggest that play consists of circumstances that may be out-of-control, and thus are inherently risky for the individual. Since the benefits of play behavior, at least in the juvenile period, are not immediate, such risks should be appropriately modified in order for play behavior to be evolutionarily selected for in the population. Risks can take on a variety of forms, including environmental, structural, or social risks, and species may modify risks in different ways according to their evolutionary and ecological history. The risk-modification strategies of three lineages of primates were examined using over 1050 total hours of focal samples of brown capuchins (Cebus apella), hamadryas baboons (Papio hamadryas), and diademed sifakas (Propithecus diadema) focusing on environmental, structural, and social risks. Both the baboons and the sifakas played the most intensely on the ground compared to behavior off the ground, suggesting a possible avoidance of physical injury risk due to environmental context. Furthermore, both capuchins and baboons exhibited self-handicapping behavior during play fights, as quantified by an intensity reduction in play fights between partners of highly disparate-ages. The trends in play partner preference, however, were not consistent across species. The formation of both play partnerships in general and strong play partnerships in the capuchins and sifakas was significantly influenced by the reciprocity of relationships, but this was not true for the baboons. In the baboons, the formation of play partnerships, both in general and strong ones, was determined by the age disparity between partners, with individuals more likely to form a play partnership if they were close in age. For the baboons, the strength of a play partnership could be significantly predicted by the strength of the affiliative relationship outside of play; however, this was not true for the capuchins and sifakas. Thus, although some modes of environmental and structural risk modification seem to be consistent across the play of primates of different lineages, the modification of social risks is highly dependent on a species' socioecology and the dynamics of the group. While the risk modification varied from species to species, all species observed showed some evidence of modifying risks of various forms during play, which may allow play behavior to be more rapidly selected for in the population.


risk modification, play behavior, training for the unexpected hypothesis, social network analysis, exponential random graph models, primates, capuchin, baboon, lemur

Access Type

Honors Thesis (Bucknell Access Only)

Degree Type

Bachelor of Science


Animal Behavior

First Advisor

Peter G. Judge