Function and Phylogeny

Abstract

Evolutionary explanations come in two basic forms. Functional explanations focus on elucidating the adaptive (i.e., reproductive) value of a particular behavior. Phylogenetic explanations focus on understanding the evolutionary history of a behavior: that is, how it originated and changed in a step-by-step manner over time. Cross-species comparisons indicate that same-sex sexuality is not an evolutionarily uniform phenomenon. Multiple analogous forms of homosexual behavior have evolved. I argue that our understanding of why same-sex sexualities evolved is contingent on the implementation of both types of evolutionary analysis. I describe research on female bonobos (Pan paniscus) to illustrate how functional investigations can help explain why some forms of primate homosexual behavior evolved, such as those that are sociosexual in nature. I then describe my research on female Japanese macaques (Macaca fuscata) to illustrate how phylogenetic investigations can help account for how other forms of homosexual behavior evolved, such as those that are strictly sexual in nature. I conclude by discussing how both functional and phylogenetic perspectives need to be fully integrated to account for these data and, by extension, the evolution of male homosexuality in humans.

Key Words:

• Primates
• Japanese macaques
• female-female sexual behavior