Monogamy in Mammals: Expanding the Perspective on Hylobatid Mating Systems

Abstract

The words monogamous and pair-bonded are widely used in the literature, yet the lack of agreement on a definition has generated misunderstanding and ambiguity. Wittenberger and Tilson (1980: 198) define a monogamous mating system as “a prolonged association and essentially exclusive mating relationship between one male and one female” (see also Komers and Brotherton 1997; Kappeler and van Schaik 2002). Several terms have been offered to better identify the specific characteristics of “monogamous groups” (Fuentes 1999; Fuentes 2000; Sommer and Reichard 2000; Kappeler and van Schaik 2002). Data presented in this review reveal that EPPs are widespread across purportedly monogamous species, and that in many species there is a high degree of individual behavioral plasticity. An overly strict definition of monogamy, therefore, while correct from a formal point of view, presents the same practical inconveniences that it was meant to solve: restricting the term monogamous (or pair-bonded) to lifelong genetically exclusive relationships rules out the great majority of species. Moreover, alternative terms such as primarily two-adult group (Fuentes 1999) do not remove subjectivity, since the point of the matter is how much flexibility, if any, is allowed, when we are considering how often extrapair individuals co-reside in the group or are involved in sexual interactions. Tolerating sporadic EPCs in a monogamous mating system seems more practical than conforming to an excessively stringent definition that applies to very few mammal (or bird) species. In any case, a basic distinction that should always be made explicit is that between social monogamy (or a monogamous social system: an adult male and female sharing a territory), genetic monogamy (an adult male and female sharing parentage of all offspring), and sexual monogamy (or a monogamous mating system: an adult male and female mating exclusively) (Kappeler and van Schaik 2002).