Dynamic Pair Bonds in Hylobatids: Implications Regarding Monogamous Social Systems

Abstract

AbstractDescriptions of monogamy in the gibbons (Hylobates spp., Hylobatidae) have generally emphasized features derived from inference rather than direct observation: 1) adults mate for life; 2) the genetic structure of groups corresponds with a ''nuclear family'' pattern of parents and their mutual offspring; 3) first mate acquisition is achieved when two ncwly dispersed, unaccompanied, young adults of opposite sex establish a new pair and territory together. The six-year history of mating associations of adults in three groups of sympatric, wild siamang (Hylobates syndactylus) and three groups of white-handed gibbon (Hylobates lar) at the Ketambe Research Station (Sumatra, Indonesia) challenge the generality of these assumptions. These longitudinal sociodemographic data suggest that gibbon pair bonds are not necessarily life-long and that adults may leave mates permanently or temporarily, sometimes in order to exploit new reproductive opportunities. Of 11 heterosexual pair bonds that existed at some time during the study, only two were terminated by death of one adult, whereas five were terminated when one adult left its current mate. In some cases, the ''deserting'' individual left its mate and joined a neighboring adult of opposite sex who had recently lost its own mate through death or desertion. This turnover in pair bond membership generated stable, ''non-nuclear'' family groups in which the replacement mate was not the parent of the resident nonadults. Mechanisms of mate acquistion observed at Ketambe included two processes that may also promote non-nuclear families. A young adult may obtain its first mate by: 1) replacing an adult of the same sex in a neighboring group or in the group in which it resides; 2) establishing a new pair bond (and territory) with another unmated young adult of opposite sex who is, however, accompanied in dispersal by juvenile conspecifics from its group. Finally, observed extra-pair copulations (in siamang) suggested an additional, but as yet unspecified potential for variable relatedness between adult males and resident nonadults. Review of other long-term histories of pair bonds in wild gibbons and of studies of socio-sexual behavior in captivity reinforce the conclusion that the apparent temporal stability and underlying behavioral inflexibility of pair bonds has been over-emphasized. Although data on behavior and reproduction are limited, they nevertheless suggest strongly that the mating and social systems of these monogamous primates are considerably more dynamic and complex than previously appreciated.