Offspring loss after male change in wild siamangs: the importance of abrupt weaning and male care.

Abstract

Infanticide by males is assumed to promote permanent male-female associations, although its importance for social monogamy is still debated. We examined the consequences of male membership change in the largest socially monogamous primate, the siamang (Symphalangus syndactylus), a species that also forms polyandrous groups and where males may provide offspring care. We examine (a) the potential risk of infanticide by documenting changes in female-offspring relationships following male change, expecting abrupt weaning; and (b) the potential importance of male care and polyandry for offspring survival. We witnessed four male changes at Way Canguk Field Station (Indonesia) in groups where the youngest offspring was estimated to be about 2years of age. We quantified aspects of mother-offspring relationships (interindividual distance, body contact, and nipple contact) and estimated the proportion of time being carried during travel (August 2007-April 2009). In the two groups into which new males immigrated, offspring were weaned abruptly, suggesting infanticide by males as a potential risk. Even though no attacks on infants by males were witnessed, both infants disappeared shortly after the male change. Another infant disappeared after its main carrier (a male) emigrated. Conversely, in a group where the resident subordinate male replaced the dominant male, the offspring experienced no change in care and survived the transition. These findings suggest that male immigration in hylobatids increases mortality risk for infants, emphasize the importance of offspring care, and cast doubts on prior classifications of 2-year-old siamangs as independently traveling individuals.