Infanticide risk and male quality influence optimal group composition for Colobus vellerosus

The threat of infanticide by males is suggested to determine upper group size limits for some folivores because large female aggregations attract immigrating males. When groups get large enough to become multimale, infanticide risk should decline because, all other things being equal, more males should deter outside takeovers and the counterstrategies of mothers and sires should lower the consequences of inside takeovers. To determine if this scenario can be generalized to all folivorous primates, we began by examining the influence of female and male group size on 4 increasing levels of infanticide risk in folivorous ursine colobus monkeys (Colobus vellerosus) at Boabeng-Fiema, Ghana. Data from 2004 to 2010 on 7 groups analyzed with generalized estimating equation models showed that infanticide risk is heightened in groups with more females and absolutely more males because male immigration is more frequent. However, when controlling for the effect of female group size on male group size, groups with relatively more males have significantly more male immigration and higher infanticide rates. Thus, multimale groups of C. vellerosus show higher infanticide risk compared with unimale groups. Resident males in unimale groups may be higher quality than those in multimale groups because they perform more energetically costly displays and win more encounters. However, males in multimale groups may also suffer from collective action problems making them unable to prevent male immigration and infanticide. This study highlights the importance of the quality of male defenders in infanticide risk and demonstrates intersexual and intrasexual conflict in group composition in C. vellerosus.