Female chacma baboons modulate their sexual receptivity in response to male intrasexual competition

Research in social mammals has revealed the complexity of strategies
females use in response to female-female reproductive competition and
sexual conflict. One point at which competition and conflict manifest
acutely is during sexual receptivity, indicated by swellings in some
primates. Whether females can adjust their sexual receptivity from cycle
to cycle to decrease reproductive competition and sexual conflict in
response to social pressures has not been tested. As a first step, this
study explores whether sexual receptivity duration is predicted social
pressures in wild female chacma baboons (Papio ursinus). Given that female
baboons face intense reproductive competition and sexual coercion, we
predicted that: females could shorten the duration of their sexual
receptive period to reduce female-female aggression and male coercion or
increase it to access multiple or their preferred male(s). We quantified
157 ovulatory cycles from 46 wild females living in central Namibia
recorded over 15 years. We found no support for our hypothesis; however,
our analyses revealed a negative correlation between maximal-swelling
duration and group size, a proxy of within-group competition. This study
provides further evidence that swelling is costly as well as a testable
framework for future investigations of ‘cycle length manipulation’.