Sexual swellings in female hamadryas baboons after male take-overs: "Deceptive" swellings as a possible female counter-strategy against infanticide

In many primate species, male infanticide is assumed to pose a serious risk for infants, and therefore female counter-strategies are expected. These c ounter-strategies may include changes in the females' reproductive behavior and physiology, e.g., mating with multiple males and prolonged receptivity . Since the risk of infanticide is particularly high when a new male enters the group or when a male rises in rank, we studied the changes in female r eproductive conditions, e.g., post-partum amenorrhea and interbirth interva ls, following group take-overs by new males in a captive group of hamadryas baboons (Papio h, hamadryas). Following take-overs, five out of six lactat ing females immediately developed sexual swellings, thus shortening their p ost-partum amenorrhea. However, none of these females conceived during the first cycles after the take-over, their reproduction was not accelerated, a nd four out of five dependent infants survived. Thus, interbirth intervals did not decrease compared to times with no group take-overs. We therefore s uggest that these situation-dependent swellings are used by female hamadrya s baboons as a counter-strategy to reduce the risk of infanticide, which ex ists in this species after male take-overs. By offering new males mating op portunities without allowing them to reproduce, females may increase their infants' probability of survival and at the same time avoid the costs of be ing pregnant and lactating simultaneously. (C) 2000 Wiley-Liss, Inc.