Does dispersal cause reproductive delays in female mountain gorillas?

Abstract

Summary Reproductive delays are a potential cost of dispersal, even when females transfer directly from one social unit to another. Delays may occur before a transfer if females avoid conception while waiting to encounter other groups, and delays following the transfer could be caused by greater stress or poorer nutrition in the new group. This paper examines the timing of reproduction and inter-group transfers of female mountain gorillas in the Virunga Volcano region in Africa. Reproductive delays were not observed for nulliparous females, who typically transfer more than a year before their first conception. For parous females, interbirth intervals were significantly longer when they contained a transfer, but limited observations did not suggest that females were avoiding conception while waiting to transfer, and immigrants did not take significantly longer to conceive than other females. We conclude that transfers may not delay reproduction of parous females, but conversely, dispersal may become more likely when successful reproduction is delayed for any reason (e.g., infertility, miscarriages, and infant mortality). We present a preliminary mathematical model to predict how much of a delay a transferring female could afford, if offspring mortality were reduced in her new group.