Timing and probability of ovulation in relation to sex skin swelling in wild West African chimpanzees, Pan troglodytes verus

Females of many catharrine primates show a periodic and often pronounced swelling of the perineum, the functional significance of which is unclear. Several hypotheses that exist to explain the function of this conspicuous trait are based on assumptions about the temporal relation between the period of maximum swelling and ovulation, and remain largely untested. We examined this relation in free-living chimpanzees of the Tai National Park, Cote d'Ivoire, and assessed the reliability of perineal swelling as an indicator of ovulation in this species. We used noninvasive urinary progestogen analysis of female reproductive status, together with observational data on swelling characteristics, to determine the variability of timing of ovulation within the maximum swelling phase in 36 cycles from 12 females. The period of maximum swelling was highly variable, lasting from 6 to 18 days. Although ovulation was virtually restricted to the second half of the period of maximum tumescence, its timing varied considerably in relation to both the onset and the end of the maximum tumescence phase. Probability of ovulation, however, was not random, but peaked on day 7 after the onset of the maximum swelling phase, and was almost 60% between days 7 and 9. Thus, in wild chimpanzees perineal swelling indicates the probability of ovulation, but does not provide sufficient information to deduce its exact timing. Given the temporal variability of ovulation relative to the last day of maximum tumescence, field workers should try to include hormonal analysis if information on timing of ovulation is required for interpretation of observational data. (C) 2003 Published by Elsevier Ltd on behalf of The Association for the Study of Animal Behaviour.