Can the patterns of sexual swelling cycles in female Tai chimpanzees be explained by the cost-of-sexual-attraction hypothesis?

Chimpanzee females of East and West African populations differ in the average number of cycles per interbirth interval. Whereas females in Gombe, Mahale, and Kibale (eastern chimpanzees, Pan troglodytes schweinfurthii) average < 9 cycles before they conceive, females at Tai (western chimpanzees; Pan troglodytes verus) average 29 cycles. We examined data from 2 different study groups (North and South) at Tai. By showing that Tai females interrupt cyclic activity for, on average, 7.4 mo between the end of the postpartum amenorrhea period (PPA) and the subsequent conception, and by calculating the number of cycles to conception based on a probability distribution of cycles over the interbirth interval, we show that Tai females average 19.4 cycles (North Group) and 11.7 cycles (South Group) to conception; therefore, the earlier calculation of 29 cycles was an overestimate. Further, at Tai young parous females have a significantly shorter PPA than those of older females, but the number of cycles to conception does not differ significantly between primiparous and nulliparous females. Some of our results are therefore not in line with the predictions of the cost-of-sexual-attraction hypothesis, which proposes that the factor ultimately responsible for the intensity of female sexual attractiveness is female intragroup scramble competition. We discuss an alternative hypothesis-the social passport hypothesis-which is compatible with the results of the study.