White-handed Gibbons of Khao Yai: Social flexibility, complex reproductive strategies, and a slow life history

Abstract

Long-term field research on wild animals is essential for understanding life history and social systems of long-lived organisms like primates. Gibbons (family Hylobatidae) live surprisingly slow lives, given their relatively small body mass. Following an approximately 7-year-long juvenile period, one of the longest among all primates, Khao Yai white-handed gibbon females begin reproducing at an average age of 10.5±1.2 years. This is much later than in monkeys of at least the same body mass and, remarkably, at about the same age as in mountain gorillas. Our long-term research also revealed remarkable social flexibility analogous to that seen in other apes. At Khao Yai, white-handed gibbons form pairs or small two-male/one-female reproductive units, although individuals may temporarily also live in single-male/multi-female groups, and here we report a novel, semi-solitary life stage of two older males for the first time. Mating patterns also turned out to be flexible, with males and females mating polygamously, including extra-pair copulations and regular polyandrous mating of females living in multi-male groups. We have also found that in accordance with this variability in male-female socio-sexual bonds, female gibbons at Khao Yai show cyclical sexual swellings that advertise the probability of ovulation without allowing males to exactly pinpoint the day of ovulation. After decades of research, we have come to recognize more clearly the importance of the gibbon community and feel confident that we understand the basic social and mating systems of the Khao Yai white-handed gibbon population, but we also continue to discover new details of the evolutionary forces that shape gibbons complex social life.