LONGITUDINAL PATTERNS OF REPRODUCTION IN WILD FEMALE SIAMANG (HYLOBATES SYNDACTYLUS) AND WHITE-HANDED GIBBONS (HYLOBATES LAR)

I present the 6-year reproductive histories of three wild female siama ng (Hylobates syndactylus) and four white-handed gibbons (Hylobates la r) at the Ketambe Research Station (Sumatra, Indonesia). Reproductive output varied considerably among females. Two females failed to gestat e: both were nulliparous young adult H. lar, one of which remained unp aired for 4 years after dispersing from her group, while the other los t her recently acquired mate to another female. Only one-(a white-hand ed gibbon)-gave birth more than once, yielding interbirth intervals of 22 and 31 months. Pair band stability or reduced interspecific feedin g competition or both factors may have contributed to the brevity of t hese intervals. The other females-one H. lar, and three H. syndactylus -each gave birth once, suggesting minimum interbirth intervals exceedi ng 4-5 years (H. lar) and 3 years (H. syndactylus) in these individual s. Even given the pronounced variation observed among H, lar, these da ta suggest that interbirth intervals may often exceed the 2- to 3-year interval commonly attributed to these two species. Sources of reprodu ctive failure were (1) maternal abandonment of the neonate due to impa ired ability to provide maternal care (H. syndactylus), (2) premature or stillbirth (H. syndactylus), and (3) pregnancy termination (H. lar) . These data and a review of information on longevity and age at menar che suggest that the actual lifetime reproductive output of a siamang or white-handed gibbon female may often fall far short of the 10 offsp ring/lifetime originally proposed for these species. Indeed, females m ay rear as few as five offspring to weaning iii a lifetime, which is a figure reminiscent of the reproductive potential of some pongids. Fin ally, variance in female reproductive success is higher than expected in these monogamous species, which suggests that females (and males) a re under strong selective pressure to exert mate choice, possibly thro ugh acquisition of (new) mates and extrapair copulations. Future resea rch must clarify the availability of opportunities for paired adults t o engage in these sociosexual behaviors.