INTERSPECIFIC AND INTRASPECIFIC VARIATION IN THE DIETS OF SYMPATRIC SIAMANG (HYLOBATES-SYNDACTYLUS) AND LAR GIBBONS (HYLOBATES-LAR)

Studies of the siamang (Hylobates syndactylus continentis) and the lar gibbon (Hylobates lar lar) where they co-occur in mainland Asia have demonstrated interspecific dietary segregation based on body size and have suggested that observed levels of frugivory represent metabolical ly based maxima for these species. I studied sympatric groups of siama ng (H. s. syndactylus) and lar gibbons (H. l. vestitus) at Ketambe in northern Sumatra (Indonesia) in order to assess the magnitude of withi n-and between-species variation in diets, The insular subspecies are c onsiderably more frugivorous (60-70% of feeding time) than mainland co nspecifics (35-50%). This is primarily because Sumatran hylobatids spe nd about twice as much time (approx. 45% of feeding) eating fig fruits (Ficus spp., Moraceae). A higher density of figs at Ketambe (compared to Kuala Lompat) may account for this behavioral difference. Enhanced frugivory has been achieved at the expense of folivory, which is much reduced in Sumatra - especially in H. lar (4% of diet) - and is limit ed almost entirely to immature foliage. The expected decline in protei n intake resulting from diminished folivory in Sumatra may be counterb alanced by observed increases in insectivory, which is especially pron ounced in lar gibbons. Interspecific dietary segregation emerges most clearly in how individuals of each species supplement their similarly fig-dominated diets. Siamang rely more on immature foliage - primarily from lianas, which generate young leaves more reliably and abundantly than trees do. Conversely, lar gibbons exploit the pulpy fruit of tre es and lianas more heavily than siamang do, This general pattern occur s where the two species coexist in Malaysia, thereby suggesting a subs tantive interspecific difference that is somewhat greater in the insul ar populations.