Primary seed shadow generated by gibbons in the rain forests of Barito Ulu, central Borneo.

Gibbons are one of the main frugivores in the forests of Southeast Asia, and consequently have long been considered to be good seed dispersers. This is the first study in which the primary seed shadow they create by their ranging and foraging activities is evaluated in detail. I studied two gibbon groups over 12 months in lowland dipterocarp forest in central Borneo. The gibbons dispersed up to 81% of the species they consumed and destroyed the seeds of only 12%. Fruit with elongated seeds (up to 20 mm wide) were more likely to be dispersed than round seeds. Considering that the survival rate of seeds in the forest to one year was 8%, the gibbons effectively dispersed 13 seedlings ha(-1) group(-1) year(-1). Their effect on germination was very variable, although most species did eventually germinate. Most seeds were deposited along their major ranging routes and close to or under feeding trees.