Periodic episodes of food scarcity may highlight the adaptive value of certain anatomical traits, particularly those that facilitate the acquisition and digestion
of exigent fallback foods. To better understand the
selective pressures that favored the distinctive dental and
locomotor morphologies of gibbons and orangutans, we
examined the foraging and ranging behavior of sympatric
Hylobates albibarbis and Pongo pygmaeus wurmbii during
an episode of low fruit availability at Tuanan, Kalimantan
Tengah, Indonesia. We found that Hylobates ranged 0.5 km day21 or 33% farther than did Pongo, but the overall daily ranging of both species did not vary as fruit availability decreased by as much as 50%. Among gibbons, we observed dietary switching to fallback foods; in particular, there was a progressively greater reliance on figs, liana products, and unripe fruit. Orangutans relied heavily on unripe fruit and fracture-resistant bark and pith tissues.