Primatologists have long viewed small fruiting trees, like figs, as the reason for gibbons’ territorial and monogamous behavior. However, at Khao Yai National Park in Thailand where gibbons are prevalent, figs are one of the largest trees in the forest. In this long-term field study, Bartlett addresses this apparent contradiction and follows gibbons as their major food sources fluctuate over time. The titles in the Primate Field Studies series impart the comprehensive results of long-term field studies to a broad audience at a critical time. Long-term field studies often have a cohesive story to tell which encompasses many different topics, from group size and food distribution, to social behavior, reproduction, and demography. The comprehensive and accessible monographs can supplement textbooks, or may be used as a stand-alone text in upper-level primatology courses.