Impact of Seasonality on Food Selection and Home Range Size of the Western Hoolock Gibbon (Hoolock hoolock)

Diet and home range use patterns provide information on the needs of species faced with changing forest quality. Many primates have diets and foraging strategies that maximize nutrient uptake and minimize energy expenditure, resulting in well-defined home ranges. Primates use their home range in varying ways depending on the spatial distribution of resources. The western hoolock gibbon (Hoolock hoolock) is a small, frugivorous ape that maintains a home range in closed-canopy forests. The species is globally Endangered and lives Bangladesh, India and Myanmar in shrinking forest patches. We studied a group of gibbons, consisting of two adult males, an adult female, an immature male, and an infant in Lawachara National Park in northeastern Bangladesh. We used scan sampling to determine their diet, home range size and daily path lengths from April 2019 to March 2020. We also mapped fruiting trees and sleeping sites within their home range from April 2019 to March 2020. The gibbons’ diet varied seasonally, but fruits (mostly figs) dominated throughout the year (fruits: 51%; leaves: 35%; flowers: 5%; buds: 6%), peaking in the monsoons. Home range size was 50.79 ha (90% Kernel Density Estimate) or 47.72 ha (Grid Cell Method). The mean daily path length was 1057.2 m (range 451–1938; n = 33). The home range was smaller during the monsoons and larger in winter. Daily path lengths decreased in the monsoon likely because of abundance of fruits, and also decreased in winter, possibly owing to low temperatures. Gibbons used multiple sites for foraging and slept in large trees with a dense canopy. They slept at a mean of 22.82 m (range 16–30 m) above ground in trees with lianas, possibly as an antipredator strategy. Continued monitoring of ecological factors that influence gibbon foraging and habitat use can help to formulate targeted conservation plans for this Endangered small ape.