Quantified diet provides suggestions for habitat restoration for the world's rarest primate

Habitat restoration for endangered species requires sufficient understanding of species' feeding ecology. Hainan gibbon (Nomascus hainanus) is the world's rarest primate species, with only one population of ca. 40 individuals living in Bawangling, Hainan Province, China. Limited habitat is the main factor restricting its population recovery. Previous studies suggested Hainan gibbons live in suboptimal habitats and recommended Lauraceae for habitat restoration. From September 2021 to December 2022, we used 5-min interval scans sampling method to collect quantitative data of feeding behavior of two Hainan gibbon groups. We totally tracked gibbons for 1477 h over 216 observation days and collected 17,249 feeding behavioral records. As a result, the two gibbon groups were observed to consume 146 plant species from 111 genera, 65 families. Fruits and figs from average 79 plant species contribute 85.36 % to the annual diet of each group. The larger number and higher proportion of fruit species in the annual diet comparing to those of other gibbon populations indicate that the habitat quality of extant Hainan gibbons is not as poor as previously thought. Meanwhile, species of Lauraceae are abundant in the habitat but contributed little to the gibbon's diet, thus should not be recommended for habitat restoration. We utilized four quantified indexes to filter three important food species lists, which can be reliable references for plant selection in habitat restoration. By clarifying these crucial misunderstandings and providing species lists for habitat restoration, our work again emphasizes the importance of quantified diet on habitat restoration for endangered species.