Diverse grouping and mating strategies in the Critically Endangered Hainan gibbon (Nomascus hainanus)

Abstract

Understanding flexibility in the social structure and mating strategies of the world’s last remaining population (35 individuals) of wild Hainan gibbons (Nomascus hainanus) is critical for developing effective management plans to aid in their population recovery. Three of the five remaining Hainan gibbon groups (A, B, and C) currently live in a social unit characterized by two or three adult males, two reproducing adult females, and offspring. A fourth group (D) contains one adult male, two adult females, and offspring, and Group E contains a single adult male–adult female pair with a young infant. In this study, we describe observations of copulations between multiple resident males and one of the two resident females in Group C. Group C is best described as a small multi-male/multi-female group. We found that this breeding female (F2) solicited copulations from two resident adult males (M1 and M2) on the same day, and also mated with each of these two males on different days. Resident males were not observed to interrupt the mating pair. Although factors such as a biased adult sex ratio, severe population disruption, and habitat degradation can help explain variation in group composition and mating strategies in Hainan gibbons, we argue that there exists considerable mating system variability across gibbon species, and that this variability offers important insights into male and female Hainan gibbon group structure and reproductive strategies.

Habitat restoration is the greatest challenge for population recovery of Hainan gibbons (Nomascus hainanus)

“…In addition, tourism has dramatically expanded on Hainan Island, from 140 000 tourists in 1987 to 2.8 million in 2002, to more than 81 million in 2021 (Xinhau 2022). Rapid economic growth and infrastructure development have had a severely negative impact on the island's flora and fauna (Li et al 2022).…”

Hainan Gibbons (Nomascus hainanus), the Most Threatened and Rarest Primate in the World

“…; Group C, 8 ind., Group D, 5 ind., and Group E, 3 ind.) including fi ve lone members [5]. The adult male-to-female sex ratio in those fi ve family groups was 1:1.2 [5].…”

“…[3]. The latest demographic investigation of the tiny remnant population of Hainan gibbon was 35 individuals in 2022 [5]. Individuals are related at the level of half-to full-siblings between social groups, and full-siblings or parent-off spring within a family group [6].…”

“…Sizes of those family groups usually vary from 2 to 10 animals [11]. And multi-male-multi female family group has been founded in this gibbon recently [5]. Hainan gibbon takes more diverse mating strategies for its living conditions than ever.…”

“…including fi ve lone members [5]. The adult male-to-female sex ratio in those fi ve family groups was 1:1.2 [5]. Two to three family group survived in the wild when the BNNR was founded in 1980 [11].…”