Spatiotemporal requirements of the Hainan gibbon: Does home range constrain recovery of the world's rarest ape?

Conservation management requires an evidence-based approach, as uninformed decisions can signify the difference between species recovery and loss. The Hainan gibbon, the world's rarest ape, reportedly exploits the largest home range of any gibbon species, with these apparently large spatial requirements potentially limiting population recovery. However, previous home range assessments rarely reported survey methods, effort, or analytical approaches, hindering critical evaluation of estimate reliability. For extremely rare species where data collection is challenging, it also is unclear what impact such limitations have on estimating home range requirements. We re-evaluated Hainan gibbon spatial ecology using 75 hr of observations from 35 contact days over 93 field-days across dry (November 2010-February 2011) and wet (June 2011-September 2011) seasons. We calculated home range area for three social groups (N = 21 individuals) across the sampling period, seasonal estimates for one group (based on 24 days of observation; 12 days per season), and between-group home range overlap using multiple approaches (Minimum Convex Polygon, Kernel Density Estimation, Local Convex Hull, Brownian Bridge Movement Model), and assessed estimate reliability and representativeness using three approaches (Incremental Area Analysis, spatial concordance, and exclusion of expected holes). We estimated a yearly home range of 1-2 km2 , with 1.49 km2 closest to the median of all estimates. Although Hainan gibbon spatial requirements are relatively large for gibbons, our new estimates are smaller than previous estimates used to explain the species' limited recovery, suggesting that habitat availability may be less important in limiting population growth. We argue that other ecological, genetic, and/or anthropogenic factors are more likely to constrain Hainan gibbon recovery, and conservation attention should focus on elucidating and managing these factors. RESEARCH HIGHLIGHTS: Re-evaluation reveals Hainan gibbon home range as c. 1-2 km2 . Hainan gibbon home range is, therefore, similar to other Nomascus gibbons. Limited data for extremely rare species does not necessarily prevent derivation of robust home range estimates.

How many remnant gibbon populations are left on Hainan? Testing the use of local ecological knowledge to detect cryptic threatened primates.

For Critically Endangered "species of extreme rarity," there is an urgent need to clarify the potential survival of remnant populations. Such populations can be difficult to detect using standard field methods. Local ecological knowledge (LEK) represents an important alternative source of information, but anecdotal reports of rare or possibly extinct species can contain uncertainty and error. The Hainan gibbon (Nomascus hainanus), the world's rarest primate species, is confirmed to only survive as a tiny remnant population in Bawangling National Nature Reserve, China, but unverified gibbon sightings have been reported from other forest areas on Hainan. We conducted a large-scale community interview survey to gather new data on patterns of primate LEK from 709 respondents around seven reserves across Hainan, to investigate the possibility of gibbon survival outside Bawangling and assess whether LEK can provide useful information for conservation management of cryptic remnant populations. Comparative LEK data for gibbons and macaques are consistent with independent data on the relative status of these species across Hainan. Local awareness and experience of gibbons was low across Hainan, including at Bawangling, but we recorded recent anecdotal gibbon reports from most reserves. A follow-up field survey at Limushan Provincial Nature Reserve did not detect gibbons, however, and documented intensive wildlife exploitation within this reserve. All other surveyed landscapes showed some statistically lower levels of respondent awareness, experience, or sighting histories of gibbons compared to Bawangling, and are therefore considered biologically unlikely to support gibbons. Unverified LEK data can provide important insights into the possible status of cryptic remnant populations when assessed carefully and critically in relation to data from known populations.

Assessing current genetic status of the Hainan gibbon using historical and demographic baselines: implications for conservation management of species of extreme rarity.

Evidence-based conservation planning is crucial for informing management decisions for species of extreme rarity, but collection of robust data on genetic status or other parameters can be extremely challenging for such species. The Hainan gibbon, possibly the world's rarest mammal, consists of a single population of ~25 individuals restricted to one protected area on Hainan Island, China, and has persisted for over 30 years at exceptionally low population size. Analysis of genotypes at 11 microsatellite loci from faecal samples for 36% of the current global population and tissue samples from 62% of existing historical museum specimens demonstrates limited current genetic diversity (Na = 2.27, Ar = 2.24, He  = 0.43); diversity has declined since the 19th century and even further within the last 30 years, representing declines of ~30% from historical levels (Na = 3.36, Ar = 3.29, He  = 0.63). Significant differentiation is seen between current and historical samples (FST  = 0.156, P = 0.0315), and the current population exhibits extremely small Ne (current Ne  = 2.16). There is evidence for both a recent population bottleneck and an earlier bottleneck, with population size already reasonably low by the late 19th century (historical Ne  = 1162.96). Individuals in the current population are related at the level of half- to full-siblings between social groups, and full-siblings or parent-offspring within a social group, suggesting that inbreeding is likely to increase in the future. The species' current reduced genetic diversity must be considered during conservation planning, particularly for expectations of likely population recovery, indicating that intensive, carefully planned management is essential.

Assessing the information-content of messy data to reconstruct population recovery dynamics for the world's rarest primate.

Understanding the dynamics of population recovery in threatened species requires robust longitudinal monitoring datasets. However, evidence-based decision-making is often impeded by variable data collection approaches, necessitating critical evaluation of restricted available baselines. The Hainan gibbon, the world's rarest primate, had possibly declined to only seven or eight individuals in 1978 at Bawangling National Nature Reserve but has experienced subsequent population growth. Past population estimates lack detailed reporting of survey effort, and multiple conflicting estimates are available, hindering assessment of gibbon recovery. We investigated all reported estimates of Bawangling gibbon population size from 1978 to 2022, to evaluate the biological signal of population trends and the extent to which noise associated with varying survey effort, reporting and estimation may mask or misrepresent any underlying signal. This longitudinal dataset demonstrates that the Bawangling population experienced a series of bottlenecks and recoveries, with three successive periods of growth interspersed by population crashes (1978-1989, 1989-2000 and 2000-2022). The rate of gibbon population recovery was progressively slower over time in each successive period of growth, and this potential decline in recovery rate following serial bottlenecks suggests that additional management strategies may be required alongside "nature-based solutions" for this species. However, population viability analysis suggests the 1978 founder population is unlikely to have been as low as seven individuals, raising concerns for interpreting reported historical population counts and understanding the dynamics of the species' recovery. We caution against overinterpreting potential signals within "messy" conservation datasets, and we emphasise the crucial importance of standardised replicable survey methods and transparent reporting of data and effort in all future surveys of Hainan gibbons and other highly threatened species.

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

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.

Comparative study on intestinal microbiome composition and function in young and adult Hainan gibbons (Nomascus hainanus).

The Hainan gibbon is one of the most endangered primates in the world, with a small population size, narrow distribution range, and high inbreeding risk, which retains the risk of species extinction. To explore the composition and functional differences of the intestinal microbiome of Hainan gibbons at different ages, the faecal microbiomes of young and adult Hainan gibbons were analysed using metagenome sequencing. The results showed that the dominant phyla in the intestinal tract of young and adult Hainan gibbons were Firmicutes and Bacteroidetes, and the dominant genus was Prevotella. Linear discriminant analysis effect size analysis showed that Firmicutes, Ruminococcus, Clostridium, and Butyrivibrio were significantly more abundant in adults than in young, whereas Bacteroidetes, Proteobacteria, Prevotella, and Bacteroides were significantly more abundant in young than in adults. In terms of gene function, the adult Hainan gibbon intestinal microbiome generally harboured a higher abundance of genes related to metabolic processes, such as carbohydrate, amino acid, and nucleotide metabolism. This may be due to adaptive advantages for adult Hainan gibbons, such as stable and mature intestinal microbiome composition, which allows them to utilise diverse foods efficiently. In summary, this study helps understand the dynamic changes in the intestinal microbiome of young and adult Hainan gibbons and plays a key role in the health monitoring and rejuvenation of their population.