Demography of a Stable Gibbon Population in High-Elevation Forest on Java

Demography is fundamental for conservation planning, but important information is often lacking. Here, we describe the demography of wild Javan gibbons (Hylobates moloch) from 2007 to 2020 in Citalahab Research Area, Gunung Halimun-Salak National Park, Indonesia. Mean group size was 3.95 ± 0.78 individuals. All groups (n = 8) included an adult pair and 0–3 immature individuals. All adults survived throughout the study, indicating that the lifespans of paired adults exceeded 24 years. Births occurred throughout the year, with intervals of 3.65 ± 0.70 years between births. Age-specific survivorship was 1.0 from birth to age 4 years (n = 12), 0.83 for 4–6 year olds (n = 6) and 1.0 for 6–8 year olds (n = 2). Six individuals disappeared between ages 4 and 10 years. One juvenile male (<5 years) is assumed to have died. One male disappeared at age 5.2 years. Four sub-adults (>8 years) experienced increasing aggression from same-sex adults before disappearing. The fates of dispersing individuals are unknown. Gibbon demography at Citalahab was similar to that of stable gibbon populations at other sites, although Citalahab is in submontane forest on a habitat edge. Most remaining Javan gibbons live in fragmented hill and montane forests, so demographic parameters from Citalahab may be helpful in population modelling for this species.