By : Kurnia Ahmaddin

The monitoring of Javan gibbons in Sawahan Hamlet, Mendolo Village began with the concern of the youth from PPM Mendolo, who suspected that the gibbons in the western hills of their village were becoming isolated. Since 2010, much of the forest area has been converted into durian orchards and non-shade coffee plantations. Through the Corridor of Life program, which started in 2022, we encouraged their concern to grow into a tree-planting initiative so that gibbons separated from their original groups could regain forest corridors for their own range. For this reason, we guided the PPM Mendolo youth to follow the movements of the Javan gibbons in the western hills of their village to address their concerns.

Forest Fragmentation in Kupel Hill

The fragmentation of forests in Kupel Hill (west of Sawahan Hamlet, Mendolo Village) cannot be easily detected from satellite imagery, as most of the hill is covered by rubber plantations owned by PTPN. A small portion (marked yellow on the map) has been left unmanaged since the last logging in 2017, which has since grown into secondary forest. The rest consists of durian and coffee plantations. Ground checks revealed that only 70 hectares of forest trees remain out of the original 200 hectares of Kupel Hill.

From January to October 2025, 74 encounter reports were recorded by the monitoring team, including research data from Nur Azizah (research intern from Biologi UIN Syarif Hidayatullah Jakarta) between April and June. Only one record noted gibbons crossing through a durian orchard; the rest were observed exclusively in forest trees. The target gibbon group consists of a breeding pair, two juveniles, and one infant. During the monitoring period, their range covered approximately 32.9 hectares. However, in August 2025, 7.29 hectares of secondary forest (yellow on the map), owned by PTPN, was logged again for timber harvest. As a result, the gibbon group’s range shrank to 25.61 hectares. Within this area, there is a natural overlap of 4.72 hectares (red on the map) with another gibbon group of four individuals.
Monitoring results show that areas without encounter points (green on the map) are durian and non-shade coffee plantations, visible as empty circles without encounter marks. Given that only one crossing was recorded in durian orchards, the target group’s range is at risk of further shrinking unless sustainable farming practices are introduced.

Behavior of the Gibbons in Sawahan

We acknowledge that our field data collection has not yet been consistent. We only recorded brief behavioral notes during monitoring, as the gibbon group in Sawahan is not yet habituated. Monitoring was conducted for half days, not from waking to sleeping. Rain and other field challenges also shortened observation hours. Behavior was recorded only once per encounter, ad-libitum, without systematic time intervals.
Behaviors noted included feeding, resting or sleeping, moving, playing, mating, grooming, and vocalizing, which we categorized as social behavior. Average encounter distance was 95.1 meters from observers. The gibbons remained cautious, often leaving when humans approached within 100 meters. Although the closest tree (Tengger tree) was about 30 meters away, the monitoring team always hid to avoid frightening the gibbons. We also wore consistent uniforms during observations to reduce disturbance. The group was active in the canopy, averaging 25.7 meters above ground.
The most frequently recorded activity was moving, likely because the group is not yet habituated. The least observed were social activities. Among social behaviors, we noted play among the juveniles, grooming, and vocalizations (great calls or alarm calls). No reproductive activity was recorded. The group was often observed resting—sitting, lying down, or napping—and frequently feeding. Due to distance, we could not yet identify the insects consumed. However, in Mendolo we have documented 27 tree species used as gibbon food sources.

Translated from original article in bahasa here :https://swaraowa.blogspot.com/2025/12/menyempitnya-wilayah-jelajah-owa-jawa.html