Feeding ecology and diet

Gibbons (Hylobates spp.) are monogamous, territorial, fru-givorous, and suspensory with elaborate duets. The nine species studied have all been shown to conform to this pattern; they live at low-biomass density in small territories, because of their focus on small, scattered but predictable sources of ripe fruit. It is for these dietary reasons, in competition with the opportunistic, frugivorous macaques (Macaca spp.) living in large social groups and with the one-male groups of langurs (Presbytis and Trachypithecus spp.) eating leaves and seeds, that they have opted for monogamous family groups defending the area containing the necessary resources.

Gibbons are fruit-pulp specialists, like the spider monkeys (Ateles spp.) of the Neotropics, and the chimpanzee (Pan spp.)

Female pileated gibbon (Hylobates pileatus) in Thailand. (Photo by Terry Whittaker/Photo Researchers, Inc. Reproduced by permission.)

of Africa. But, unlike most primates, especially cercopithecids (Macaca spp. of Asia), gibbons compete more with large birds such as pigeons and hornbills for the small, colorful, sugary fruit. The monogamous family groups focus on small fruiting trees to avoid competition with the large multimale, multi-female groups of macaques and the large orangutan (Pongo pygmaeus in Borneo and north Sumatra in Indonesia).

Gibbons differ from other primates in not having a markedly bimodal pattern of daily activity, with feeding peaks early and late in the day, and a long mid-day siesta. After active bouts of feeding, gibbons continue foraging in the cooler lower levels of the canopy through the heat of the day; they retire early for the night, usually several hours before sunset.

Gibbons are active for 9-10 hours each day on average in the evergreen rainforest, but for only 8-9 hours in Bangladesh. Lar gibbons tend to be active for a shorter time than siamang, with a 40-50% activity period for siamang compared to 30-40% for lar gibbons. Hoolock gibbons actually feed for about 40% of the active period.

Between 57% and 72% of feeding time is spent eating the reproductive parts of plants, such as fruit and flowers, except for the larger siamang (44%). About 25% of the fruit intake is figs (nearly 40% in siamang). Young leaves are important for most gibbons, especially the siamang, but not for the Kloss gibbon (where the soils are poor and the leaves are better defended chemically). Animal matter, mostly invertebrate, provides an important source of animal protein (about 10% of feeding time). More recent studies have confirmed that the gibbons of the more seasonal forests are finding as much, if not more, of such fruit, compared with those in the evergreen forests. Hoolock gibbons spend 79% of their feeding time on fruit.

Ketambe (Sumatra), unusually rich in fruit, has a very high biomass of primates. It was found that figs predominated in the diets of siamang and lar gibbons (44%). With 61% of the diet being fruit, the siamang had an intake of only 17% leaves, whereas the lar gibbon ate 71% fruit and only 4% leaves. Both fed on small fruit patches (the lar gibbon finding more of them) that were seasonally variable. There was more feeding competition in the lar group, hence its greater dispersion, lower cohesiveness. Tree fruit were more abundant than liana fruit, but only 37% of trees fruited annually, compared with 58% of lianas. Trees produced more young leaves seasonally, whereas lianas provided a more continuous supply. Both species consumed more than 20% animal matter.

The consumption of fruits and dispersal of seeds is a key feature of the coevolution of animals and plants in the natural regeneration of forests. Different animals remove seeds of different sizes, which can be related to a suite of characters that distinguish fruit whose seeds are dispersed by primate or bird or rodent. For some plant species, gibbons are key seed dispersers; for others, especially those dispersed by several bird species, gibbons are less important dispersers.

Mueller's gibbon (Hylobates muelleri) is found only in Borneo. (Photo by Tom & Pat Leeson/Photo Researchers, Inc. Reproduced by per-

The density of monogamous family groups of gibbons (usually four individuals) varies from 1.5 (two species in Malaya) to 6.5 (Thailand) groups/mi2 (km2); the combined biomass of siamang and lar gibbons in Malaya was 278 lb/mi2 (l26 kg/km2), with 75 lb/mi2 (34 kg/km2) for Mueller's gibbon in Kalimantan, and 229 lb/mi2 (l04 kg/km2) for lar gibbons in Thailand. Thus, there are not fewer gibbons in the more seasonal forests further north. Biomass density relates more closely to food availability, presumably at times of scarcity.

Pileated, Mueller's gibbons, and siamang travel 0.49-0.56 mi (0.8-0.9 km) daily on average, while the others travel 0.74-0.93 mi (1.2-1.5 km). Siamang have been seen to travel as little as 490 ft (150 m) a day (when fruit were scarce) and as much as 9,380 ft (2,860 m) a day; hoolock gibbon day ranges vary 919-11,155 ft (280-3,400 m); other gibbons show comparable variation from about 1,312-8,202 ft (400-2,500 m). These changes reflect variation in food distribution, but in the monsoon forests, where leaves are not such a viable alternative for the smaller gibbons, increased day ranges may reflect a wider search for sufficient fruit.

Home range varies between 39.5 ac (l6 ha) for lar gibbons in Thailand and 42 ac (l7 ha) for moloch gibbons, to 111 or more mi (45 or more ha) for hoolock in Bangladesh, 138 ac (56 ha) for lar gibbons in Malaya, where siamang home ranges are also large at 74-99 ac (30-40 ha). Where there are two species of gibbon, which always involves the larger siamang, it is likely that the homes ranges of both are larger than when alone, because of competition for particular fruit trees.

Of the home range, 62% (siamang) to 94% (moloch gibbon) are defended as territories for the exclusive use of the resident group; most are clustered around 75%. Again, the gibbons of the more seasonal forests are not traveling further around a larger area, but they are defending 80-90% of the home range. Thus, home ranges average about 86.5 ac (35 ha), of which about 75% is defended. While siamang travel around their home range in single file, the smaller gibbons more often scatter to forage as they move between food sources in which they all feed.

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