A field study on Sumatran orang utans (Pongo pygmaeus abelii Lesson 1827) : ecology, behaviour and conservation

Abstract

The results of a three year research project on the ecology, behaviour and conservation of the Sumatran orang utan are discussed. The 150 hectares Ketambe study area lie within the boundaries of the Gunung Leuser. reserve in Aceh Tenggara, and consists of mixed rainforest typical of hilly regions.The orang utan's food is irregularly distributed in quantity, space and time within this area. Although the orang utan is frugivorous he also eats leaves, insects, 'bark' and several other less important foods, such as bird's eggs. By far the largest part of the 114 food plants collected is typical primary rainforest growth. Figs make up an important part of the diet, particularly those known as the 'strangling' Ficus spp. As the fruiting seasons of the various strangling fig species are staggered over the whole year, and such trees are often enormous in size, they form particularly suitable food sources. Orang utans appear to have a good topographical knowledge of their 'home range'. There are indications that fig trees bearing ripe fruits are sometimes found by using the flight paths of flocks of hornbills, which also feed on figs. With respect to figs in particular, the orang utan must compete for its food with many other species, notably primates.The study area was regularly used by 22 individually recognized orang utans. Both males and females live in homeranges which overlap considerably and are 2-10 km sq. in extent. The population density of the Ketambe area is considerably higher than that reported for other areas in Indonesia, being approximately 5 individuals per square kilometre. Orang utans live a 'limited gregarious' lifestyle, that is, adult males spend the largest part of their time alone, and avoid encounters with other adult males. The adult females and their offspring stay together for the first 4-5 years of life (the birth interval is at least 3 years.). Adolescent social groups form after the mother-offspring bond becomes weaker. This social phase continues for each individual until either (a) the sexual dimorphistic characteristics of males are clearly distinguishable (at approximately 15 years of age), or (b) the female's first young is born (approximately 10 years of age). Adults also have social moments in their lives. Several individuals in an area can come together to form a 'temporary association' in certain fruit trees. It may be seen from their behaviour that they know each other well and that a network of relationships exist which is partly based on competitive dominance. A peaceful co-existence between adult males however, has not been observed; meetings between adult males had invariably an agonistic character. The sexual behaviour of the orang utan is roughly divisable into: (a) 'raping' of some females by sub-adult males; and (b) cooperative matings, where females often take the intitiative by presenting themselves to, usually, adult males. It appears that only the latter makes an essential contribution to reproduction.The social behaviour of the orang utan is considered and described in detail. Comparison of the behavioural repertoire of this species with that of the (gregarious) chimpanzee shows that the two species are closely related. There are clear differences in social organisation, however, which appear to be mainly of degree, rather than of quality. The social organisation of an animal must be a factor that fits its situational (i.e. environmental) context, but is based on phylogenetic characteristics. Comparison of the habitats of these two apes reveals that they both live in a similar ecological condition with respect to food. The reasons for the exceptional life-style on the part of the orang utan are possibly to be found in the predation pressure the species is exposed to, particularly by man. Man has been present in regions of South-EastAsia since the Pleistocene. It is plausible that the orang utan has been persecuted by man continuously from this time. A comparable influence on the chimpanzee is much more recent. The orang utan is the heaviest arboreal creature and has an exceptional social organisation for a primate. If it may be assumed that high intelligence is a biological outcome of complex group life then, on grounds of intelligence, and from the distinct social organisation and current arboreal life-style it maybe postulated that the orang utan has evolved from a groundliving social ape. This original form was supplanted from its niche by a better adapted ground-living social ape, that is, man. It is possible that a stricly arboreal life style for an animal as heavy as an adult orang utan is not compatible with a social life-style. Moreover, it is likely that refraining from a high degree of sociability and inconspicuousness are the best strategies against human predation.Since one possibility is that the hunting pressure in ancient (and current) times had a great influence on the biology of the orang utan, I have paid particular attention to hunting behavio