Abstract
Since the earliest reports, it has been known that orang-utans are fruit eaters. Frugivory has important implications for many aspects of an animal’s biology and in particular its way of life. Thus, the quantitative and qualitative distribution of the ape’s fruit diet in time and space will be a major determinant of its ranging behaviour, population density and, consequently, social organisation. Field researchers during the 1970s habituated for the first time small populations of resident apes in particular, well-studied areas, and since then biologists have acquired some insight into the complex lifestyle of the orang-utan.
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In 1981 Sugarjito and Nurhada observed an orang-utan devouring what they thought was a dead infant gibbon, but which was probably a slow loris. In the 1990s the capture and eating of slow loris individuals was observed several times by Ms. Sri Suci Utami at Ketambe (Azwar: Orangutan kanibal di Gunung Leuser, Mutiara 03.12.1993) and by Mr Bahlias P.G., at Suaq Balimbing, in Sumatra in 1994 (van Schaik, pers. comm.).
Tool use by captive orang-utans is well known, and in experimental situations the red ape deploys tools as intelligently as, if not more so, than the chimpanzee (Lethmate, 1977); tool use by wild orang-utans has been observed during locomotion (‘fishing’ for a branch or liana which was out of reach), during insect foraging (Rijksen, 1978), honey collection (Van Schaik et al., 1996), protection against bees, and for flushing out of tree-holes young squirrels and geckos (Rijksen, unpubl.) which can be caught and eaten. Insect foraging and the opening and processing of ripe Neesia fruits by orang-utans in the swamp forest of the southwestern Leuser Ecosystem (Van Schaik et al. 1996) has also been recorded as a cultural routine.
An ecotone is an interface where two areas of different ecological (e.g. geomorphological, hydrological) conditions meet; a‘border zone between two habitats’ (Fitter and Fitter, 1967); alluvial areas, such as flood plains or a river valley, are also ecotones.
The ‘best’ soil types, supporting the highest species diversity, are deep ultisol/acrisols and humult ultisols; diversity declines when a humus podzol becomes shallower (Bruenig, 1996).
The 50 ha permanent plot at Pasoh in West Malaysia contains 660 species (224 genera in 67 families) of trees with a diameter at breast height (DBH) of 10 cm or larger (Soepadmo, 1995); the forests of Sumatra and Bomeo are usually sligl3tly richer in species; any sample plot has between 100 and 250 species of trees/ha of > 10cm DBH (M.MJ. van Balgooy pers. comm.; van Valkenburg, 1996) and between 250–350 species of smaller diameter (Whitmore, 1995; Bruenig, 1996).
For a fruit tree, a harvesting visit of orang-utans may indeed be a calamity because the apes are usually very destructive gatherers. At Ketambe the highly preferred Durio oxleyanus and Heritiera elata trees required at least six years to recuperate and bear fruit again (see also Leighton, 1982: 145). Under domestic conditions a durian tree produces fruit annually or once in two years.
This lean period is not fixed between the months of November and March. In recent years it appears to have shifted so as to begin in ebruary and end in April (Van Schaik, in Sugardjito, 1987).
The cultivation of rice and marketing of fruit (e.g. for durian, rambutan) also moves like a wave over large geographical regions in western Indonesia (Bottema, 1995).
As noted earlier, this is not what Suzuki (1989) found in the burned forests of the Kutai area in East Kalimantan. However, it is not unlikely that the resident orang-utans in the Sanggata study area realised that there was no chance of finding a better spot for survival under the circumstances; after all, the forest had been burned over enormous areas. Such a hypothesis is corroborated by the observations concerning orang-utans in the open-pit KPC concession mine area in East Kalimantan, who apparently saw no options for moving in the midst of the boundless devastation.
Mothers with dependent offspring usually travel between 500–700 metres/day; adult males travel slightly further than adult females and often show crudely circular ranging patterns covering anything between 600–800 metres on average per day (except when being chased). Of all the age-groups, adolescent and young adult females without offspring range farthest, travelling in an erratic wide-ranging pattern while covering anything between 600–1000 metres/day, while sub-adult males commonly trava almost as far as young females, yet more directionally and often far beyond the range of their natal deme.
In view of this fate, it is perhaps understandable in ontogenetic terms why sexually mature (i.e. sub-adult) males actively enforce copulations (i.e. rape); it may be their only chance in life to reproduce, no matter how unlikely the copulation is to result in conception.
Social relationships and bonds with other females are developed mainly during the years of adolescence and pre-motherhood.
High-ranking sub-adult females also can ‘rape’ lower ranking, newly encountered females, by manual intromission of the victim, usually while masturbating (see also Rijksen, 1978).
Under experimental conditions in captivity, however, fully adult orang-utan males, after separation from a female for some time, revert immediately to rape, enforcing copulations (Nadler, 1995).
It is remarkable in this respect that in comparison to that of the bonobo and the chimpanzee, the erect penis of the orang-utan is small and inconspicuous.
It is assumed that such a situation in e.g. langurs (Hrdy, 1977), humans (Devereux, 1976; Kempe, 1976) and gorillas (Fossey, 1983) can lead to infanticide; as yet there is no indication that infanticide occurs in orang-utan demes under natural conditions.
Such an instance of temporary ‘refugee crowding’ due to logging was reported from conversion forest near Moara Wahau (East Kalimantan).
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© 1999 Stichting Tropenbos/H.D. Rijksen / H.D. Rijksen and E. Meijaard
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Rijksen, H.D., Meijaard, E. (1999). Ecology and Natural History. In: Our vanishing relative. Springer, Dordrecht. https://doi.org/10.1007/978-94-010-9020-9_4
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DOI: https://doi.org/10.1007/978-94-010-9020-9_4
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