Fine-scale habitat use by orang-utans in a disturbed peat swamp forest, central Kalimantan, and implications for conservation management.

This study was conducted to see how orang-utans (Pongo pygmaeus wurmbii) were coping with fine-scale habitat disturbance in a selectively logged peat swamp forest in Central Kalimantan, Borneo. Seven habitat classes were defined, and orang-utans were found to use all of these, but were selective in their preference for certain classes over others. Overall, the tall forest classes (≥20 m) were preferred. They were preferred for feeding, irrespective of canopy connectivity, whereas classes with a connected canopy (canopy cover ≥75%), irrespective of canopy height, were preferred for resting and nesting, suggesting that tall trees are preferred for feeding and connected canopy for security and protection. The smaller forest classes (≤10 m high) were least preferred and were used mainly for travelling from patch to patch. Thus, selective logging is demonstrated here to be compatible with orang-utan survival as long as large food trees and patches of primary forest remain. Logged forest, therefore, should not automatically be designated as 'degraded'. These findings have important implications for forest management, forest classification and the designation of protected areas for orang-utan conservation.

Meat-eating by a wild Bornean orang-utan (Pongo pygmaeus).

We present the first evidence for consumption of meat by a wild Bornean orang-utan (Pongo pygmaeus). Meat-eating has been reported in Sumatran orang-utans, specifically the hunting and consumption of slow lorises (Nycticebus coucang), but loris-hunting behaviour has not been observed in the Bornean species and meat of any species is essentially absent from their diet, with only two anecdotal reports of vertebrate meat consumption prior to this current finding in over 40 years of study. In August 2012 an unhabituated adult flanged male orang-utan was observed eating an adult horse-tailed squirrel (Sundasciurus hippurus) carcass in the Sabangau peat-swamp forest, Central Kalimantan. We suspect this to be a case of scavenging, never reported previously in a Bornean orang-utan.

The orang-utan mating system and the unflanged male: A product of increased food stress during the late Miocene and Pliocene?

The orang-utan is unique among apes in having an unusually long male developmental period and two distinct adult male morphs (flanged and unflanged), which generally, but not exclusively, employ different reproductive strategies (call-and-wait vs. sneak-and-rape). Both morphs have recently been shown to have roughly similar levels of reproductive success in the one site where such a study has been conducted. This is in stark contrast to the unimale polygynous gorilla, in which dominant males sire almost all infants. Despite this, evidence on sexual dimorphism, life history, diet, and socioecology of extant and extinct apes, as well as the ontogeny, reproductive morphology, and physiology of extant apes, all indicate that the orang-utan's present-day mating system most likely evolved from a gorilla-like base, with one dominant male guarding a harem of females. The available evidence indicates that, due chiefly to the likely onset of the El Niño Southern Oscillation (generally regarded as the trigger for mast fruiting in dipterocarps) approximately 3-5Ma, southeast Asian forests would have begun to experience longer and more severe periods of low food availability. This change in food availability would have meant that full-time gregariousness was no longer energetically tolerable and, as a result, females dispersed more widely in search of food and adult/flanged males were no longer able to effectively guard a harem of females. A niche for a quiet, quick, opportunistic "sexual predator" (i.e., the unflanged male) then became available. This finding implies that, despite being anatomically quite chimpanzee-like, the ancestral hominoid probably had a social and mating system more similar to the gorilla than any other living ape.

Sexual swellings of female gibbons.

Female gibbons were observed for labial swellings from March 2002 to August 2003. Data were collected on length of time each female was swollen, changes in swelling period as females matured, and cycle length (number of days between onsets of consecutive swellings). Data were also collected on the behaviour of the males when the females were swollen. There was a significant relationship between age of the female gibbon and number of days she had labial swelling. There was no significant relationship between the age of the gibbon and the cycle length, nor between species (Hylobates agilis albibarbis and Hylobates muelleri) and number of days swollen. Copulation was more common when females were swollen. Swelling in female gibbons may be related to copulatory behaviour and serve to provide the male with some certainty of paternity through fertilisation insurance.