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'Froehlich JW, Schillaci MA, Jones-Engel L, Froehlich DJ, Pullen B, 2003: A Sulawesi Beachhead by Longtail Monkeys (Macaca fascicularis) on Kabaena Island, Indonesia. Anthropologie (Brno) 41, 1-2: 17-24'. |
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| Abstract | | The Island of Sulawesi in Indonesia, located east of Borneo on Wallace's Line of faunal separation, is notable for the presence of eight distinct species of macaques. This represents 40% of the species diversity in the genus Macaca in less than 2% of its historical range, roughly half the size of California. These monkeys were derived from two separate, overseas migration events by a pigtail macaque (M. nemestrina) ancestor beginning about one million years ago. Almost everywhere else in Indonesia, and also in the Philippines, the longtailed monkey (M. fascicularis) now predominates. Despite this recent history of successful dispersion, there is no record of feral populations of longtail monkeys on the Island of Sulawesi. An exception to this exclusion is the small volcanic Island of Kabaena, off the southwest coast of Southeast Sulawesi, where there apparently are no recent records of M. brunescens, the very short-tailed monkey that occupies the adjacent two islands, Muna and Buton. Thirteen of the Kabaena animals were measured in 2002. Because most of these pets were juveniles, growth regression lines were used to compare them with a wild-shot sample of longtailed macaques in Thailand. In comparison with this natural population, the Kabaena females are markedly dwarfed, but the males appear to have an accelerated growth trajectory that predicts an unexpected greater degree of sexual dimorphism on this small island than in continental Thailand. For whatever reason, this remarkable adaptation implies that the longtail monkeys have resided on the island for some time. Data on ranging patterns suggest that the longtailed monkeys only occupy coastal mangrove swamps and gallery forests to an elevation of about 200m. This leaves more than 75% of the island for the anticipated presence of M. brunescens, but informants noted that they had not been seen for at least 50 years. There are four hypotheses for this apparent, island-wide competitive exclusion of Sulawesi monkeys from Kabaena. Possibly, they were never present on Kabaena, though the shelf of Sulawesi must have absorbed Kabaena, as it did Muna and Buton, during Pleistocene sea level regressions. Secondly, they may have been extirpated by human habitat degradation, as much of the island has secondary grasslands and tall stands of bamboo, but the longtails would seem to be more susceptible to these anthropogenic factors than animals in the interior forested valleys, surrounded by peaks up to 1500m. Thirdly, they may have lost out to the dwarfing and increased reproductive capacity of the longtails, presumbly excluding them from some essential resource, but this seems unlikely in the relatively small area occupied by the longtails. Finally, they may have suffered from an epidemic spread by the arrival of the longtail monkeys, or one the longtails were already immune to in the human hosts that introduced them to Kabaena. Much like the colonization of the Americas since Columbus, this disease could spread throughout the island, and may still endanger the rest of Sulawesi. Reciprocally, however, competitive exclusion by diseases endemic in Sulawesi monkeys might be a hypothetical mechanism to explain why Kabaena is the only beachhead for longtail monkeys in Sulawesi, almost surrounded as it is by natural populations of these ubiquitous and highly successful monkeys throughout most of Indonesia. | | | | Keywords | | Macaques - Competitive exclusion - Island biogeography - Dwarfing - Disease epidemics | | |
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