Living together in the night: abundance and habitat use of sympatric and allopatric populations of slow lorises and tarsiers

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Throughout much of Asia, slow lorises (Nycticebus) and tarsiers (Tarsius) live allopatrically, but on several islands, including Sumatra and Borneo, they occur in sympatry. Dwindling habitats could result in resource competition within these sympatric populations, as the diets of these species overlap. To assess the possibility of resource competition, we gathered data from the literature on the abundance and microhabitat structure of slow loris and tarsier species throughout their ranges. We also estimated abundances of Bornean lorises Nycticebus menagen- sis and western tarsiers Tarsius bancanus borneanus and investigated their habitat use in Sabah, Malaysian Borneo. We predicted that species living sympatrically would favor different heights, and thus different microhabitat structures from congeners, and have lower abundances compared with allopatric species because of limiting factors. Across their ranges, loris density did not vary regardless of whether they were allopatric or sympatric. However, across sympatric and allopatric tarsier ranges, there were significant differences in densities (sympatric: 3!27 ind. km !2 , allopatric: 57!268 ind. km !2 ). Microhabitat use varied significantly between sympatric and allopatric loris populations (p = 0.036) but not between sympatric and allopatric tarsier populations. Our results, although based on a limited amount of data, suggest that tarsiers are impacted by the presence of slow lorises in their habits, and these populations should be monitored, especially as habitat sizes dwindle and resources continue to become scarce.