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With 9 figures in the text)Lepilemur edwardsi and Avahi occideiitali.7 are both small, nocturnal folivores, but live sympatrically over a large part of their range, suggesting that niche differentiation has occurred. In order to establish whether ranging behaviour and activity patterns have contributed to niche differentiation, a field-study was carried out at Ampijoroa, Madagascar, within the area of range overlap. Four individuals of each species were fitted with radiocollars and tracked, dusk to dawn, over I8 months. Data were collected on home-range size, travel distances, activity and social behaviour. Neither showed the markedly biphasic activity patterns which have often been described for nocturnal prosimians; this might be explained by the combination offolivory and small body size. L. edwurdsi spent long periods inactive, but engaged in considerable social interaction during the night, including extensive long-range calling. The mean home range was I .09 ha, and ranges were rather evenly used. Home ranges of females overlapped extensively. and the range of the single adult male which could be caught overlapped with the ranges of several females, as is common in nocturnal prosimians. The mean nightly travel distance was very low. only 343 metres. On the other hand, social interactions in A. occidenrdis were primarily within a stable monogamous family unit, and long-range calling was very limited. Home ranges were rather larger than in L. edwurdsi, but less evenly used, and mean nightly travel distances were much greater, 117.5 m. Differences in range-use patterns may be explained by the contrast between Avuhi's selective feeding on relatively high-quality young leaves and flowers, versus Lepilemur edrrardsi's unselective feeding on relatively low-quality, but ubiquitous and evenly-distributed. mature and old leaves. Similarly, while overlapping female ranges may be adequate when food resources are evenly distributed, monogamy in Avahi may be a response to selective feeding on a patchy food resource.

A well-known principle holds that where food supply is limited, two sympatric species with the same diet, morphology, and behavior will compete, leading to exclusion of one species or differentiation of their ecological niches. Avahi occidentalis and Lepilemur edwardsi are two saltatory, nocturnal folivorous lemurs of similar body size which live sympatrically over much of their range and may often be seen feeding in the same trees. We report a study of their locomotor behavior, carried out in order to determine to what extent locomotion might contribute to niche separation. Avahi lives at a higher level in the forest and is the more committed leaper, with a much larger home range and nightly travel distance, while Lepilemur climbs more and has a very small nightly travel distance. Avahi uses small, low-angled supports more often, while Lepilemur makes more use of large, high-angled supports (however, neither species uses vertical supports at a frequency greater than the availability of such supports in the forest stratum in which they live). Both species tend to land on larger-diameter supports on longer leaps, but Avahi also takes off from larger-diameter supports in longer leaps, reducing loss of energy to the substrate during takeoff. We suggest that the differences in locomotion and ranging behavior between them are consistent with niche differentiation on the basis of food quality, Avahi investing more energy in harvesting better quality but more patchy food, Lepilemur minimizing expenditure in harvesting a less selective, quantitatively poorer diet.

Lepilemur edwardsi and Avahi occidentalis are two species of nocturnal, folivorous ‘vertical clingers and leapers’ (VCL). They have a similar body mass and share the same morphological adaptation for leaping. In a field study under sympatric conditions at Ampijoroa, Madagascar, comparison of support use with support availability using Jacobs' D preference values (Jacobs, 1974) showed that both species actively chose or avoided branches with certain qualities. However, while both species showed a preference for small oblique and horizontal branches, they selected them at different heights in the forest and with varying degrees of preference and avoidance for the other available supports. Despite their traditional locomotor assignation, both species showed a surprisingly strong preference for horizontal supports. These striking variations in detail of support preference may aid the maintenance of species segregation and niche difference.

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