During the past few years results of field studies on nocturnal primates showed that many species are socially and spatially pair-living (e.g., Fietz, 1999; Müller and Thalmann, 2000; Schülke and Kappeler, 2003). Indeed, the study presented here once started with the premise that Avahi and Lepilemur provide an exemplary case for behavioral and ecological comparisons between—at the time—a monogamous and a solitary species eliminating body mass, gross diet, and environment as interfering variables. Meanwhile, in the light of all new results, it became clear that it is more appropriate to use the term pair-living instead of monogamous for the grouping pattern, thus separating genetic and social descriptors for social organizations, and solitary-but-social instead of solitary, because all nocturnal primates seem to live at least in some kind of social network (e.g., Müller and Thalmann, 2000). In more rigorous terminology, the spacing pattern between individuals was described as gregarious or cohesive for group-living species, and dispersed for

100 200 300m

Figure 9. Home ranges with range centers (harmonic mean) for Lepilemur individuals in 1996 based on direct observations and sleep tree localizations. Most data were available for the focal animals L1m and L2f, and L18f. All individuals were either neighboring or occupied overlapping ranges when belonging to a range. Gaps between individuals and/or range associations are most probably due to differences in observation intensity.

species where individuals are mostly encountered alone during their active period but obviously live either in the same range or belong to the same social network.

The goals of the study were (1) to provide basic descriptive data on aspects of behaviour and ecology for A. occidentalis and L. edwardsi to test whether it is justified to use them as a field model to compare a pair-living and a solitary-but-social species, (2) explore possible links between behavior, ecology, and seasonality in view of a supposed change in availability of food resources over the year, and (3) to determine if Avahi show different adaptations than do Lepilemur in this respect.

The collected descriptive data showed both similarities and differences between the two species. Most surprising was certainly the finding that both species are obviously pair-living, although in different ways. In both species, male and female had coinciding ranges during consecutive years, and ranges were stable in location (Figures 7-10), to the exclusion of other neighboring groups or range associations. Avahi lived in spatially gregarious family groups, and it is most probable

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