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a Size differences within individuals are most probably due to differences in observation intensity.

a Size differences within individuals are most probably due to differences in observation intensity.

feeding during the day. Avahi seems to have some kind of potential for diurnal activity or cathemerality. Lepilemur was never seen feeding during the day but sometimes animals peered out of their tree hole. Both species start their activity shortly after sunset and this is very likely triggered by the low light intensity as indicated by the high positive correlation. However, cessation of activity is not related to sunrise. Thus, it seems reasonable to suggest that, once all needs of individuals are met, activity stops whether it is still comfortably dark or not. The available time for foraging is not entirely exploited. When Lepilemur return early to their sleeping hole, they often remain immediately outside of it, and survey the forest before they retire for the day. The length of the active time is related to night length and does not differ over the year between the two species (Figure 4). The positive correlation between night length and active time is lower in Lepilemur than in Avahi, and it might be possible that Lepilemur do adjust their active time more to food availability than do Avahi to conserve energy during the leanest months of the dry season (July-October). Lepilemur do not use the longer nights to spend more time foraging but instead seem to reduce active time from July to October (after the length of the active time seems to increase till June in line with night length).

Activity Budget. Obvious differences are present in the activity budget in that Avahi spends significantly more time feeding than does Lepilemur. An increased sample would probably also reveal significant differences in resting time during the activity period and grooming, especially grooming between individuals ("partner grooming"). This would corroborate subjective impressions that the Lepilemur male and female groomed each other considerably more often when they met than did the Avahi male and female who were always in close spatial relationship. The detailed investigation of food resources (Thalmann, 2001) confirmed that both species are folivores but that there is obviously no direct competition for food. Indeed, dietary overlap is minimal, especially if seasonality and resource size is accounted for. As Avahi chooses more high-quality leaves compared to Lepilemur (Ganzhorn, 1993), it might be hypothesized that Avahi anticipates or finds high-quality food more efficiently than does Lepilemur. Indeed, it seemed that Avahi returned more often and regularly to the same feeding localities as long as they provided food, and more often chose the very same feeding trees in consecutive years. Whether this matches differences in mapping capacities in the two species is speculative at this time. It may also be hypothesized that Avahi have a generally higher basal metabolic rate and a lower ability to adjust their metabolic rate to environmental conditions.

Lepilemur show several behavioral traits, notably complex vocalizations that correlate significantly with environmental variables (Table 2). These complex vocalizations increase dramatically towards the rainy season, evidently in line with increasing rainfall, leaf cover (increased food availability), and simultaneously with an increase in Nightly Path Length and range use. Both sexes show basically the same pattern. This is not too surprising for the complex vocalizations, as they constitute most probably pair-specific duets (Thalmann, unpublished data). More interestingly are the questions as to why such coordinated vocalizations occur at this time of the year, what their function is, and whether they have the same function in both sexes. Several functions have been proposed for coordinated pair vocalizations, including enhancing/advertisement of the pair bond, and territorial advertisement (e.g., Geissmann, 1999). If enhancing and advertisement of the pairbond is the only function, why then should these complex and coordinated vocalizations occur outside of the mating season which is from May to July? If the main function is territorial in nature, what is actually defended? It seems unlikely that these are food resources as they are most probably available in abundance for L. edwardsi (Thalmann, 2001). Is it the territory with the female that is defended by the male, or, quite unlikely, vice versa? The complex vocalizations do, however, overlap to some degree with the birth season (September - November) and continue over a period when newborn Lepilemur are cached and do not follow their parents. Hence, it could be suggested that this coordinated vocal behavior occurs in the context of infanticide prevention, as infanticide has been reported in L. edwardsi (Rasoloharijaona et al., 2000). This might explain the participation of both sexes in such vocalizations during this particular time of the year. Further investigations are certainly needed to investigate this particular behavior, which is obviously not present in A. occidentalis where the mother always carries the offspring until it can move independently.

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