Like all Eulemur species, brown lemurs exhibit the unusual activity pattern known as cathemerality (i.e., active during both day and night; Tattersall, 1987). All known brown lemur populations forage and move during daylight hours, but the extent of nocturnal activity varies regionally (Rasmussen, 1999; Overdorff and Johnson, 2003). In the west, brown lemurs tend to increase night activity during the dry season (Rasmussen, 1999; Kappeler and Erkert, 2003), while eastern and northern populations remain active across the daily cycle throughout the year (Overdorff and Rasmussen, 1995; Freed, 1996; Johnson, 2002). In semidecidu-ous western forests, the most important proximate determinant of nocturnal activity appears to be light availability, which is dependent on lunar phase (Donati et al., 2001; Kappeler and Erkert, 2003; but see Rasmussen, 1999), while no such relationship is apparent in the dense humid forests of the east (Overdorff and Rasmussen, 1995). The origins for this unusual circadian rhythm have yet to be determined: it may represent a phase in an ongoing transition from nocturnality to diurnality (e.g., van Schaik and Kappeler, 1996; Kappeler and Erkert, 2003) or a stable behavioral pattern, perhaps primitive for day-active lemurs (Tattersall, 1982). Several proposed functions of cathemerality have received support from brown lemur field studies. There is evidence that increased nocturnal activity is an antipredator strategy (largely in response to diurnal raptors), particularly in seasonally defoliated habitats in the west (Rasmussen, 1999; Donati et al., 2001). In addition, cathemerality may be a thermoregulatory response (i.e., to maintain body temperature by moving at night, especially during colder months; Overdorff and Rasmussen, 1995; Donati et al., 1999; Kappeler and Erkert, 2003). Cathemeral activity patterns may also function to mitigate feeding competition with other day-active lemur species (Rasmussen, 1999; Vasey, 2000). Finally, Engqvist and Richard (1991) posited that cathemerality serves to increase feeding time on relatively low-quality food items, such as fibrous leaves, when preferred high quality food items are scarce. Lemurs are relatively small-bodied and lack typical digestive tract specializations for folivory and therefore may be required to consume large quantities of leaves over long periods of time (i.e., across the 24-hour cycle) (Engqvist and Richard, 1991). This function has thus far received little empirical support (Overdorff and Rasmussen, 1995; Donati et al., 1999). However, there was a correlation between nocturnal activity and increased unripe fruit consumption in E. f. rufus at Ranomafana (Overdorff and Rasmussen, 1995). It is important to note that all but the last of these proposed functions are based on the assumption that cathemerality represents a shift by an originally diurnal species toward increased nocturnal activity — when instead the reverse may be the case (Kappeler and Erkert, 2003).
Brown lemurs across Madagascar also exhibit variability in time allocation for specific activities. While the most common behavior overall is resting, the frequency varies greatly across populations (47-77%; Sussman, 1974; Overdorff,
1991; Vasey, 1997; Rasmussen, 1999; Gerson, 2000; Johnson, 2002). The large (and variable) proportion of time devoted to resting is undoubtedly partly a response to cathemerality: minimum daily feeding requirements and social demands are more easily met when brown lemurs are active throughout the 24-hour cycle. Brown lemurs also exhibit marked variation in time devoted to feeding (10-26%), travel (6-30%), and social or other activities (1-28%) (Sussman, 1974; Overdorff, 1991; Vasey, 1997; Rasmussen, 1999; Gerson, 2000; Johnson, 2002).
Differences in activity budgets have been suggested to minimize interspecific competition between brown lemurs and other sympatric lemurs (Sussman, 1974, 1977; Overdorff, 1991, 1996; Vasey, 1997). Intrinsic habitat characteristics such as spatiotemporal variation in resources may also affect these behaviors in brown lemurs. During food scarcity at Ranomafana, E. f. rufus increased feeding time while reducing time spent traveling and resting (Overdorff, 1996). In some scarce periods, E. f. albifrons at Masoala minimized energy expenditure by increasing time spent resting and feeding and traveling less; in other seasons of low resource availability, this population adopted the opposite strategy due to the patchiness of resources and increased direct interspecific competition for particular food items (Vasey, 1997). Vevembe E. albocollaris exhibit the latter approach (although without competition from other day-active frugivorous lemurs), while the associations between activity and the availability of important food items are not apparent in the Andringitra brown lemurs (Johnson, 2002). Thus, the relatively low-quality habitat at Vevembe may compel brown lemurs to shift their activities according to changes in resource base and feeding requirements, while the Andringitra hybrids are less constrained (Johnson, 2002).
Ranging patterns also differ across Madagascar. In western dry forests, home ranges are very restricted (0.75-1.0 ha in E. f. rufus; Sussman, 1974; 7-16 ha in E. f. fulvus; Harrington, 1975; Rasmussen, 1999). Eastern rainforest brown lemurs tend to have larger ranges, but there is tremendous variation (12-100 ha) (Overdorff, 1991; Vasey, 1997; Johnson, 2002). Partly in conjunction with home range differences, daily travel also varies considerably among populations. Again, western brown lemurs have relatively short daily path lengths (E. f. rufus: 125-150 m at Antseranomby; Sussman, 1974; 213-368 m at Anjamena; Gerson, 2000; E. f. fulvus: 447 m in the wet season at Ampijoroa; Rasmussen, 1999). In the eastern forests, E. f. rufus and E. f. albifrons both have much longer daily path lengths (962 and 978 m, respectively) despite clear differences in total home range area (85-100 versus 16 ha) (Overdorff, 1996; Vasey, 1997). Divergent patterns are also apparent within sites: Andringitra hybrid groups vary between 286 and 744 m in daily path length, which in this case correlates with home range size (Johnson, 2002). It is difficult to link these patterns with particular ecological constraints or strategies (e.g., in the Andringitra hybrid zone, where ranging differences are not associated with variation in diet, resource availability, population density, or lemur community structure; Johnson, 2002).
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