The brown lemur complex has been characterized as highly flexible in its ecology, in accordance with its broad geographic distribution which spans many distinct habitat types and elevational zones (Tattersall and Sussman, 1998; Goodman and Ganzhorn, 2004). Brown lemurs have relatively high dietary diversity (Tattersall and Sussman, 1998) and have demonstrated niche contraction and expansion in response to community structure (Vasey, 2000).
Guided by community ecology theory, nearly all studies of brown lemur ecology have assessed adaptations in juxtaposition to those of sympatric presumed competitors. Accordingly, most researchers have investigated brown lemurs in conjunction with a sympatric lemur species (Lemur catta: Sussman, 1974, 1977; Eulemur rubriventer: Overdorff, 1991, 1993; E. coronatus: Freed, 1996; Varecia variegata rubra: Vasey, 1997, 2000, 2002; E. mongoz: Rasmussen, 1999). Many mechanisms have been proposed to allow brown lemurs to share their habitats with these potential competitors. These include: greater dietary diversity (Vasey, 2000), utilizing higher (Sussman, 1974; Freed, 1996) or lower (Vasey, 2000) vertical microhabitats, feeding in smaller patches (Vasey, 2000), and/or consuming lower-quality food items (leaves: Sussman, 1974; mature leaves, unripe fruit, and flowers: Overdorff, 1993; higher levels of toxic compounds: Ganzhorn, 1988).
Except in some highly folivorous western E. f. rufus groups (Sussman, 1974, 1977), fruit is the primary resource among brown lemurs, comprising between 66 and 95% of the diet (Tattersall, 1977; Overdorff, 1991, 1993; Freed, 1996; Vasey, 1997, 2000; Rasmussen, 1999; Johnson, 2002). Interestingly, relatively proximate and closely related populations represent the lower and upper limits of fruit dependence: E. albocollaris at Vevembe and E. albocollaris x E. f. rufus hybrids at Andringitra, respectively. Andringitra groups consumed fruit in quantities similar to those observed in lemur species generally considered to be more strictly frugivorous than brown lemurs (e.g., Eulemur rubriventer: 81%; Overdorff, 1993; Varecia variegata: 71-90%; Morland, 1991; White, 1991; Balko, 1998; Vasey, 2000).
Most brown lemurs also supplement their diets with substantial quantities of secondary food sources (< 34% of diet), including leaves (< 26%) and flowers and nectar (< 20%), as well as other less common items (fungi, soil) (Sussman, 1974, 1977; Tattersall, 1977; Ganzhorn, 1988; Overdorff, 1991, 1993; Freed, 1996; Vasey, 1997, 2000, 2002; Rasmussen, 1999; Donati et al., 2002; Johnson, 2002). Very rare occurrences of invertebrate (Overdorff, 1993) and vertebrate (Pitts, 1995; Mizuta, 2002) prey have also been recorded in brown lemur diets. Diversity appears to be the norm for most brown lemur populations; only E. f. sanfordi and Andringitra hybrids infrequently consumed secondary food items (9 and 4-13%, respectively) (Freed, 1996; Johnson, 2002).
Given the differences observed in feeding ecology, as well as the variation in habitat types and food availability across regions, it may also be instructive to examine seasonal influences on diet among the brown lemur groups. Surprisingly, it is often difficult to link diet choices with fluctuations in resource base. In E. f. fulvus at Ampijoroa, there was no correlation between seasonal variation in frugivory and fruit production, nor between secondary resources (leaves and flowers) and their availability (Rasmussen, 1999); however, these brown lemurs did feed more on preferred individual species according to availability (Rasmussen, 1999). In the Masoala Peninsula, E. f. albifrons and sympatric Varecia also remained highly frugivorous during seasonal food scarcity (Vasey, 2000). Vertical stratification and direct contest competition likely served to minimize niche overlap between these species during these periods. Dietary demands of reproductive females in both species also mitigated seasonal diet separation (Vasey, 2000). At Ranomafana, E. f. rufus had greater dietary diversity when fruit was most available and least available (Overdorff, 1993). However, the most dramatic behavioral response during scarcity was migration: red-fronted lemur groups moved 4-5 km from their normal home ranges to more productive areas (Merenlender, 1993; Overdorff, 1993), a ranging pattern that has never been observed in other brown lemurs (e.g., Freed, 1996; Johnson, 2002). In addition, niche separation between E. f. rufus and sympatric E. rubriventer fluctuated according to resource seasonality. During peak fruit scarcity, dietary overlap was lowest and the two species varied greatly in time spent exploiting common plant species (Overdorff, 1993). Nevertheless, frugivory remained high for both species in periods of low fruit availability (Overdorff, 1993). This pattern is similar to the year-round obligate frugivory seen in the Andringitra hybrids (Johnson, 2002). There were positive correlations between frugivory and fruit availability in some Andringitra hybrids (as well as in pure E. albocollaris at Vevembe), but an inverse association was recorded in other areas of the contact zone (Johnson, 2002).
Thus, it appears that seasonal shifts in resource availability do not directly control food choice in brown lemurs. However, year-round food production may have a substantial impact on diet. For example, availability of all food resources (fruit, flowers, and leaves) is much higher at Andringitra than at Vevembe (Johnson, 2002). This higher overall productivity may permit both the very high population density and the greater degree of frugivory recorded in the Andringitra hybrid populations — despite the richer lemur community at this site (Johnson, 2002; Irwin et al., 2005). Yet, overall resource production may be even higher at Ranomafana (Overdorff and Wright, unpublished manuscript), while diet breadth in E. f. rufus here is more like that observed in E. albocollaris at resource-poor Vevembe (Overdorff, 1993; Johnson, 2002). Therefore, the abundant and highly frugivorous brown lemur population at Andringitra may be better explained by more specific characteristics of the habitat, such as the availability of critical resources.
The use of specific resources during scarce seasons is important in many primate communities throughout the tropics. For example, keystone resources — particularly asynchronously fruiting Ficus species — likely sustain the primates of Cocha Cashu, Peru, during the dry season (Terborgh, 1986). However, the universality of Ficus as a keystone resource has been questioned (e.g., Gautier-Hion and Michaloud, 1989). Ficus is rare in Madagascar, with 24 total species and only up to 12 species within individual forests (Goodman and Ganzhorn, 1997). Overall fruit availability is also lower in Madagascar, and there is evidence that seasonality is less predictable and more extreme (Goodman and Ganzhorn, 1997; Wright, 1999). Furthermore, the density of individual Ficus trees tends to be low in Malagasy forests (Goodman and Ganzhorn, 1997), and thus, rarely consumed by frugivores (Goodman et al., 1997). Therefore, the lack of an important keystone resource like Ficus may contribute significantly to the rarity of frugivorous mammals, such as brown lemurs.
Despite the overall rarity of Ficus and its presumed effects on frugivores, there may be considerable variation in this pattern within the southeastern rainforest region. At Andringitra, hybrid groups exhibited a striking dietary shift to Ficus fruit (< 79% of feeding time), especially in July-September (Johnson, 2002; Dalecky et al., 2003). This period coincides with cold winter months and typically the end of a long period of reduced fruit availability, although eastern rainforests demonstrate great variability in the timing and magnitude of resource production (Hemingway and Overdorff, 1999; Wright, 1999). The apparent abundance of figs in this area may help sustain the very dense brown lemur population during seasonal resource crashes (Johnson, 2002; Irwin et al., 2005). In contrast, Overdorff (1993) found no such seasonal switch to Ficus in the diet of E. f. rufus at Ranomafana. The seasonal migrations (likely to areas of cultivated or invasive Psidium) (Overdorff, 1991, 1993) suggest that Ficus cannot sustain the brown lemurs during peak scarcity at Ranomafana. Also unlike in the Andringitra hybrid zone, the relatively low-density E. albocollaris populations at Vevembe appeared to rely on Pandanus flowers as keystone resources (Johnson, 2002).
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