The growing body of research on brown lemurs summarized here demonstrates the striking diversity of this group. Recent studies of cytogenetics and molecular genetics have addressed this diversity, elevating the two southeastern taxa (E. collaris and E. albocollaris) to distinct species (e.g., Djlelati et al., 1997; Wyner et al., 1999; but see Pastorini et al., 2000). The validity of the traditionally recognized subspecies of
E. fulvus may also be in question, as molecular research has identified distinct clades within both E. f. fulvus and E. f. rufus (Pastorini et al., 2000). Studies of parapatric southeastern populations suggest that boundaries may be stable and the presence of hybrid zones does not necessarily indicate substantial gene flow between parent taxa, evidenced by the novel genetic variants found in the Andringitra contact zone (Wyner et al., 2002). Morphological variation in this species complex includes eco-geographic variation in body size (Albrecht et al., 1990), as well as population differences in levels and direction of sexual dimorphism (Kappeler, 1996; Gerson, 2000; Johnson et al., 2005).
Ecological and behavioral differences are present as well. There is clear variation across populations in feeding behavior (from folivory to near exclusive fru-givory), dietary diversity, and diet switching during resource scarcity (e.g., Sussman, 1974; Overdorff, 1993; Vasey, 1997). Brown lemurs also vary in other niche dimensions, including activity rhythms and ranging. These differences may in some cases be related to particular habitat characteristics, including community structure and seasonal and year-round variation in resource availability. While social behavior in many brown lemur taxa remains poorly understood, distinct patterns of social organization are evident. Group composition is similar across populations, with an even sex ratio — a highly unusual pattern for multimale/ multifemale groups among primates (Kappeler, 2000; Ostner and Kappeler, 2004). In contrast, group cohesion differs among brown lemurs. In most populations, groups are stable and cohesive but fission-fusion group structure is found in E. albocollaris and, at least at some times and localities, in E. f. fulvus and E. f. rufus (Tattersall, 1977; Overdorff et al., 1999; Johnson, 2002).
Such variation in behavior and ecology may reflect a high degree of adaptability or, alternatively, localized evolution of individual brown lemur populations or taxa. The relative success of this group in occupying Madagascar's remaining natural habitats, coupled with the apparent evolutionary divergence of some populations, suggests that this complex represents an ongoing adaptive radiation. Nonetheless, several distinct populations face imminent risk of extinction. Conservation concerns for brown lemurs are also elevated because, as prominent frugivores in a broad range of habitats, they may serve critical roles in the maintenance and regeneration of ecosystems across Madagascar (e.g., Bollen et al., 2004). Finally, the ongoing behavioral, ecological, and genetic divergence within this complex represents a dynamic evolutionary process. Thus, preserving brown lemur populations and habitats safeguards both present and future biodiversity in Madagascar.
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