Social Structure And Organization

Brown lemurs live in multimale/multifemale social systems with group size ranging from 4 to 17 individuals (Sussman, 1974; Harrington, 1975; Overdorff, 1993; Freed, 1996; Vasey, 1997; Johnson, 2002). Long-term demographic studies in the east (Overdorff et al., 1999) and genetic analyses in the west (Wimmer and Kappeler, 2002) indicate that E. f. rufus is primarily characterized by female philopatry and male dispersal, although females may also migrate from natal groups (Ostner and Kappeler, 2004). Preliminary data suggest a similar pattern in E. f. albifrons (Vasey, 1997) but conclusive information is unavailable for other brown lemur populations.

Equal sex ratios — or perhaps a slight bias in the number of males — appear to be the norm across the brown lemur complex (Overdorff et al., 1999; Kappeler, 2000; Johnson, 2002; Ostner, 2002). Such a proportionally high number of males is unusual among polygynous primates and may be maintained by equal birth and mortality rates for males and females (Overdorff et al., 1999; Kappeler, 2000). Moreover, estrus synchrony may limit male potential for monopolization of females, reducing incentives for males to exclude others from the group (Ostner and Kappeler, 2004). The ultimate causes for even or male-biased sex ratios remain obscure. Overdorff et al. (1999) suggest that increased numbers of males may result from the relatively greater energetic demands of females. Males may also perform some group services such as increased vigilance or aid in intergroup conflicts (Overdorff et al., 1999). Other potential services include support for females in intragroup agonistic conflicts and protection against infanticide, but there is as yet little empirical support for these potential functions (Kappeler, 2000). Indeed, no cases of infanticide have been recorded in brown lemurs. Male strategies may also account for brown lemur sex ratios, as the increased number of males in social groups may be due to the benefits of social and/or mating cooperation among males or joint transfer (Kappeler, 2000, Ostner and Kappler, 2004).

The composition of multimale/multifemale brown lemur groups may also reflect a recent evolutionary development: a transitional stage from nocturnality to diurnal activity (the evolutionary disequilibrium hypothesis) (van Schaik and Kappeler, 1996). In this scenario, the larger, equal sex-ratio social groups are fusions of multiple pair bonds (Kappeler, 2000). Strong bonds between individual males and females within social groups also have been suggested to serve as a mechanism for the prevention of infanticide (van Schaik and Kappeler, 1993). However, in E. f. rufus at Ranomafana, Overdorff (1998) found male-female dyads did not consistently maintain greater proximity during critical reproductive seasons (mating and birth) and mating was not exclusive to the dyad (Overdorff, 1998). In addition, subgroups did not exclusively consist of adult male-adult female pairings (Overdorff, 1998), which was also the case in fission-fusion E. albocollaris (Johnson, 2002; see below). Ostner and Kappeler (1999) also found no evidence for strong affiliative or mating dyads within E. f. rufus social groups at Kirindy. Thus, pair bonding components of the evolutionary disequilibrium and infanticide prevention hypotheses currently lack support from field studies of brown lemurs.

While the composition of brown lemur social groups appears to be similar across populations, the cohesiveness of these units may vary. Typically, brown lemurs are found in stable groups that maintain close spatial proximity (Vasey, 1997; Overdorff and Johnson, 2003). However, E. f. fulvus on Mayotte and E. albocollaris maintain loose, fission-fusion communities (Tattersall, 1977; Johnson, 2002). In addition, after a 10-year study period wherein social groups were cohesive, E. f. rufus groups at Ranomafana recently underwent a complete transition to fission-fusion community structure (D. Overdorff, personal communication). Such fluid group structure has previously been noted only during permanent group division events (Overdorff et al., 1999). This variable expression across populations suggests that flexible grouping patterns are latent in the brown lemur complex. Fission-fusion may be a mechanism for reducing intragroup feeding competition in response to acute resource scarcity in some southeastern brown lemur populations (Johnson, 2002; D. Overdorff, personal communication).

Social dynamics within brown lemur groups may also be fluid. Fixed dominance hierarchies are not present in either sex (Pereira and Kappeler, 1997). Notably, females are not dominant to males in aggressive contexts, as is the case in most other lemurs (Pereira et al., 1990). Wild populations of brown lemurs have been characterized by low rates of agonistic behavior (Kaufman, 1996). Even in captive groups where aggression may be relatively common, brown lemurs lack consistent submissive signals and asymmetry in aggressive relationships between dyads is rare (Pereira and Kappeler, 1997). Pereira and McGlynn (1997) suggest that pair bonding within brown lemur social groups may preclude female dominance in these taxa. Females may accrue benefits such as increased foraging efficiency from coalitions with individual males, thus reducing the impetus to dominate all males, while males may pursue a strategy of special relationships with individual females for increasing mating opportunities (due in part at least to concealed ovulation in these females) (Pereira and McGlynn, 1997). However, the apparently more complex nature of dyadic relationships — or sometimes lack of such special relationships — in wild brown lemurs (Overdorff, 1998; Ostner and Kappeler, 1999) casts some doubt on the importance of male-female pair bonds in determining group social dynamics. Alternatively, consistent dominance relationships may be affected by frequent nocturnal activity, during which social interactions may be constrained (Rasmussen, 1999). However, there are indications that strong male-male competition may persist in this context. For example, recent studies of E. f. rufus at Kirindy have demonstrated that a single male may monopolize social interactions and dominate in agonistic encounters within the group - and apparently achieve greater reproductive success (Ostner and Kappeler, 1999; Wimmer and Kappeler, 2002).

Interactions among social groups vary distinctly across brown lemur populations. Brown lemurs are generally described as nonterritorial, with extensive overlap in home ranges (Sussman, 1974; Harrington, 1975; Overdorff, 1991; Vasey, 1997; Gerson, 2000). However, the nature of intergroup encounters can range from passive to highly aggressive. For example, in E. albocollaris at Vevembe and in the Andringitra hybrids, the majority of intergroup encounters were neutral, with little or no direct interaction between the groups (57-67%; Johnson, 2002), while in neighboring populations of E. f. rufus at Ranomafana, 65% of associations with con-specifics were hostile, with threat displays, chases, and/or direct fighting between groups (Overdorff, 1991, unpublished data). As most conflicts appear to involve food resources, the population differences imply different strategies based on the distribution of preferred food items.

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