Early field studies noted the variability in sifaka group composition (e.g., Petter et al., 1977). However, they suggested that ". . . the ancestral group structure of Propithecus is monogamous, and that a normal group was composed of a pair of adults and two to three offspring of different ages" (p. 379), proposing that the larger observed group sizes may be due to habitat disturbance (possibly representing aggregations of multiple family groups). However, field studies have confirmed for eastern sifakas (as did Jolly, 1966, and Richard, 1978, for western sifakas) that monogamous groups are not the rule.
P. edwardsi at Talatakely (Ranomafana) live in groups of three to nine individuals, with a mean of 4.61 (Wright, 1995; Pochron et al., 2004), while those at the Vatoharanana trail system live in groups of 2-8, with a mean of 4.3 (Hemingway, 1995). Groups of 3-6 individuals (mean = 4.8) were observed for P. diadema at Mantadia (Powzyk, 1997), while P. diadema at Tsinjoarivo have been observed in groups of 4-7 (mean = 4.9; Irwin, 2006, unpublished data). P. tattersalli have slightly larger group sizes (3-10; Meyers, 1993), similar to those of western sifakas (Richard, 1978). Such intermediate group sizes open the door for several group types. Assuming that stable groups have at least one breeding male and female, four distinct types are possible: polygynous, polyandrous, polygynan-drous, and monogamous pairs. Such variability is less likely in larger groups (usually polygynandrous) or smaller groups (usually monogamous).
Indeed, Pochron and Wright (2003), using data from 46 group-years for P. edwardsi at Talatakely (Ranomafana), found an average of 3.2 adults per group and a surprisingly even distribution of the four possible group types. Since the competitive regime would be expected to vary greatly in different group types, these differences may have profound effects on other aspects of social life. However, Pochron and Wright (2003) found no effect of group type on infant birth rate and survival. They argue that feeding competition limits group size, causing small, nonuniform social groups, but mating may occur more freely across group boundaries (as in P. verreauxi: Richard, 1985). However, mating season influxes have not yet been observed in eastern sifakas to the same degree known
1Totals include infants and only data from December or January were used whenever possible.
in P. verreauxi, and preliminary genetic data provide no evidence for extragroup paternity (Morelli and Wright, in preparation).
The dataset of Pochron and Wright is by far the largest for eastern sifakas, but as it derives from a disturbed (selectively logged) site, one must consider the possibility it does not represent the "natural" state. However, data from all other behavioral studies conducted in pristine forests (Meyers, 1993; Hemingway, 1995; Powzyk, 1997; Irwin, 2006) show similar groups sizes and variable composition. It therefore appears that the variable social structure described by Pochron and Wright is typical of eastern sifakas, though further study is necessary to better understand the causes and consequences of this variability.
As with social organization, patterns of natal dispersal do not follow any hard-and-fast rules. In most primates, one of the sexes tends to be philopatric (Pusey and Packer, 1987); only in a few species do both sexes commonly disperse. Based on available evidence, eastern sifakas seem to rank among those rare species having no sex bias in dispersal. In P. edwardsi, roughly half of males and females disperse, usually at 4-5 years of age (though females may travel greater distances); this dispersal is usually "motivated" by targeted aggression from adults (Wright, 1995; Pochron et al., 2004). Other individuals of both sexes remain, and reproduce, in their natal group.
Pochron et al. (2004) suggest that this opportunism may be due to slow reproduction and high infant mortality. The combination of small groups and slow reproduction means that a given sifaka generally cannot have enough same-sex kin to form the social networks seen in other male- or female-bonded primates. Therefore, animals may be equally willing to stay in their natal group should breeding opportunities become available, or disperse to find breeding opportunities elsewhere. This opportunism may also apply throughout adult life for males; secondary dispersal has been observed among males, but not females (Pochron et al., 2004).
Behavioral studies at other sites have not lasted long enough to provide definitive confirmation of this pattern; the natural rarity of dispersals means that only longer-term studies (i.e., >5 years) can provide a balanced view of dispersal. However, it is worth noting that among P. diadema at Tsinjoarivo, the three observed dispersal events have involved two females and one male (Irwin, 2006; Irwin and Raharison, unpublished data).
Sifaka groups tend to have relatively stable dominance relations among individuals (e.g., Meyers, 1993; Hemingway, 1995), but the patterns of dominance vary from group to group. Intersexual relations are difficult to quantify for two reasons: first, aggression rates are extremely low (with a high percentage of undecided encounters), and second, the variable social structure may lead to different social environments and therefore different dominance relationships (Overdorff and Erhart, 2005). Available evidence from various sites is, however, consistent with the definitions of female feeding priority and true female dominance (Pochron et al., 2003), but only under certain circumstances; the situation is still less clear-cut than for some lemurs (e.g., Lemur catta). For example, in groups with multiple adult females, the dominant female appears to be consistently dominant over males, but the same is not always true for subordinate females (e.g., Hemingway, 1995).
Patterns of association (as measured by proximity) among males and females are also variable from group to group, and therefore difficult to categorize (Meyers, 1993; Hemingway, 1995). This aspect of group life may also be strongly influenced by the variation in group composition and relatedness of same-sex animals.
Infanticide has been observed in P. edwardsi (Wright, 1995; Erhart and Overdorff, 1998), always perpetrated by newly immigrant males. Although infanticide would seem less likely to be adaptive among seasonal breeders, the life history of sifakas (see below) is such that early loss of an infant could increase the chances of conception in the following breeding season.
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