After consistently proving to be unique and enigmatic in all aspects of their ecology discussed thus far, it would seem unfitting if sifakas had an ordinary life history; indeed recent studies have proved that this is not the case. Like all extant lemurs, the reproductive schedule of sifakas is tightly constrained seasonally (most primate species reproduce year-round or show more moderate seasonality; e.g., Koenig et al., 1997). In P. edwardsi and P. diadema mating occurs in December and January, while birth occurs between May and July, with the majority in June (Wright, 1995; Pochron et al., 2004; Irwin, 2006). P. tattersalli breeds slightly later (Meyers, 1993), like western sifakas: mating in January-February, and births in late July.
Average interbirth interval (IBI) at Talatakely, Ranomafana, is 1.56 years (Pochron et al., 2004); in other words, 1 year for roughly half of births and 2 years for most other cases. This is in contrast to many smaller lemurs, which give birth every year. The roughly even split between 1- and 2-year IBIs suggests a possible pressure to speed up infant development (i.e., reduce time to weaning). Sifakas who are metabolically ready to conceive 12 months after a previous conception can do so, but those who become ready at 13 or 14 months must wait until the 24th month postbirth, due to the strict estrous seasonality. This delay might have led to selective pressures to reduce mothers' postbirth metabolic costs, thereby reducing "recovery time."
In terms of infant development, Godfrey et al. (2004) showed that indriids have a slow somatic growth rate relative to other lemurs. This is contrary to the expected pressures of seasonal reproduction, as well as the predictions of the risk aversion hypothesis of Janson and van Schaik (1993). This hypothesis suggests that more folivorous taxa should have rapid development, because the relative lack of food competition lessens the starvation risks associated with rapid growth. However, the slow body growth seen in Propithecus and other indriids is paired with an unusually fast rate of dental development (this family is unusual in the extent to which somatic and dental development rates are decoupled). Godfrey et al. (2004) suggest that accelerating the development of adult dentition at the expense of other body tissues may get infants to independence as soon as possible (the high-fiber diet of Propithecus requires more dental competence than the softer diet of frugivores). By achieving dental competence earlier than other similar-sized primates, sifakas shorten the dependency period and the mother's overall postbirth metabolic cost, thereby promoting her own survival. It has further been proposed (Wright, 1999) that reproduction is timed to place weaning at the season of peak food availability (March), further reducing the mother's costs at this time. This explanation for sifakas' unusual combination of life history traits fits nicely with what is known about Madagascar's impoverished environments, and with the "Energy Conservation Hypothesis" of Wright (1999).
However, while mothers may succeed at shortening their infants' march to independence, the infants themselves do not fare that well. Roughly half of infants die before 1 year of age at Talatakely, and only about one quarter of females reach reproductive age (Pochron et al., 2004). The only reason this population sustains itself seems to be a long reproductive life span (>20 years; Wright, 1995; Pochron et al., 2004). Adult females seem to follow the "bet-hedger" strategy of Richard et al. (2002), slowing down their reproductive output and reducing investment in individual offspring to aid their own long-term survival. This fits well with what is known about both the paucity and the unpredictability of Madagascar's environment; reducing investment makes reproduction possible in average years, and a long life span makes it possible to wait out bad years (Godfrey et al., 2004). However, the unfortunate combination of fast acquisition of adult dentition and a long life span bodes poorly for the state of teeth in elderly individuals. Indeed, observations at Ranomafana indicate that some older animals may suffer from extreme tooth wear, and an impaired ability to feed (King et al., 2005).
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