Madagascar is a refugium insomuch that many conservative eutherians that colonized the island have since undergone spectacular radiations (Eisenberg, 1981; Yoder, 2003; Yoder and Flynn, 2003). Indri indri may well be the largest folivore on the island and in this chapter we have detailed numerous adaptations that reveal the ecological specificity of this remarkable indriid. One of the major selection pressures acting upon Indri is its preference for a food source that likely yields less energy when compared to a diet of fruit. This may predispose Indri to a lifestyle that can be described as an "energetic minimizer."
Indri does not actively patrol its territorial boundaries, nor does it scent mark judiciously from its glandular anogenital region but achieves territorial defense (and individual recognition) through long calling. The rate of anogenital scent marking by P. diadema spiked during the breeding season at Mantadia, and remarkably Indri showed this same spike but in the number of long calls given (Powzyk, 1997). In contrast to other lemur species, Indri lack scent glands on the throat, chest, head, arms, and wrist, with glandular fields in evidence around the anus region (Petter et al., 1977; Pollock, 1975a,b; Powzyk, 1997). Indri does appear to cheek mark with salvia throughout their territory (Powzyk, 1997), a marking system that can also be found in Microcebus with its unspecialized salivary glands (Glatston, 1983). Although anogenital scent marks by Indri were observed, they were significantly lower in frequency than for the sympatric P. diadema (Powzyk, 1997).
Indri appears to rely more on the auditory (long calling) rather than the more lemur-typical olfactory (scent gland marking) form of communication. The two types of signals can achieve similar results, yet vocalization may be far less costly (energetically) for a 6.5-kg animal that must travel via vertical clinging-leaping throughout a relatively large (34-40 ha) territory. In studies of bird vocalizations, the cost of singing was found to be relatively "cheap" when compared to most other activities, especially patrolling territorial boundaries (Gaunt, 1987). Remarkably, when the olfactory region of the Indri brain was examined, researchers (Stephens and Andy, 1969, 1970; Stephens et al., 1982) found that Indri showed the highest reduction of all 20 prosimians tested. Therefore, Indri has less olfactory tissue with which to interpret scent marks when compared to other lemur species, further evidence that it may be emphasizing a territorial defense that is less costly energetically when compared to the territorial behaviors of P. diadema.
Many of Indri's behaviors relate to its dependence on immature leaves. Although these food items are ubiquitous within the Mantadia study site (Powzyk, 1997), they may levy a cost in that it predisposes an animal to limited movement and protracted bouts of resting which in turn enhance fermentation efficiency.
Overall, Indri indri has a body type well suited to life in the trees. It is an exceptional leaper with the ability to move through and search for palatable food in its forested home. Coupled with these external morphological features, Indri has internal specializations that allow it to efficiently convert its fiber-rich diet into assimilable energy. In this chapter we have detailed Indri's feeding behaviors and dietary preferences, all of which have had major repercussions on other aspects of its behavioral repertoire.
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