Indri indri is a large-bodied lemur with numerous adaptations to facilitate its ecological niche of folivory within the rainforests of eastern Madagascar. More precisely, Indri is well adapted to life in the trees: its limb morphology allows for a highly arboreal lifestyle, while digestive specializations permit a diet that is almost extensively folivorous with limited frugivory. An adult male and female live together as a monogamous pair and quickly identify their presence in a forest with a deafening duet of long calling. This remarkable vocalization can last over 3 minutes and is typically introduced with a communal "roar" followed by a song proper of both ascending and descending notes, with a male and female timing their phrases to attain a stable dueting pattern (Powzyk and Thalmann, 2003). The long call is termed "contagious": as one group finishes their long call, a neighboring group commences and so the calling continues sequentially through the forest. Yet despite their loud vocalizations, Indri can be difficult to view since they rely heavily on crypsis. Crypsis works well to camouflage a lemur that spends long periods of the daylight hours resting on tree branches in the dappled light of a forest. This has earned Indri the reputation of "ghost of the forest," often heard
Joyce A. Powzyk • Department of Biology, Wesleyan University, Middletown, CT 06459 Christopher B. Mowry • Department of Biology, Berry College, Mt. Berry, GA 30149
but not seen. Yet, once sighted, Indri is easily identified with its distinctive black, gray, and creamy white pelage, dark, cublike ears (often tufted), and a penetrating gaze from its light yellow irises. In addition, Indri's near lack of a tail (tail length just 5-7 cm) sets it apart from all other lemur species (Glander and Powzyk, 1995). Adult pelage coloration can vary; some individuals are almost entirely black, while others have extensive amounts of white on their arms, legs, lower back (inverted V in pygal region), top and back of the head and on their facial disk. Born nearly all black except for a light pygal patch, an Indri typically gains more white in its overall coloration as it matures.
Indri indri is able to move through the trees with alacrity as its long arms and even longer legs assist them in what is termed "ricochetal leaping" (Oxnard et al., 1990), a form of locomotion that also comes under the description of "vertical clinging and leaping" (Napier and Walker, 1967). Their body is held in a near vertical position during the leap with the arms extended where the lateral skin serves as a sort of "flying membrane" (Oxnard et al., 1990). In order to grasp their substrate, both hands and feet show extensive elongation in their digits (i.e., metatarsal and phalange bones) (Tattersall, 1982), and are similar to gibbons in having a deep cleft separating the thumb from the other digits, an adaptation that may facilitate climbing large vertical supports (Cartmill, 1974) with a "vicelike grip." The head is balanced over the neck vertebrae in a position that facilitates the head having a wide range of movement, especially when scanning the forest in a vertical cling position (Napier and Napier, 1985; Rigamonti et al., 2005). All of these adaptations make Indri one of the most arboreal of lemurs with a heightened avoidance of the ground. Even when descending a tree for geophagy, Indri often maintains a vertical cling position while straining its neck to consume soil from an exposed bank (Powzyk, 1997).
Although Indri indri has long been heralded to be the largest extant lemur (Petter and Peyrieras, 1974), reported body weights of 12.5 kg were based on visual estimations (Pollock, 1977). We previously described the sympatric Indri indri and Propithecus diadema as the two largest extant lemurs on Madagascar with mean body weights of 6.48 kg (n = 4) and 6.50 kg (n = 11), respectively (Powzyk and Mowry, 2003; Powzyk and Thalmann, 2003). When sexes were compared, Indri females averaged 7.14 kg (n = 2) while males averaged 5.83 kg (n = 2) (Glander and Powzyk, 1995). Britt et al. (2002) weighed eight adult Indri (four females, four males) and recorded a mean body weight of 6.93 kg, although females averaged 1.1 kg heavier than the males (overall range: 6.1-8.8 kg). Therefore, Indri may well be the largest extant lemur, but additional body weights of both Indri and P. diadema need to be collected from the field to substantiate this claim.
Indri indri has been studied extensively at three sites. One site is located in the Eastern Domain, within the Betampona Nature Reserve (low-altitude secondary humid forest), while the Central Domain has two Indri sites, Mantadia National Park (midaltitude dense rainforest) and Analamazaotra Special Reserve (low-altitude secondary humid forest) (Mittermeier et al., 1994). Only the Mantadia study site, which was approximately 100 ha in size and located in the northwest sector of the park, had been undisturbed by human activities such as trail blazing, logging,
Figure 1. Current range of Indri indri in Madagascar indicated by crosshatching. Locations of Betampona, Mantadia, and Analamazaotra study sites are indicated by circles. (Redrawn from Lemurs of Madagascar by Mittermeier et al., 1994.)
and/or rice cultivation. Current population numbers of Indri are unknown but Figure 1 indicates Indri's distribution throughout Madagascar and the location of major study sites, while Table 1 provides a summation of important parameters on data collection and physical makeup of the Indri sites.
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