The family Indriidae contains three genera: Avahi, Propithecus, and Indri. The first one is small (roughly 1 kg) and nocturnal, the latter two are large (3–9 kg) and diurnal, and include the largest of all living lemurs, the indri (Indri indri) and the diademed sifaka (Propithecus diadema). All three genera are “vertical clingers and leapers,” an unusual postural and locomotor complex among primates. This is characterized by resting postures in which the trunk of the body is held vertically on upright supports and locomotor behavior that involves leaps between vertical supports, sometimes quite spectacular and up to 10 m between trees. Suspensory postures are also a feature of this behavioral complex, especially when feeding and moving in peripheral branches and twigs of trees. Some species also regularly descend to the ground, where they jump bipedally, often with the arms held above the head. This amazing behavior, especially among the smaller sifakas (e.g., Propithecus verreauxi, Propithecus coquereli), has now become quite well known internationally, and is one of the highlights of a visit to Madagascar. All indriids have greatly elongated legs compared to their arms and trunks, and they are highly distinctive and difficult to confuse with other species. Indeed, they are among the most unusual of all primate families. The only real possibility of confusing any indriid with a member of another lemur family is at night, when Avahi might be mistaken for Lepilemur or possibly Cheirogaleus.
Avahi Jourdan, 1834 Woolly Lemurs
The genus Avahi contains the only nocturnal members of the family Indriidae. The common English name refers to the curly or woolly appearance of the dense fur, while the generic name is a transcription of the animal’s high-pitched vocalization (Thalmann, 2003). Woolly lemurs are much smaller than the indri or any of the sifakas, usually weighing around 1 kg. Nonetheless, they are typical, thigh-powered vertical clingers and leapers, possessing hindlimbs proportionately longer than the trunk and forelimbs. Like the sifakas they have long tails, typically longer than their combined head and body length.
At a distance and at night, Avahi is often difficult to distinguish from species of Lepilemur, which are similar in size and also nocturnal. At reasonably close range, however, woolly lemurs are readily identified by the white patches on the backs of their thighs, which serve as highly visible identification marks when the animals assume their characteristic vertical resting posture. Indeed, this character is so distinctive that it has given rise to one of the most common names for woolly lemurs in Madagascar, “fotsife,” which means “white leg.” At first glance, Avahi may also be confused with Cheirogaleus, especially the larger eastern rain forest species. However, dwarf lemurs do not exhibit the vertical resting posture and are much smaller. Woolly lemurs also frequently rest closely huddled together in pairs or small family units of three or four, behavior rarely observed in the other similar-sized nocturnal genera.
Avahi was originally regarded as a single species, Avahi laniger, with an eastern and a western subspecies (Schwarz, 1931; Tattersall, 1982). Rumpler et al. (1990) subsequently elevated the subspecies to full species: Avahi laniger and Avahi occidentalis. Thalmann
and Geissmann (2000) later described a third species, Avahi unicolor, from the region of the Ampasindava Peninsula, and more recently a fourth, Avahi cleesei, from the Tsingy de Bemaraha region (Thalmann and Geissmann, 2005). These were followed by two more in 2006, when Zaramody et al. described Avahi peyrierasi and Avahi meridionalis, at the same time suggesting that A. cleesei may be a junior synonym of A. occidentalis. The validity of A. cleesei was subsequently reaffirmed by Andriantompohavana et al. (2007). Although A. meridionalis was originally described with two subspecies—A. m. meridionalis and A. m. ramanantsoavanai–Andriantompohavana et al. (2007) elevated both to full species. In 2007, Andriantompohavana et al. also described the new species Avahi betsileo. Lastly, Lei et al. (2008) described yet another, Avahi mooreorum, bringing the current number of described Avahi species to nine. However, there are strong indications that still more species of Avahi remain to be described, so we are not yet finished with this genus.