The indri has been studied in the forests of Analamazaotra (Pollock, 1975a, 1975b, 1977, 1979a, 1979b, 1986a) and in the nearby Mantadia National Park (Powzyk, 1996, 1997; Powzyk and Mowry, 2003). In this region it lives in groups of 2–6 individuals, normally consisting of a monogamous adult pair, although changes in partners have been observed, and changes in group composition are quite frequent (V. Sorrentino, pers. obs.). Although groups in fragmented habitat have been reported to be larger than those in more extensive, undisturbed areas (Pollock, 1979a, 1979b; Powzyk, 1997), this is not always the case (V. Sorrentino, pers. comm.) The female appears to be the dominant member of the pair, normally feeding higher in the trees than the male, and having priority access to food sources. The diet consists primarily of immature leaves supplemented by flowers, fruit, seeds and bark, which vary in proportion according to season. Every day the indri descends to the ground to eat soil, which may help it detoxify seeds that it has eaten (Powzyk, 1997; Britt et al., 2002; Powzyk and Thalmann, 2003). Although this behavior was once rarely seen by the tourist groups at Analamazaotra, it has become increasingly common in recent years as the animals have become fully habituated to the daily tours (R. A. Mittermeier, pers. obs). Individuals at Analamazaotra and Anjozorobe also sometimes travel bipedally on the ground in the same way that Verreaux’s sifakas (Propithecus verreauxi) do at Berenty (F. Hawkins, R. A. Mittermeier, pers. obs.). Home ranges average 18 ha in the fragmented forests of Analamazaotra, but can be as large as 40 ha in the more pristine forests of Mantadia, where day ranges of 300 to 800 m are common. Before dusk the group retires to a sleeping tree, bedding down at heights of 10–30 m, the female typically with her offspring and separated from the male by 2–50 m.
Spacing between groups may be conditioned by the famous loud morning calls, which are answered from as far as 3 km away. Indris call most during the austral summer, typically between 06:00 and 13:00 hours (and sometimes as early as 04:45 in the warmest months, R. A. Mittermeier, pers. obs.), with most activity occurring between 7:00 and 11:00 hours, and occasional bouts between 14:30 and 16:30 (Powzyk, 1997; Garbutt, 2007), but vocal activity sometimes lasts throughout the night (F. Hawkins, pers. obs.). These calling bouts appear to be effective in maintaining territorial spacing, and may help explain the relatively small degree of range overlap between neighboring groups. Moreover, acoustic-structural differences in the song of males and females can convey sex information that is potentially meaningful to the other group members or to other groups (Rendall et al. 2004; Norcross et al., 1999; Giacoma et al., 2010).
Females give birth every two to three years. Reproduction is highly seasonal, with the birth of a single offspring in May or June after a gestation period of 120–150 days. Infants ride on the mother’s ventrum up to the age of four to five months, then transfer to the back. By eight months they are moving independently, but they stay close to the mother until well into their second year (Pollock, 1975a, 1975b). Reproductive maturity is reached between seven and nine years of age (Pollock, 1977a, 1977b). This slow reproductive rate means that indris, in theory, have more difficulty bouncing back from hunting pressure than smaller, more rapidly maturing species, and is a critical factor to take into account in designing conservation programs on their behalf.