The family Indriidae contains three different types of animals divided in three genera: Avahi, Propithecus and Indri. Within the Indriidae the largest and most beautiful lemurs are found. Some species are diurnal or crepuscular, others are nocturnal.
All indrids move around by jumps made with the hind legs by sudden movement and they appear to bounce from trunk to trunk keeping their bodies in upright position, comparable to Lepilemur. All indrids have a long, thin muscular tail, except Indri, which has only a rudimentary tail.
Cytogenetic research on the animals revealed the existence of numerous more developed chromosomes. On the other hand, in the karyotypes of the three genera are a large number of small mediocentric chromosomes, which can not be found in the karyotypes of other Malagasy lemurs and which indicate a certain degree of distinctness. Comparison between the chromosomal formulas of the three genera presents an evolutionary stratification of the karyotypes of which the most primitive formula belongs to Avahi laniger and the most evolved to Indri. Situated between those two extremes, Propithecus has formulas closer to Indri than to Avahi, certainly those belonging to P. diadema, which formula is more specialized than P. verreauxi.
The body size of the animals differs depending on the corresponding regions and study of the skulls reveal characters, which also have a similar differentiation.
The form of the lower jaw of the Indriidae is very different from other lemurs, especially the depth of the horizontal part, the large size of the angular apophysis, the orientation of the symphysis and the form of the condyle. As stated by Tattersall, 1974, the insertion zone of the anterior part of the digastric muscle is very large and progresses in size from Avahi to Propithecus to Indri.
Together with Daubentonia, the Indriidae differ from the other lemurs by the arrangement of the orbital bones (the frontal reaches the maxilla and the palate just reaches the lacrimal bone), but they resemble the Lemuridae by the arrangement of the carotid circulation.
The dental characters more clearly separate the Indrids from the other Prosimians. They have only four teeth in stead of six in the dental comb of the lower jaw (four incisors or two incisors and two canines) and only two upper premolars. With Avahi and Propithecus, P3 is longer than P4. These two premolars are of equal size with Indri. The family Indriidae has a dental formula of 2123/1123 with a total of 30 teeth.
The Indrids are also unique among the Prosimians by the possession of four deciduous lower premolars (Schwartz, 1974). Several other small dental details separates them from the other lemurs (Hill, 1953).
Moreover, the Indriidae are the only ones, which have the permanent upper premolars in a more proximate position in the mouth than the lower premolars (Tattersall and Schwartz, 1974)
The indrids possess elongated hind limbs, but do not have special development of the tarsus. The forelimbs are reduced and the hand is somewhat alternated, with a reduced thumb. The indriid lemurs have long, pentadactyl feet, with digits webbed basally. The hallux is large and the pollex is short.
The palmar and plantar pads are characteristic for this family. A similar evolutionary stratification as observed with the karyotypes can be found. Evolution manifests itself by a progressive fusion of pads. Avahi laniger has palms and soles with more isolated pads, whereas Indri indri has the most fused pads.