Aye-ayes are the largest nocturnal primate. Their pelage consisted of two layers: the short, soft underlayer is light in color and thick on the back; the outer guard hairs are coarse, dark brown to black at the roots, and gray-white at the tips similar to a didelphid North American possum (Didelphis vir-giniana). Some aye-aye guard hairs have measured 7 in (18 cm). Fur above the eyes and on the throat is often light yellow or beige. Eyes are amber and are not frontally oriented.
The body mass of males and females is not significantly different, with an average of 6 lb (2.7 kg). Body length averages at 16 in (40 cm), tail length at 22 in (55 cm). The tail is very bushy, more like a fox, than a primate. The ears are bare, flexible, and very large: 4 in (10 cm) length by 2.8 in (7 cm) width, probably the largest for the body size of any primate. The evergrowing incisors have enamel on the buccal side only, with a long dental gap before the molars, like a rodent, not a primate. The molars are flat and wear down quickly. Legs and arms are about the same length and aye-ayes walk on all fours. The third and fourth fingers of aye-ayes are elongated and the last knuckle on the middle digit has a ball and socket joint, allowing rotation. The female mammary glands are located iguanally, between her legs, not under her arms as seen in other primates. Aye-ayes have a nictitating membrane, a character shared with reptiles and birds but few other mammals. The male aye-aye has a penis bone that is 1.2 in (3 cm). Aye-ayes have the largest brain to body weight ratio of any prosimian. The isolated incisors and the arrangement of hand bones for percussive foraging suggest the robust extinct aye-aye (D. robusta) had a similar locomotion and lifestyle to the living smaller species.
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