The endangered lemur that is the world’s largest nocturnal primate is the aye-aye.
The aye-aye, Daubentonia madagascariensis, is a distinctive lemur native to Madagascar. Its rodent-like incisors grow continuously, and its unusually thin middle finger helps it extract grubs from wood. It locates hidden prey by tapping on branches, listening for hollow spaces, gnawing an opening, and pulling out larvae.
Aye-ayes are arboreal and largely nocturnal. They spend the day sleeping in leafy nests and emerge after dark to forage, travel, and interact. Their large ears, sensitive whiskers, and strong sense of smell are useful adaptations for life in darkness.
The answer is not indri or sifaka, which are generally active during the day, nor the diurnal ring-tailed lemur. “Largest nocturnal primate” refers to body size among primates active at night; it does not mean the largest lemur overall. Habitat loss and persecution have contributed to the aye-aye’s endangered status.