Strepsirrhini
Behavior

Approximately three-quarters of all extant strepsirrhine species are nocturnal, sleeping in nests made from dead leaves or tree hollows during the day. All of the lorisoids from continental Africa and Asia are nocturnal, a circumstance that minimizes their competition with the simian primates of the region, which are diurnal. The lemurs of Madagascar, living in the absence of simians, are more variable in their activity cycles. The aye-aye, mouse lemurs, woolly lemurs, and sportive lemurs are nocturnal, while ring-tailed lemurs and most of their kin, sifakas, and indri are diurnal. Yet some or all of the brown lemurs (Eulemur) are cathemeral, which means that they may be active during the day or night, depending on factors such as temperature and predation. Many extant strepsirrhines are well adapted for nocturnal activity due to their relatively large eyes; large, movable ears; sensitive tactile hairs; strong sense of smell; and the tapetum lucidum behind the retina. Among the adapiforms, most are considered diurnal, with the exception of Pronycticebus and Godinotia from Middle Eocene Europe, both of which had large orbits that suggest nocturnality.

Reproduction in most strepsirrhine species tends to be seasonal, particularly in lemurs. Key factors that affect seasonal reproduction include the length of the wet season, subsequent food availability, and the maturation time of the species. Like other primates, strepsirrhines are relatively slow breeders compared to other mammals. Their gestation period and interbirth intervals are usually long, and the young develop slowly, just like in haplorhine primates. Unlike simians, some strepsirrhines produce two or three offspring, although some produce only a single offspring. Those that produce multiple offspring tend to build nests for their young. These two traits are thought to be plesiomorphic (ancestral) for primates. The young are precocial (relatively mature and mobile) at birth, but not as coordinated as ungulates (hoofed mammals). Infant care by the mother is relatively prolonged compared to many other mammals, and in some cases, the infants cling to the mother's fur with their hands and feet.

Despite their relatively smaller brains compared to other primates, lemurs have demonstrated levels of technical intelligence in problem solving that are comparable to those seen in simians. However, their social intelligence differs, often emphasizing within-group competition over cooperation, which may be due to adaptations for their unpredictable environment. Although lemurs have not been observed using objects as tools in the wild, they can be trained to use objects as tools in captivity and demonstrate a basic understanding about the functional properties of the objects they are using.
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