Because they are nocturnal and not widely distributed throughout their range, the social behavior of slow lorises has historically been difficult to study. Nonetheless, several studies have been conducted and are now invaluable sources of information.
Males and females generally forage separately, coming together only to breed. Females leave their young behind when they venture out in search of food. As such, the slow loris is considered to be a solitary species, although family groups have occasionally been found together.
Males are suspected to have a slightly larger home range than females, and are considerably more territorial. Lorises communicate with each other by urine-marking, a process in which they urinate on their hands and then wipe them on branches. (It is also thought that this makes their hands sticky and thus allows them to grasp better). Urine marks apparently convey important messages to others, as well as staking out each animal’s territory, so that individuals almost never cross paths.
Slow lorises can produce a variety of sounds, ranging from a low hiss or growl when disturbed, to a high whistle made by females in estrus. Animals may also scent mark themselves when under stress, using glands on the underside of their arms.