in the same tree or in nearby trees and are shared by neighboring giant mouse lemurs, fork-marked lemurs, and the introduced black rat (Rattus rattus). Unlike most other nocturnal lemurs, giant mouse lemurs do not appear to sleep in tree holes. Social structure Both species usually are solitary foragers, although the northern giant mouse lemur tends to be the most social, possibly due to its higher population density. Up to eight (typically four) adult males, adult females, and juveniles may be found in a northern giant mouse lemur nest, whereas Coquerel's giant mouse lemurs do not nest communally, except when females share their nest with their offspring. Males do groom and call to females when they come into contact, and according to radio-tracking and direct observations at Analabe near Kirindy, they form pair bonds, sometimes briefly traveling together during the dry season. However, most interactions between adults are infrequent and typically occur later at night and particularly during the dry season in overlapping core areas, often involving chases and other agonistic behavior, and only rarely social grooming. During the mating season, males act aggressively towards one another, pulling out fur around the head and shoulders and biting the head. Giant mouse lemurs use at least eight vocalizations, the most common of which are contact calls, which sound like "hum" or a "hein" and are used when moving and when meeting familiar individuals. A "mother-infant meeting call" used at dawn before returning to the nest consists of short, modulated whistles. Both males and females have distinct single note calls used in territorial behavior; the female call sounds like "pfiou" and the male call is a short, loud whistle. Both sexes use an alarm call, which sounds like a "croak", and an agonistic call, which consists of repeated "tisk-tisk-tisk" sounds. Females exhibit a "waking call sequence", sometimes referred to as "loud calls", which start when foraging commences and then switch to quieter "hon" calls possibly to indicate their position to their neighbors. A long "sexual call sequence" consisting of soft whistle and several modulated, hoarse "brroak" calls is used by both sexes during estrus. Studies of captive individuals have found other vocalizations, but their purpose has not been determined. The northern giant mouse lemur appears to be the most vocal of the two species. Although vocalizations are the primary form of social communication, they also scent mark using saliva, urine, and secretions