Giant mouse lemurs were first studied in the wild by Petter and colleagues in 1971. His observations were secondary to his primary research interest, the fork-marked lemurs north of Morondava. Both northern and southern populations were studied intermittently between 1978 and 1981, and in 1993, long-term social and genetic studies began in Kirindy Forest. Behavioral studies of captive individuals have also been performed at the
Duke Lemur Center (DLC) in
Durham, North Carolina during the 1990s.
Population density and territory Before the recognition of more than one species, differences in
population density were noted between southern forests like Kirindy and northern forests near
Ambanja. Later, it was recognized that Coquerel's giant mouse lemur was found in lower densities than the northern giant mouse lemur. The former range between 30 and 210 individuals per square kilometer (250 acres), with lower densities in open areas of the forest, while the latter has been recorded with 385 to 1,086 individuals per km2. However, in the case of the northern giant mouse lemur, populations were found in more isolated forest fragments and it is thought that their consumption of
introduced cashew and
mango help sustain these higher populations. According to studies of Coquerel's giant mouse lemur,
home ranges of both sexes vary from with frequent overlap, particularly on the periphery of their range. Individuals most heavily use and aggressively defend a smaller core area within their range. Individuals can have up to eight neighbors. Home ranges of males tend to overlap with those of both females and other males, and typically expand to four times the size during the mating season. Female home ranges show no variability in size, and can remain stable for years. At Kirindy Forest, genetic studies showed that the home ranges of related females tend to clump closely together, while unrelated males may overlap their range, suggesting male
dispersal and migration is responsible for
gene flow.
Activity patterns Both species are strictly
nocturnal, leaving their nests around sunset to stretch and
self-groom for a few minutes. Both species typically forage between above the forest floor, though Coquerel's giant mouse lemur has been observed on the ground. They primarily move by
quadrupedal running and occasionally leaping between branches, and use the same feeding postures as mouse lemurs, such as clinging to tree trunks. When moving through the trees, giant mouse lemurs scurry rapidly like mouse lemurs, unlike dwarf lemurs, which use more deliberate movement. Slow movements are usually seen in lower, denser foliage when hunting for insects, while more rapid motion and leaping is typically seen at moderate heights of . Surveillance of the home range involves slower movements in lighter foliage near the tops of large trees, while movements along the border of a home range is more rapid and occurs at a lower height. Similar movement patterns have been observed in captivity as well. Giant mouse lemurs begin foraging moments before the sun disappears, occasionally participate in social activities during the last half of the night, and return to one of their nests prior to sunrise. Cold temperatures cause them to leave the nest later and return early, sometime during the second half of the night. During the first half of the night, giant mouse lemurs are more likely to rest for an hour or more, usually at the expense of social activities, but not feeding time. Rest periods are longer when temperatures are low. Unlike many other cheirogaleids, they remain active all year and do not enter daily or seasonal
torpor.
Nesting Both species sleep in round nests up to across made of interlaced
lianas, branches, leaves, and twigs gathered from nearby trees and woven using the mouth and hands. Nests are typically between above the ground in the fork of large tree branches or surrounded by dense lianas. Trees covered in thick lianas as well as trees with year-round leaf cover (e.g.
Euphorbiaceae) are favored for nest construction, though large bare trees may be used by building the nest higher. In addition to nesting in dense lianas, individual giant mouse lemurs will rotate between 10 and 12 nests every few days to avoid predators. Only females have been observed building nests in the wild, though males, females, and young have been observed building nests in captivity. Multiple nests are sometimes built 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 from the
anogenital scent gland on small branches and other objects.
Reproduction Reproduction starts in November for Coquerel's giant mouse lemur at Kirindy Forest; the estrous cycle runs approximately 22 days, while estrus lasts only a day or less. The mating season in this southern population is limited to a few weeks, whereas the northern giant mouse lemur is thought to breed throughout the year, a trend seen in only two other species of lemur: the
aye-aye (
Daubentonia madagascariensis) and the
red-bellied lemur (
Eulemur rubriventer). The northern giant mouse lemur had been observed breeding year-round in captivity if their litter did not survive or was removed, but at the time this population was thought to be Coquerel's giant mouse lemur. One to three offspring (typically two) are born after 90 days of
gestation, weighing approximately . Because they are poorly developed, they initially remain in their mother's nest for up to three weeks, being transported by mouth between nests. Once they have grown sufficiently, typically after three weeks, the mother will park her offspring in vegetation while she forages nearby. After a month, the young begin to participate in social play and grooming with their mother, and between the first and second month, young males begin to exhibit early sexual behaviors (including mounting, neck biting, and pelvic thrusting). By the third month, the young forage independently, though they maintain vocal contact with their mother and use a small part of her range. Females start reproducing after ten months, while males develop functional testicles by their second mating season. Testicle size in the northern giant mouse lemur does not appear to fluctuate by season, and is so large relative to the animal's body mass that it is the highest among all
primates. This emphasis on sperm production in males, as well as the use of
copulatory plugs, suggests a mating system best described as
polygynandrous where males use
scramble competition (roaming widely to find many females). In contrast, male Coquerel's giant mouse lemurs appear to fight for access to females (contest competition) during their breeding season. Males disperse from their natal range, and the age at which they leave varies from two years to several. Females reproduce every year, although postpartum estrus has been observed in captivity. In the wild, the lifespan of giant mouse lemurs is thought to rarely exceed five or six years, though in captivity they can live up to 15 years. ==Ecology==