Silky Sifaka - Behavior

Behavior

Until the 21st century, brief observations and lemur surveys had merely documented the presence of the silky sifaka in special reserves and national parks. More recently, a 14-month study and two short-term studies in Marojejy National Park have revealed previously unknown details about its behavioral biology, communication, and feeding ecology. The silky sifaka has a variable social structure and is known to live in male-female pairs, one-male groups, and multi-male/multi-female groups. Group sizes range from two to nine individuals, while the home ranges are estimated to range from 34 to 47 ha (0.13 to 0.18 sq mi), varying in size by location.

According to some studies, the silky sifaka spends most of its day resting and feeding (approximately 44.4% and 25% respectively). It also devotes approximately 16.8% of the day to social behavior, such as personal grooming, social grooming (allogrooming), and play. The rest of the day is spent traveling and sleeping. Other sources report that the species divides half of the day between traveling and foraging, while the rest is spent resting. Daily foraging usually starts at dawn unless delayed by rain. Group movement is usually led by females, and groups usually travel 700 m (2,300 ft) per day, and may climb 500 m (1,600 ft) along vertical slopes. Although the species spends its time in the trees, terrestrial play is not uncommon, even among adults, with play sessions lasting 30 minutes or more. Like other sifakas, it uses a type of arboreal locomotion known as vertical clinging and leaping. Dominance hierarchies have not been documented among female sifakas, including the silky sifaka, although seasonal hierarchies are known to occur. Aggression, which is generally infrequent, occurs primarily during feeding, where females take priority over males, although submissive signals are not always obvious.

The silky sifaka's diet is similar to that of other eastern rainforest sifakas, consisting primarily of leaves (folivory) and seeds (seed predation). It is highly varied and includes many plant species. A two-month study from the mid-2000s showed that the silky sifaka can feed on as many as 76 species of plant from 42 families. Its favorites included primarily tree species, but also some lianas. The most prominent plant families in the diet were Moraceae (20.30%), Fabaceae (12.87%), Myrtaceae (12.65%), Clusiaceae (10.13%) and Apocynaceae (9.49%). In the study, feeding upon these four plant families took up as much as 37.06% of the total feeding time for the silky sifaka: 16.09% on the fruit of Pachytrophe dimepate, 8.43% on the seeds of Senna sp., 6.52% on the young leaves of Plectaneia thouarsii, and 6.02% on the fruit of Eugenia sp. In sum, folivory accounted for 52% of the feeding time, while frugivory (fruit-eating) accounted for 34%, and seed predation made up 11%. The consumption of flowers, as well as soil (geophagy), was rarely observed in this study. Prior to this, preliminary studies had reported that folivory account for 75% of the diet, while fruits and 15% was seed predation, 7% was flower consumption, and bark and soil made up the remainder.

Like all other lemurs, the silky sifaka is a seasonal breeder, and it is thought that it mates only one day a year during the start of the rainy season sometime in December or January. Infants are born six months later in June or July. Females typically give birth to a single infant once every two years, although births in consecutive years have been observed. Infants initially cling to the fur of their mother's chest for nearly four weeks and then switch to riding on her back. As with other eastern rainforest sifakas, the infants of this species develop rapidly. This may be due to assistance in the care of infants by all group members (known as alloparental care)—a trait typical among all sifakas. Non-maternal care usually takes the form of allogrooming, but also playing, occasional carrying, and in rare cases, nursing. The dispersal of offspring is thought to be similar to that of other eastern rainforest sifakas, with both males and females transferring out of the group at sexual maturity. However, dispersal has only been observed once with a young adult male, who left its natal group and proceeded to oust an older male from a group in which he had been a member for seven years. Female dispersal and group transfer has not yet been observed.

The only documented predator of the silky sifaka, other than humans, is the fossa, a cat-like carnivore found only on Madagascar. Although no aerial predators are known, the silky sifaka often watch the sky and emit loud "aerial disturbance" roars at the sight of the large Madagascar buzzard (Buteo brachypterus) and other small birds. Another more general alarm call is the loud, sneeze-like "zzuss" vocalization, which are emitted in response to terrestrial disturbances, calls from lost group members, and aggression by other group members. Acoustic analyses of the "zzuss" vocalization have shown that the call's acoustic structure differs between individuals and by gender.

Adult eastern sifakas have a moderately sized vocal repertoire of about seven call types. It is uncertain if their vocalizations have specific or varied contexts, and as with other primates, arousal level may play a role in the acoustic structure of its calls. Despite the limited size of the silky sifaka's vocal repertoire, it does exhibit high call rates of seven calls per hour, on average. Even infants are known to have several specialized vocalizations. The most frequently emitted silky sifaka vocalizations are low-amplitude, low-frequency, tonal "hum" and "mum" vocalizations. These contact calls are used in a variety of circumstances including group movement, affiliation, foraging, and while resting.

The silky sifaka uses well-developed olfactory (smell-based) communication, as with all other prosimian ("pre-monkey") primates. Like other eastern rainforest sifakas, it has several specialized glands for scent-marking, including a sebaceous gland on the chest, found only in males, and mixed apocrine-sebaceous glands on the genitals in both sexes. Unlike the true lemurs of the genus Eulemur, the silky sifaka does not directly scent-mark its conspecifics (allomarking), although it does scent-mark its territory. Both sexes will often urinate while scent-marking. Males and females scent-mark in different ways: females rub their genital glands in an up-and-down motion against trees, while males may use their chest gland, genital glands, or a combination of both. Males will also use their specialized prosimian toothcomb to gouge trees before scent-marking with their chest—a behavior that leaves long-lasting visible marks. The gouging is thought to serve a role in communication and has no dietary component, since males do not eat the bark or tree gum.

Males scent-mark more frequently than females, as much as two or three times as often. Males also respond to female scent-marking by overmarking with their own scent glands, usually by combining chest and genital marking. They will also overmark other males, although less quickly and less often. In a one-year study, males responded to 71% of the marks made by females within an average of 61 seconds while only 17% of male marks received a response from other group members. Because males overmark frequently, this results in "totem-tree marking", where certain trees become covered by male scent and gouge marks. However, no observations of widespread home range border scent-marking have been reported. Scent-marks are usually left on trees in the core area of the home range, as opposed to the territorial boundaries.