Propithecus candidus has been discussed in several survey reports (Safford et al., 1989; Duckworth et al., 1995; Sterling and McFadden, 2000), and two short-term studies (Kelley and Mayor, 2002; Queslin and Patel, 2008). A 14.5-month study was carried out by E. R. Patel in Marojejy National Park, including a detailed 12-month dietary study (E. R. Patel, pers. comm.). In Marojejy and in Anjaraharibe-Sud, animals are encountered in mountainous primary rain forest at altitudes between 700 and 1,900 m. They may sometimes be found in sclerophyllous forest, near their highest elevations. The southernmost silky sifakas in Makira adjacent to the Antainambalana River inhabit an unusual, low-elevation forest fragment at altitudes of 300 to 600 m. Despite its extreme rarity, this species has the greatest elevational range of any sifaka.
As with other eastern sifakas, P. candidus has a variable social structure, living in both pair-bonded and multi- male/multi-female groups of up to nine individuals. Home range size varies by site, ranging from 34 to 47 ha (Patel and Andrianandrasana, 2008).
Approximately 25% of the day is spent feeding and 44% resting, with 31% being devoted to other activities such as social behavior, travelling, and sleeping. Long bouts of terrestrial play involving adults are not uncommon (Patel, 2006b). Rates of aggression are low, and mainly occur during feeding. Females have feeding priority over males. Like other eastern sifakas, P. candidus is a folivorous seed predator that eats a very large variety of plant species. A recent 2-month study documented feeding from 76 species in 42 families (mainly trees, but many lianas and epiphytes as well). The most important plant families in the diet were Moraceae, Fabaceae, Myrtaceae, Clusiaceae, and Apocynaceae. Fruit from Pachytrophe dimepate, seeds from Senna sp., young leaves from Plectaneia thouarsii, and fruit from Eugenia spp. were the four foods most preferred, and accounted for roughly 37% of total feeding time. Overall, 52% of feeding time was spent eating leaves, 34% fruit, and 11% seeds. Flowers and soil were eaten rarely (Queslin and Patel, 2008). Preliminary results from an ongoing, detailed 12-month dietary study suggest general similarity in diet with P. diadema and P. edwardsi (E. R. Patel, pers. comm.).
Mating is believed to occur on a single day each year in December or January, with infants born in June or July. Generally, females give birth to a single infant every two years; only occasionally in consecutive years (Patel, 2006b). Infants initially grasp the fur on their mother’s belly, and about four weeks later begin to ride jockey-style on their mother’s back. As is typical of Propithecus, all group members interact affiliatively with infants.
Grooming is the most frequent form of non-maternal infant care, followed by playing and occasional carrying as well as nursing in a few remarkable instances (Patel, 2007a). Although in eastern sifakas generally both male and female group members disperse, female dispersal has not yet been observed in this particular species (E. R. Patel, pers. comm.).
The fossa (Cryptoprocta ferox) is the only documented predator of the silky sifaka, other than human beings (Patel, 2005). No aerial predation attempts by raptors have ever been observed, although these sifakas sometimes stare skyward and emit “aerial disturbance” roars in the presence of the Madagascar buzzard (Buteo brachypterus) (Patel et al., 2003a). Acoustic analyses have revealed sex and individual differences in the structure of the silky sifaka zzuss vocalization (Patel et al., 2005b, 2006). The adult vocal repertoire has seven different calls, a much smaller number than has been observed in Lemur catta and Varecia (Patel et al., 2005c).
Eastern sifakas have several scent-marking glands that include a sebaceous chest gland only found in males, and mixed apocrine-sebaceous genital glands in both sexes. Sifakas, in contrast to Eulemur, do not allomark by directly scent-marking conspecifics. Females scent-mark trees by rubbing their genital glands against them in a rhythmic vertical motion. Males, for their part, scent-mark trees in several different ways, by rubbing them with their chest gland, with their genital glands, or with a combination of the two. Males routinely gouge trees with their toothcombs just prior to chest-marking, which leaves long-lasting visible marks (Patel and Girard-Buttoz, 2008). Both sexes often urinate while scent- marking. Although males scent-mark two or three times as often as females, female scent marks are responded to far more often and more quickly than male marks. Males commonly overmark a female’s mark, followed by males overmarking the scent-marks of other males (Patel, 2006a; Patel and Girard-Buttoz, 2008; Ritchie and Patel, 2006).