The diademed sifaka forms groups typically of two to ten individuals, which may include multiple male and female adults. Each troop defends an exclusive home territory of 25 to 50 hectares (62 to 125 acres) using perimeter scent territorial marking by both the males and females. Although the diademed sifaka defends the group's territory strongly against other members of their same species, it will share territory with other species such as the red-bellied lemur and the common brown lemur. P. diadema is thought to traverse the greatest daily path distance relative to other members of its family in its patrolling and foraging, attaining a typical travel distance in excess of 1.6 kilometres (one mile) per day. To accomplish this it consumes a diet high in energy content and diverse in plant content, each day consuming over 25 different vegetative species. This diurnal lemur further diversifies its diet by consuming not only fruits, but certain flowers, seeds and verdant leaves, in proportions that vary by season.
For a large lemur, the diademed sifaka is rather athletic, being capable of lateral aerial propulsion of up to 30 kilometers per hour, a result of muscular leg thrusting action pushing off from a vertical tree trunk. It is possible, although not proven, that its vigorous health characteristics are enhanced from high consumption of two plants which contain high concentrations of alkaloids. This species is arboreal, and only rarely are seen on the ground; moreover, it is a vertical clinger and lateral leaper.
The diademed sifaka is classified as critically endangered on the IUCN Red List, and is listed in CITES Appendix I.[2] As of the year 2002, population estimates for the species range between 6,000 and 10,000 individuals. The primary threat is habitat reduction due to shifting cultivation by native peoples. This threat is also present even within designated national parks, which are sufficiently distant from the center of government, that enforcement of existing national laws protecting P. diadema habitat is problematic. Pressures of population growth in central and eastern Madagascar are causing many of the rural poor to seek subsistence by seizing whatever forest lands are available and undertaking slash-and-burn tactics as their initial step in a shifting cultivation system. Returns from such land use are usually meager, yielding small amounts of charcoal, firewood or grass crop for grazing of zebu.
The diademed sifaka and three other sifaka species form a tight species group within the genus Propithecus. The other three species are Milne-Edwards' sifaka (Propithecus edwardsi), Perrier's sifaka (P. perrieri), and the silky sifaka (P. candidus). All of these species have luxuriant silky coats and are powerful leapers. They share similar characteristics of gestation length (four months), age of sexual maturity, female dominance, life expectancy (18 years) and propensity for sunbathing while stretched out on a branch. They differ distinctly in colouration and markings, except for having universally totally black faces.
