Von der Decken’s Sifaka, Propithecus deckenii
VON DER DECKEN'S SIFAKA
Geographic Distribution and Habitat
The Von der Decken’s sifaka (Propithecus deckenii), or simply Decken’s sifaka, is one of nine distinct species of sifakas endemic to Madagascar. It and three other species that live on the western half of the island were once considered subspecies of the Verreaux sifaka (Propithecus verreaux). Recently, closer anatomical research has determined each of these is, in fact, its own distinct species.
Renowned for their resilience and adaptability, sifakas often live in environments that are inhospitable to any other primates but themselves. Von der Decken’s sifakas exemplify these specialized lemurs’ unique capabilities for survival in unforgiving environments. In the dry deciduous forests of northwestern Madagascar between the Mahavavy and Manambolo rivers where they live, members of this species traverse the peculiar landscape known in Malagasy as “tsingy.” With their sharp edges and needle-like spires, these forests of limestone spires (the like of which are found nowhere else in the world) would prove impractical for most living creatures. Yet the Von der Decken’s sifakas manage these dangerous obstacles with incredible grace and ease.
Near the Manambolo River, Von der Decken’s and crowned (Propithecus coronatus) sifakas populations do significantly overlap. Sifakas with color patterns characteristic to both species have been reported in the forests of Bongolava Massif, suggesting the possibility of a hybrid species. Whether or not this is the case remains to be properly studied.
This species is also found on the island of Nosy Be, off the northwestern coast of Madagascar.
Size, Weight, and Lifespan
On average, a Von der Decken’s sifaka measures 18 inches (45 cm) at maturity. Her long tail doubles her overall length. Males and females are roughly the same size, ranging in weight from 7–13 pounds (3–5 kg).
It is currently unclear how long the average individual may live in the wild.
Von der Decken’s sifakas’ stark black faces are round and hairless. Two yellowish-orange eyes bulge beneath a smooth brow. A stout but prominent muzzle forms the nose and mouth and two black semi-circular ears protrude from the top of the head.
Their torsos and limbs are covered in white fur. Beneath this ivory coat, black skin comes subtly through, making them appear light gray. This effect is especially apparent on the chest and tail and around the base of the fingers and toes. Tufts of brown fur grow in subtle halos at the top of their chests and around the armpits.
Their large, lanky limbs are packed with muscle. Long, padded fingers and toes protect them from injury as they traverse the forest and the sharp rock ledges of the tsingy landscape. Hook-shaped hands and opposable big toes help them to grasp tree trunks firmly.
Sifakas are known to eat a variety of leaves, flowers, fruit, buds, and tree bark that grow seasonally within their ranges. Their strictly vegetarian diets are digested thoroughly by an enormous digestive tract that, uncoiled, would stretch 14–15 times longer than their bodies. An enlarged caecum—a pouch that is considered to be the beginning of the large intestine—is home to a slew of specialized bacteria that help break down the sifaka’s fiber-rich meals. Some of these bacteria are even responsible for degrading the toxins found in several of the plants sifakas eat.
The details of the Von der Decken’s sifaka’s diet currently needs more research.
Behavior and Lifestyle
During the day, Von der Decken’s sifakas go in search of food. Leap by leap, they negotiate the complicated terrain of the tsingy landscape. Powerful hindlegs launch them from perch to perch. Small parachute-like folds open between their arms and body midair, giving their fall a style and grace unique to sifaka locomotion. Landing, they use their hook-like fingers and opposable big toes to firmly grasp whatever substrate—rock or tree—they contact before calculating a next move. Their unique adaptations, including the pads on their hands and feet, give them the ability to move quickly and nimbly despite the dangerous sharpness of the tsingy’s limestone escarpments. The presence of such unique obstacles occasionally forces them to betray their arboreal lifestyle. Indeed, Von der Decken’s sifakas descend to the ground more often than other sifaka species.
At rest on a perch, they generally assume an upright position. They may sit vertically on a branch, or cling with limbs or digits wrapped around a branch or tree trunk. Asleep, they drop their head in their chest, bury it beneath their arms and rolls up their tail in a spiral between the legs.
Sifakas are generally renowned for having tame and silent demeanors compared to other types of lemurs. This does not mean that they are never aggressive or territorial, however. They mark their territories using special scent glands and keep a watchful eye out for invading sifakas. When an offender is spotted, groups quickly raise the alarm and go on the defense. Group conflicts sometimes end in complete takeover by an opposing sifaka group, during which only adult females are likely to be spared from death.
Despite their general unfriendliness towards outsiders of their own species, sifakas are often known to live peacefully alongside some other lemur species.
The Von der Decken’s sifaka is one of the few creatures capable of navigating the uncompromising and dangerously sharp limestone formations known in Malagasy as tsingy.
Von der Decken’s sifakas have been found living in eucalyptus trees in the middle of Soalala town, suggesting the species may be quite adaptable.
Daily Life and Group Dynamics
There is still much to be learned about the lives of Von der Decken’s sifakas in the wild. Navigating the tsingy terrain where Decken’s sifakas live is uniquely challenging for humans, making any proper and thorough study of this species extremely difficult. For now, what we know about less elusive sifaka species may give us insight into the lives of the more enigmatic Decken’s sifakas.
To avoid the oppressive Madagascar heat, sifakas are most active at the beginning and end of the day. During these times groups may seek out and consume food. When the sun reaches its peak, they take to the shade, where they can relax and digest their fibrous meals. Come dusk, they prop themselves up on comfortable branches and sleep.
Sifakas live in small groups of 3–10 members. These groups are matriarchal and travel together to find food. Sifaka social structures undergo drastic shifts during the mating season, when subordinate males vie for more dominant positions in the hopes of having a chance to mate.
The name “sifaka” derives from the “tschi-faak” call made uniquely by these species. This is one of many alarm calls sifakas use to warn group members of proximal threats.
Research has shown the acoustic qualities of these calls can vary between species. The tschi-faak calls made by the Von der Decken’s sifakas living at Bemahara, for example, are notably longer and deeper in pitch compared to those of the crowned sifakas living at Antrema to the north. More comparisons need to be made between different sifaka populations, however, to determine the full truth of this assertion. If it is so, researchers surmise it is most likely a difference in the anatomical structures of the species’ vocal tracts rather than a consequence of their environments.
Like other social primates, sifakas communicate through body language and facial expressions. By grooming each other, group members form closer social bonds. Scent marking is a crucial method for communicating territory and attracting mates.
The specific methods Von der Decken’s sifakas use to communicate are yet to be researched.
Reproduction and Family
The reproductive behaviors of Von der Decken’s sifakas are not yet researched, though they are probably similar to those of other sifaka species.
Sifakas mate seasonally. Mating season tends to be a time of significant social upheaval as subordinate males vie for more dominant positions in their own, or neighboring, sifaka groups.
A female and male may use scent-marking to gain one another’s attention. When they have mated, the female carries their offspring in her womb for 130–141 days. Females rarely birth more than a single infant at once.
The helpless newborn attaches himself to his mother’s belly. Clutching handfuls of her fur as she traverses the forest in leaps and bounds, he remains in this position for 2–3 months before moving to her back. At 6 months old, he is weaned and begins to learn the more complex behaviors of a sifaka. Through play, he develops the motor skills demanded of his environment. By imitating adults in his group, he learns survival skills and how to navigate the intricacies of sifaka social life.
He reaches his adult size as early as 21 months but remains immature until about 2.5 years. While female sifakas remain with their natal group their whole lives, a male who has reached sexual maturity may leave his natal group if he finds opportunity for improving his social rank by joining another.
Sifakas play a vital role in their ecosystems as seed dispersers.
While the limestone spires of the the tsingy are themselves uninhabitable, the deep canyons and rifts between these escarpments are riddled with pockets of life. As one of the few animals capable of navigating the tsingy landscape, Von der Decken’s sifakas transport the seeds of the fruit they ingest between these otherwise isolated miniature ecosystems.
Conservation Status and Threats
The Von der Decken’s sifaka is classified as Critically Endangered by the International Union for Conservation of Nature (IUCN, 2017), appearing on the IUCN Red List of Threatened Species. Deforestation rates have increased in recent years across Madagascar. Habitat loss is the principal threat faced by this species. Forests within its range have declined dramatically over the last few decades and continue to be burned to provide pasture for livestock, cut for charcoal production, and logged. Habitat destruction of varying degrees was reported at every single Von der Decken’s sifaka site surveyed in 2014. Although traditional taboos against killing them exist in some regions and were previously considered to offer protection across much of the species range, hunting for meat does occur at many locations and was reported at between 66 and 83% of the sites surveyed during that same time. Some live capture for the illegal pet trade was also recorded in 2017.
Much of Madagascar’s wildlife—90% of which is found nowhere else in the world—is threatened by habitat loss. In the last 2,000 years, human activities have reduced the island’s forest cover by more than 90%. Of that reduction, almost half occurred between the years 1950 and 2000! Despite these harrowing figures, habitats continue to be dismantled in the wake of human development. The poor regenerative powers of Madagascar forests makes this situation bascially irreversible.
As a result of this mass deforestation, 90% of lemur species are currently threatened with extinction—Von der Decken’s sifakas among them. So long as these unique primates lose their habitat to human-driven farming and mining projects, their populations will remain in steady decline.
Human encroachment also drives forest fragmentation. As this occurs, groups of Von der Decken’s sifakas grow more isolated from one another, limiting their social exchanges. By interrupting gene flow, this trend has the ability to weaken the viability of individuals. Inbreeding renders animals more vulnerable to contracting diseases and playing host to deadly parasites, thus decreasing their chances of survival.
In spite of these bleak statistics, there is some hope for the Von der Decken’s sifaka.
For now, a local taboo against killing sifakas protects the Von der Decken’s sifaka from local hunters. If this taboo were to disappear, however, conservationists fear it could be disastrous for the species.
The inhospitability of the tsingy inhibits almost all human activities from taking place there. As much as this difficult terrain challenges conservation efforts and much-needed research of the Von der Decken’s sifakas who live there, it also prevents further habitat loss.
This species already lives in several protected areas, including the Baie de Baly, Tsingy de Bemaraha, and Tsingy de Namoroka national parks. Von der Decken’s sifakas also roam the Tsingy de Bemahara Strict Nature Reserve as well as four special reserves in Ambohijanahavy, Bemarivo, Kasijy, and Maningoza.
There is still a great deal of work and research that needs to be done in order to keep the Von der Decken’s sifaka from extinction. Developing unique methods for monitoring this elusive species will shed more light on their population trends and behaviors, aiding conservation efforts. For instance, more taxonomic research is needed to better understand the limits of the species’ geographic distribution. In turn, a better understanding of Von der Decken’s sifakas diets and ecological roles can take shape.
Written by Zachary Lussier, March 2020. Conservation status updated July 2020