Scaly-Tailed Squirrels: Anomaluridae

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SCALY-TAILED SQUIRRELS: Anomaluridae

LORD DERBY'S ANOMALURE (Anomalurus derbianus): SPECIES ACCOUNT

PHYSICAL CHARACTERISTICS

The scaly-tailed squirrels range in size from 7.3 to 18.5 inches (18.5 to 46 centimeters) along their head and bodies, with tail length measuring between 5.4 and 18.4 inches (13.8 to 45 centimeters). They generally weigh between 7 ounces and 4 pounds (200 to 1,800 grams). The family Anomaluridae (from words meaning "strange-tailed") look very much like regular squirrels (family Sciuridae) from the outside because they have adapted to similar environments, but major differences in their skulls, teeth, and other internal items show that they have no close relationship. Scaly-tailed squirrels, unlike regular tree squirrels, have a furred "gliding membrane" on each side of their bodies that stretches in a square shape between the front legs and the back legs and also between the hind legs and the tail. Only one genus, the mainly diurnal (active during the day) Zenkerella, lacks this membrane and cannot glide. The membrane is supported in front by a strut-like, rigid section of cartilage that extends from the elbow joint, rather than from the wrist, as in the true flying squirrels. They are the only gliding mammals in Africa. Scaly-tailed squirrels are so named because of the double rows of overlapping, spiky scales on the underside of the tails for one-third of its length along the base. When the animals land after a glide, the scales help to keep them from skidding on tree trunks, and also help them climb up trees. Their silky tails are bushy on top and have strongly colored tufts. They have strong digits for manipulating food and climbing, and very long whiskers and large ears for their mainly nocturnal activity. Their heads are large and placed forward on the face, providing excellent binocular vision for finding prey and good landing places.

GEOGRAPHIC RANGE

Scaly-tailed squirrels are native to the middle region of Africa, and live mainly south of the Sahara Desert in west, central, and east Africa. Countries in which they appear regularly are Sierra Leone, Kenya, Angola, Mozambique, Ghana, Liberia, Senegal, Congo, Cameroon, Gabon, the Central African Republic, and Ivory Coast.

HABITAT

Scaly-tailed squirrels prefer the open woodlands of east, central, and west Africa and the rainforests of west and central Africa.

DIET

Larger scaly-tailed squirrels eat bark and twigs from more than a dozen species of tree, but their favorites are miombo, velvet tamarind, ironwood, owala oil, and awoura. They occasionally also eat insects and gum (tree sap). The smaller squirrels eat almost nothing besides gum and insects.

BEHAVIOR AND REPRODUCTION

Because of the remoteness of their habitats and the animals' secretive nature, scientists know relatively little about the anomalurids, members of the Anomaluridae family. However, it has been observed that the scaly-tailed squirrels clear out small branches that obstruct their habitual gliding paths. In doing so, along with their method of pruning the tops of non-food trees to keep them from crowding out their favorite food trees, the squirrels perform important functions in their ecosystems. They dislike coming to the ground, and when forced to do so move in a clumsy, kangaroo-like fashion to the nearest tree. Their gliding membranes fold away neatly when not in use, and do not prevent the squirrels from quickly scurrying along tree branches like their familiar garden-variety counterparts.

INTESTINAL FORTITUDE

Because bark has few nutrients, the scaly-tailed squirrels who eat it have very long intestines so that their bodies have a longer time to extract nourishment. In fact, about half of an adult scaly-tailed squirrel's body weight and mass is made up of its intestines. To compensate for this heavy load, the animals' skeletons are extremely light so that they can still glide effectively.

Anomalurids compete with hornbill birds for dens, which they typically make in old, hollowed out trees up to 131 feet (40 meters) high. They also battle eagles, which sometimes come in to snatch their young for prey. Females have litters of up to three pups, which are born with open eyes and thick fur. Their parents wean them from milk onto solid food by feeding the pups already chewed food from special cheek pouches. The squirrels communicate largely by scent, and use large glands in their groins to mark areas, but observers have heard them making a twittering noise as well. Field biologists believe that scaly-tailed squirrels may reach population densities of 500 individuals per 1.2 square miles (1 square kilometer). They often spend their days clinging to the side of a tree. The squirrels usually associate in pairs, but some species have been seen collected into large groups within a single den.

SCALY-TAILED SQUIRRELS AND PEOPLE

The mammals are sometimes accused of raiding oil palms for their nuts, but in general they have very little interaction with humans. Conservationists have worked to limit or stop the harvesting of the squirrels' food trees, many of which are valuable sources of high-quality commercial lumber.

CONSERVATION STATUS

Despite logging of their food trees and a general decline in habitat quality and quantity, scaly-tailed squirrel species are not considered threatened.

LORD DERBY'S ANOMALURE (Anomalurus derbianus): SPECIES ACCOUNT

Physical characteristics: Although Lord Derby's anomalure's appearance varies across its range, this species is generally grey to a rich reddish brown with silver throughout. All of the 16 subspecies share the same facial pattern, however, of white cheeks, forehead, and snout with a black band over the nose, around the eyes, and on the back of the head. The fur on their heads is dense and soft, and the silky fur on their bodies can be up to 1 inch (25 millimeters) long. They also all have six pairs of scales underneath their tails. Their tails are shorter than their bodies, the last half being black. Lord Derby's anomalure has furless, pink ears.

Geographic range: This species is widely distributed in an area across central Africa, from Sierra Leone in the west to Kenya in the east, and, less commonly, from Angola in the north to Mozambique in the south.

Habitat: This anomalure favors habitat in moist rainforests and seasonally dry woodlands from sea level to 7,875 feet (2,400 meters), although they are particularly attracted to areas that contain their favorite food trees. With regard to shelter, any tree will do for roosting as long as it is hollow in places. Roosting holes have been observed at both the tops and bottoms of trees, with entryways just large enough for anomalures to fit through. The dens are constructed so that temperature and humidity remain fairly consistent.

Diet: Like most of the larger anomalurids, Lord Derby's anomalure eats the bark of such trees as the miombo, velvet tamarind, ironwood, owala oil, and awoura, preferring the thickest portions of the main truck and large branches. The animals forage among several different trees, taking a thin strip from a tree one night and returning to take another strip during the next night's feeding, stopping when the removal site reaches about 6 inches (15 centimeters) wide to prevent permanent damage to the tree. Lord Derby's usually start a feeding site at natural wounds in a tree's bark caused by growth splits, elephant damage, or falling branches. Interestingly, the trees and this species have evolved together for so long that unlike most other trees, the rodents' food tree species can grow replacement bark. The anomalures supplement their nutrient-poor bark diet with insects, tree sap, nuts, fruits, and flowers.

Behavior and reproduction: Although biologists have yet to study the social aspects of Lord Derby's anomalure behavior, they do know that the animals tend to share dens, and may crowd up to eight individuals into one roosting hole. They are mainly active at night, but seem to enjoy lying in the sun in the early morning and late afternoon. Using their gliding membrane to move across longer distances, they perform a final abrupt upturn to cause their flight to stall and allowing them to land safely. Biologists have measured the glide distances of Lord Derby's anomalure females at 1,770 feet (540 meters), but most are under 328 feet (100 meters). Males typically achieve even greater gliding distances and move through more territory. The Lord Derby's are fairly quiet animals, with vocalizations that include a variety of twitters and purrs along with growling and hissing when threatened or disturbed.

For Lord Derby's anomalures living in the rain forest, breeding occurs year round. For residents of the dry forests, breeding occurs seasonally. Pregnant females often move to a special nursery den, where they give birth to an unknown number of pups. Although the young are large, well formed, and able to move in a coordinated fashion soon after birth, they stay with the female parent until they are almost fully mature. After weaning, they receive chewed-up food from both parents. Observers have noted that mothers and pups will often glide among trees and chase each other playfully.

Lord Derby's anomalures and people: Although biologists have long sought to learn more about this secretive and easily frightened species, there are no records of any significant interactions between Lord Derby's anomalure and humans.

Conservation status: Despite their specialized diet and habitat requirements, Lord Derby's anomalure is not threatened. However, in Ghana, their population is on a conservation watch list due to habitat destruction and degradation. ∎

FOR MORE INFORMATION

Books:

Kingdon, J. The Kingdon Field Guide to African Mammals. Princeton: Princeton University Press, 1997.

Nowak, Ronald M. "Family Anomaluridae: Scaly-tailed Squirrels." Walker's Mammals of the World Online 5.1. Baltimore: Johns Hopkins University Press, 1997. http://www.press.jhu.edu/books/walkers_mammals_of_the_world/rodentia/ (accessed on June 14, 2004).

Wilson, D. E., and D. M. Reader, eds. Mammal Species of the World: A Taxonomic and Geographic Reference, 2nd ed. Washington, DC: Smithsonian Institution Press, 1992.

Periodicals:

Dawson, M. R., et al. "Rodents of the Family Anomaluridae (Mammalia) from Southeast Asia." Annals of Carnegie Museum 72, no. 3 (2003): 203–213.

Julliot, C., et al. "Anomalures (Rodentia, Anomaluridae) in Central Gabon: Species Composition, Population Densities and Ecology." Mammalia 59 (1995): 441–443.