Scaly-Tailed Squirrels (Anomaluridae)

(Anomaluridae)

Class: Mammalia

Order: Rodentia

Suborder: Sciurognathi

Family: Anomaluridae

Thumbnail description
Squirrel-like animals with a flexible furred membrane between the fore- and hindlimbs (except in one genus); long, silky fur and very long whiskers; long tail with two rows of pinecone-like scales one-third of the way along the underside

Size
Anomalurus: Head and body 7.3–18.5 in (18.5–46 cm); tail 5.4–18.4 in (13.8–45); weight 7 oz.–4 lb (200–1,800 grams); Idiurus and Zenkerella: Head and body 2.5–9 in (6.5–23 cm); tail 2.7–6.6 in (7–17 cm); weight 0.5–7.8 oz (14–220 g)

Number of genera, species
3 genera; 7 species

Habitat
Open dry forest and rainforest

Conservation status
Lower Risk/Near Threatened: 4 species

Distribution
Africa

Evolution and systematics

Anomaluridae is an exceedingly ancient rodent family, with a lineage extending back some 30 million years (late Eocene). Formerly much more diverse, some 20 fossil genera have been identified. The external resemblance to squirrels is misleading and, instead, anomalures are thought to resemble closely the rodent stock from which today's porcupines, rats, and squirrels are descended. Anomalures are an evolutionary enigma, very ancient and yet at the same time highly specialized. The anomalure lineage is thought to have survived for so long because they have specialized in a substance that is easily found but that few other mammals can eat: tree bark. Their family name, means "the strange-tailed ones." All members use the furred membranes for gliding between trees, sometimes very adeptly. This ability is thought to have arisen originally for moving between the widely spaced trees of dry seasonal forests, not unlike today's Brachystegia "miombo" woodlands. A pliable fur-coated membrane extends between the legs and also extends between the hind legs and the tail. The presence of a similar membrane in 14 genera of flying squirrels (Aeretes, Aeromys, Belomys, Biswamoyopterus, Eupetaurus, Glaucomys, Hylopetes, Iomys, Petaurillus, Petaurista, Petinomys, Pteromys, Pteromyscus, Trogopterus) and the marsupial genus Petaurus is a remarkable example of convergent evolution.

The systematic position of this family has been much debated. In the past they have variously been grouped with Old World porcupines, the mouse-related rodents, and the squirrel-related rodents. Sometimes, they are included in the separate suborder, Anomaluromorphia, and superfamily, Anomaluroida. This also includes another family, the Zegdoumyidae, which is extinct. Unlikely as it may seem, the closest relative to the anomalures may well be the springhare (Pedetes capensis), a thoroughly terrestrial resident of southern Africa's semi-arid grasslands. It too has a very ancient lineage.

Physical characteristics

There are two major forms in this family, the large anomalures and the smaller flying mice (one of which cannot fly). With their roughly square gliding membranes extended, members of the former group may be larger than a tea tray, while in the same condition the latter are smaller than the cover of a paperback novel. A cartilaginous extension of the elbow keeps the flying membrane rigid, while allowing the front limbs greater freedom for manipulative tasks. This strut is one of the anomalurids' unique features. The membrane, though thickly furred on top, is relatively sparsely furred on the underside. Eyes are large and forward facing, providing excellent biocular vision. Hearing is acute and may extend to ultrasonic frequencies. The two rows of spiked scales under the tail act as anti-skid devices on landing and also provide extra grip during climbing, or support while resting vertically on a trunk. For extra grip, claws are robust and sharp. The membrane is very flexi ble, and forms a loose folded extension of the flank when not in use. It does not hinder movement and all anomalurids can run along branches like the garden-familiar Sciurus squirrels. Anomalures do not come voluntarily to the ground. When placed on it they are clumsy, moving away in clumsy kangaroo-like hops.

Distribution

Anomalurids live in Africa south of the Sahara, but are absent from southern Africa.

Habitat

These animals live in the rainforests of West and Central Africa and the open drier woodlands of East Africa. The anomalures' distribution closely maps the distribution of five of the most preferred food trees: Brachystegia (miombo), Cynometra (ironwood), Dialium (velvet tamarind), Julbernadia (awoura), and Pentaclethra (owala oil).

Behavior

They are the only gliding mammals in Africa and fulfill a similar ecological role to gliding squirrels elsewhere. Anomalures remove small branches that obstruct their traditional gliding lines out from their denning tree. The long duration of the Anomalure lineage has led to some remarkable examples of coevolution with the tree species they feed on, including the deliberate gnawing off of the tops of any young non-food trees nearby, thus reducing competition with their food trees and favoring food tree growth. Anomalures and hornbills compete for dens/nest holes. Eagles are occasional predators. Scent is important in communication and large glands in the groin region produce a variety of strong-smelling secretions. Field studies of anomalures have been made difficult by the remoteness of their habitat and by the fact that they are sensitive to the red light biologists usually use to observe nocturnal animals. Anomalures are not rare, but they are hard to find. Recent population estimates in the Foret des Abeilles, central Gabon, indicate that there may be up to 500 anomalures (of three species) per 1.2 mi² (1 km²).

Feeding ecology and diet

The large anomalures gnaw bark and small branches from more than a dozen types of tree. They may also eat gum and insects. The smaller ones (flying mice) are believed to eat insects and gum almost exclusively. Bark is a low-nutrient food source. To compensate, anomalures have long guts, allowing the extraction of the maximum nutrients. These comprise half the weight and much of the body volume of an adult anomalure. Gut contents regularly exceed 12% of body weight. In order that they may still glide effectively, the anomalure skeleton is very lightweight.

Reproductive biology

The young are born with thick fur and open eyes (precocial). Litter size may be up to three. They are weaned onto solid food via finely chewed food held in the parents' cheek pouches.

Conservation status

Four species are Lower Risk/Near Threatened (Anomalurus pelii, Idiurus zenkeri, I. macrotis, and Zenkerella insignis). The remaining three species are not threatened.

Significance to humans

Occasionally accused of robbing oil palms for their nuts, but not regarded as a major pest. The large anomalures are highly dependent on particular species of tree and require tall trees, with a clear sub-canopy and enough old trees to provide an abundance of holes. Hence they are vulnerable to habitat disturbance. Many of their food tree species are valued commercially for timber.

Species accounts

Resources

Kingdon, J. The Kingdon Field Guide to African Mammals. New York: Academic Press, 1997.

Adam, F., L. Bellier, and L. W. Robbins. "Deux nouvelles captures d'Idiurus macrotis Miller (Rodentia, Anomaluridae)." Mammalia 34 (1970): 716–717.

Aellen, V., H. Heim de Balsac, and R. Vuattoux. "A propos des Anomaluridae de Côte d'Ivoire." Mammalia 34 (1970): 159–160.

Aellen, V., and J. Perret. "Sur une nouvelle trouvaille de Zenkerella insignis Matschie, 1898 (Rodentia, Anomaluridae)." Saugetierkunde Mittelunde 6 (1995): 21–23.

Jones, C. "Notes on the Anomalurids of Rio Muni and Adjacent Areas." Journal of Mammalogy 52 (1971): 568–572.

Julliot, C., S. Cajani, and A. Gautier-Hion. "Anomalures (Rodentia, Anomaluridae) in Central Gabon: Species Composition, Population Densities and Ecology." Mammalia 62 (1998): 91–121.

Perez de Val, J., J. Juste, and J. Castroviejo. "A Review of Zenkerella insignis Matschie, 1898 (Rodentia, Anomaluridae). First Records in Bioko Island (Equatorial Guinea)." Mammalia 59: (1995): 441–443.

Roche, J. "Capture de Zenkerella insignis (Rongeurs, Anomaluridés) en République Central-Africaine (Equatorial Guinea)." Mammalia 36 (1972): 305–306.

[Article by: Adrian A. Barnett, PhD]