Named by: Waterhouse, 1843
Taxonomy: Synapsida, Sphenacomorpha, Sphenacodontia, Sphenacodontoidea, Therapsida, Theriodontia, Cynodontia, Probainognathia, Mammaliamorpha, Mammaliaformes, Mammalia, Theria, Eutheria, Placentalia, Euarchontoglires, Glires, Rodentia, Anomaluromorpha, Anomaluridae
Included species: A. derbianus (type, as A. “fraseri”), A. beecrofti, A. pelii, A. pusillus
Gliding locomotion has evolved several different times in mammaliaforms. Among living mammals, well-developed gliding has arisen independently in the genera Acrobates and Petaurus among marsupials, the colugos among archontans, and the flying squirrels among rodents. (A few primates such as sifakas have been suggested to be capable of some degree of gliding, but they lack the extensive membranes connecting the front and hind limbs that the aforementioned groups have.) What is less frequently appreciated, however, is that there exists a second group of gliding rodents restricted to the forests of central Africa, where there are no flying squirrels to be found. These poorly known gliders are the anomalures.
The “official” common names of anomalures used by mammalogy organizations tend to be some variation of “scaly-tailed squirrel”. I for one don’t understand why this is the case when “anomalure” is a perfectly serviceable name that additionally emphasizes the uniqueness of these animals. Despite superficial similarities, anomalures are not at all closely related to squirrels beyond the fact that both are rodents. The closest living relatives of anomalures are another group of African rodents, the springhares, whose lineage diverged from theirs around 57.1 million years ago. Today, springhares are rodents of open country that get around primarily by hopping, whereas anomalures are forest dwellers that spend most of their time in trees.
Though “squirrel” misses the mark as a common name for anomalures, the “scaly-tailed” descriptor does not. All anomalures have two rows of scales underneath their tail, which provide them with extra grip while sitting or moving in the trees. With the exception of one species (Zenkerella insignis), most extant anomalures are gliders, and it is said that the slap of their tail scales against tree bark can be audible when they come in for a landing. Another unusual feature of the gliding anomalures is the cartilaginous spur that extends from each of their elbows and helps support their gliding membrane. Similar structures are found in flying squirrels, but those instead extend from the wrist. The gliding anomalures are divided into two genera, and here we will focus on the larger species, those in the genus Anomalurus.
The largest species of Anomalurus is A. pelii, which can weigh up to 2 kg (4.4 lb). In both size and gliding capacity Anomalurus are comparable to the largest flying squirrels, with documented gliding distances of up to 100 m (328 ft). Some species (especially A. derbianus) are known to prune branches around their preferred food sources to keep their glide paths clear.
Anomalures are largely herbivorous and Anomalurus are known to eat a variety of plant-based foods, including sap, leaves, bark, fruits, and flowers. A. derbianus often feed on the bark of the tree genera Strombosia, Klainedoxa, and Neoboutonia, and A. beecrofti feed on the pulp of palm fruits. A. beecrofti have a notably narrow snout, which may be an adaptation to this feeding behavior given that ripening palm fruits are protected by short spines.
Young Anomalurus are born with their eyes open and a complete covering of fur, at least in species (mainly A. derbianus) for which relevant observations have been made. In A. derbianus, both parents have been seen provisioning the young with chewed-up food, and the young remains in a hollow tree until it is almost fully grown.
During the day, Anomalurus retire to their nests, which are usually situated in tree cavities. Individuals may maintain up to six different nests at once. They are known to reuse the same nests over periods of months or even years. A. derbianus have also been seen resting in abandoned beehives, and A. beecrofti often sleep in dense vegetation at the tops of trees. Anomalurus are known to share their nesting hollows with other small mammals, including other species of anomalures as well as bats. However, the giant A. pelii aggressively deter larger potential nestmates such as hornbills by growling, hissing, and snapping their teeth. This is also their response to being disturbed by predators, and it has been suggested that the contrasting black and white coloration of this species warns potential aggressors of their belligerence.
Some species of Anomalurus do venture out before dark. A. derbianus regularly sunbathe during early mornings and occasionally evenings, whereas A. beecrofti have been seen clinging vertically to tree trunks during the day (sometimes even sleeping in this position). This does not necessarily make them any easier to observe, however. A. beecrofti have been credited with “exceptional powers of concealment”, likely because their mottled fur provides excellent camouflage. Their pelt has even been claimed to be “leaf green” in life, but this coloration allegedly fades at night and after death, and is thus not apparent in preserved specimens. The mechanism of this color fading has never been studied in detail, but it would appear to be a particularly unusual case of mammalian coloration.
- Happold, D.C.D. (ed.). 2013. Mammals of Africa Volume III: Rodents, Hares and Rabbits. A&C Black Publishers Ltd., London. 784 pp.
- Heritage, S., D. Fernández, H.M. Sallam, D.T. Cronin, J.M. Esara Echube, and E.R. Seiffert. 2016. Ancient phylogenetic divergence of the enigmatic African rodent Zenkerella and the origin of anomalurid gliding. PeerJ 4: e2320. doi: 10.7717/peerj.2320
- Jackson, S. 2012. Gliding Mammals of the World. CSIRO Publishing, Clayton. 232 pp.