Dassie Rats

Perception Channels: tactile ; chemical

Other Physical Features: endothermic ; bilateral symmetry

Key Reproductive Features: gonochoric/gonochoristic/dioecious (sexes separate); sexual

The dassie or rock rat is 27-38 cm long and weighs 100-300 g. It resembles a squirrel, but its hairy tail is not bushy and the soles of the feet are bare and have pads. The head and skull are flattened. The yellowish nose tends to stand out. The animal has short ears, long, black facial whiskers and no underfur. The dental formula is 1:1; 0:0; 1:1; 3:3 with narrow incisors and hypsodont (high crowned and rooted) cheek teeth. The animal has narrow, clawed feet with 4 digits on the forepaws have 4 digits and 5 on the hindpaws. The fur grows in clusters of 3-5 hairs, appearing bristly, but generally feels smooth and soft to touch. The body blends in with the rocks. The dorsal side can be brown, grey, greyish-tawny or almost black; the ventral side is grey or yellow. The very flexible axial skeleton lets the animal slip through narrow rock cracks for protection and shelter. With its short legs and squat build, it is more adapted to run over rocks, rather than jumping. The male's testes are semi-internal and usually inconspicuous. The female's teats are positioned high on the sides of the body, level with the scapula, so the young can nurse from the sides when they are crammed in a narrow rock crevice.
The dassie rat tends to live in crevices and rocky cavities and outcrops in arid and semi-arid stony deserts, escarpments and adjoining mountainous areas, inselbergs and pro-Namib plains in Namibia, parts of southwest Angola and and northern Namaqualand in western South Africa up to an altitude of 1,200 m. Suitable habitats have an average annual rainfall above 25 mm, and habitat boundaries are often limited by areas of moist woodlands or cold, wet winters. The Namib dunes and very arid unvegetated mountains and plains west of the 50-mm isohyet restrict its spread west and the Kalahari sands limit its spread east. The largest part of the distribution range is between the 50-600 mm isohyet, but the animal may occur along canyons entering the Namib Desert, where average rainfall is below 35 mm (1).

The animal rests and suns itself on warm rocks beneath overhangs to protect itself from flying predators. It can squeeze into extremely narrow crevices, due to its flattened skull and flexible ribs. It animal nests in rock crevices, while piled-up boulders provide nesting sites, which have accumulations of sticks and other vegetable matter. In Augrabies Falls National Park, South Africa, the animal inhabits rock shelters that hyraxes cannot enter (2), due to its broad, flattened skull. The animal is diurnal and usually lives in small colonies, but up to 22 were recorded in an area of 170 m x 70 m (1).
The dassie rat is a herbivore and seeks food on the ground or in low bushes. It feeds mainly on grassy parts, but may eat fruits, seeds, leaves, blossoms of desert and steppe plants, greens, berries and fruits. It may disperse seeds.
It is attacked from the air by birds of prey, which it avoids by often foraging beneath rocky overhangs, out of view from birds above. It is protectively coloured, blending in well with its rocky surroundings. If a dassie rat sees a predator, it may emit a single whistling note as a warning signal to other dassie rats. Its parasites include the nematodes Acanthoxyurus shortridgei monnig and Heteroxynema cafer.
The dassie rat mates once a year, usually in early summer (November-December). The breeding season may be related to the onset of the rainy season or be endogenous to the animals. Females give birth to 1-3 young (usually 2) in late December or early January after a gestation period of @ 3 months (1). This is an unusually slow reproductive rate among rodents[Bishop, 1984]. The young are precocial, rather large and covered with hair. They to eat solid food at @ 14 days, weaning @ 7 days later. They reach adulthood near the age of nine months. A captive was still alive after 7.1 years (3).
The IUCN Red List Category is Least Concern, due to its wide distribution, presumed large population, occurrence in several protected areas, including the Etosha, Skeleton Coast, Ai-Ais, Namib-Naukluft, Iona and Augrabies National Parks. It is unlikely to be declining fast enough to qualify for listing in a more threatened category. There is no special status on the Federal List or CITES. There are no major threats to the species, but predators include domestic cats in urban areas. Research is being done on the species in the Irongo Mtns, Omaruru.
Taxonomy: The dassie rat is the only living member of its genus, Petromus, and family, Petromuridae. It is named after the "dassie" or hyrax, which lives in similar habitats. Petromus means "rock mouse," but dassie rats are sometimes called rock rats. The family and genus names are sometimes misspelled as Petromyidae and Petromys. The family Petromuridae was a diverse family that appeared in the Oligocene of Africa. Morphological and molecular studies suggest the closest living relatives to the dassie rats are the African cane rats (Family Thryonomyidae). These two families, with related fossil families such as the Phiomyidae represented an important early radiation of rodents in Africa.