This family contains 12 species in 5 genera. Its members are found in Africa south of the Sahara. These rodents are small to medium in size, ranging from around 80 gms to perhaps 600 gms in body weight.
Bathyergids are diggers. Their bodies are fusiform, their legs short and fairly powerful, and their eyes and external ears are small. Most have exceptionally loose skin, permitting them to reverse directions easily in a very narrow space. They see poorly or not at all; the surface of the eyes may serve instead to detect air movement (as would happen if a normally-sealed burrow entrance were opened). Most excavation is done with the mouth, so the legs are not as strongly modified as they are in fossorial species such as moles. As in other burrowing rodents, their lips close tightly behind their portruding incisors, preventing loose earth from entering the mouth. The feet are used to move earth freed by the incisors, and the hind feet are broad in most species. All bathyergids except Bathyergus have short claws; Bathyergus, which uses its feet rather than its incisors for excavation, has exceptionally long claws. Both forefeet and hindfeet end in 5 digits.
Perhaps because they live in an environment without light, bathyergids seem to rely heavily on their sense of touch. Their tails are short but clearly used as a tactile organ when the animals are backing. Their pelage also serves a sensory function, and many bathyergids have long, sensitive hairs scattered around their bodies. These are the only hairs present in naked mole rats (Heterocephalus), but most bathyergids have thick and soft pelage.
The skulls of bathyergids are wide, flat, and robustly built, as might be expected in animals that dig with their heads. They are hystricognathus but not hystricomorphous; the infraorbital foramen is relatively small and does not transmit much or any of the medial masseter, nor does it contain a groove or separate foramen for the passage of nerves or blood vessels. The zygomatic arch is heavy but fairly simple, and the jugal does not touch the lacrimal. Other cranial characteristics include small to fairly large auditory bullae and a short but distinct paroccipital process. Bathyergids also have enlarged angular processes and mandibular fossae, perhaps as a result of the need for enlarged masseters to power the incisors when they are used for digging.
Dentally, the teeth bathyergids are distinctive. The dental formula, 1/1, 0/0, 2-3/2-3, 0-3/0-3 = 12-28, reflects the great variation in cheek tooth number seen in this family. The cheek teeth are hypsodont but rooted. Their occlusal surfaces are simple, either ring or 8-shaped. In most genera the incisors are heavy, procumbent (portruding), and curiously, lack the yellow pigment that characterizes most rodents.
Mole rats are highly fossorial, and they are primarily (but some species not exclusively) vegetarian. Other than that, it is difficult to generalize about their habits. Most are solitary; Heterocephalus, on the other hand, appears to be one of the most highly social mammals in existence, forming colonies in which most individuals specialize in different tasks (defense, digging, food gathering, etc.) and forego reproduction. Where several species of mole rats occur in the same area, they appear to segregate by soil type.
Bathyergids are known from as early as the Oligocene. Their phylogenetic relationships are controversial; here, we include them within the Hystricognathi.
Families of Order Rodentia Suborder Sciurognathi Family Aplodontidae (mountain beaver, sewellel) Family Sciuridae (squirrels) Family Castoridae (beavers) Family Geomyidae (pocket gophers) Family Heteromyidae (kangaroo rats, pocket mice, and allies) Family Dipodidae (birch mice, jumping mice, jerboas) Family Muridae (familiar rates and other rodents) Family Anomaluridae (scaly-tailed squirrels) Family Pedetidae (spring hare, springhaas) Family Ctenodactylidae (gundis) Family Myoxidae (dormice and hazel mice) Suborder Hystricognathi Family Bathyergidae (mole rats, blesmols, and rats) Family Hystricidae (Old World porcupines) Family Petromuridae (rock rat or dassie rat) Family Thryonomyidae (cane rats or grasscutters) Family Erethizontidae (New World porcupines) Family Chinchillidae (Chinchillas and viscachas) Family Dinomyidae (pacarana, branick rats, false paca) Family Caviidae (cavies and guinea pigs) Family Hydrochaeridae (capybara) Family Dasyproctidae (agoutis, acouchis) Family Agoutidae (pacas) Family Ctenomyidae (tuco-tucos) Family Octodontidae (degus, coruros, rock rats) Family Abrocomidae (chinchilla rats, chinchillones) Family Echimyidae (spiny rats) Family Capromyidae (hutias, zagouties, cavies, Indian coneys) Family Heptaxodontidae (Quemi, giant hutias) Family Myocastoridae (nutria, coypu)
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