African Mole-Rats (Bathyergidae)

(Bathyergidae)

Class: Mammalia

Order: Rodentia

Suborder: Hystricognathi

Family: Bathyergidae

Thumbnail description
Small- to medium-sized rodents that show anatomical adaptations to living underground; bodies are cylindrical in shape and sturdy, limbs and tails (except for naked mole-rats) are short; eyes and external ears are very small, and noses flattened; prominent white incisors lie outside the lips and are used in digging

Size
Head and body length: 1.2–31 oz (34–896 g); mean mass: 3.2–11 in (83–281 mm)

Number of genera, species
5 genera; at least 14 species

Habitat
Arid regions, savanna, grasslands, and open woodlands

Conservation status
Vulnerable: 1 species; Data Deficient: 6 species

Distribution
Sub-Saharan Africa

Evolution and systematics

The classification of the Bathyergidae has long been problematic. There is now good support, using morphometric parameters and molecular phylogeny, for grouping them with the phiomorph rodents (the Old World lineage of the suborder Hystricognathi). Their closest relatives are the rock rats (Petromuridae), cane rats (Thyronomyidae), and Old World porcupines (Hystricidae).

The New and Old World Hystricognathi probably separated from their common ancestor during the Eocene, 33–49 million years ago (mya). The Bathyergidae are monophyletic, that is, all taxa can be traced back to a single common ancestor. However, fossil evidence and molecular data indicate an early divergence of Heterocephalus from the other genera in the family. Their fossil history is sparse but they most likely had an ancient African origin. The earliest fossil finds, in East Africa and Namibia, are from the early Miocene, around 25 mya.

The five modern genera are grouped into two subfamilies. The Bathyerginae contains one genus, Bathyergus, and two species, characterized by having grooved upper incisors and enlarged forefeet with strong digging-claws. The Georychinae contains four genera, Georychus, Heliophobius, Cryptomys, and Heterocephalus, all with ungrooved upper incisors and forefeet and claws that are not enlarged. All the Georychinae, except Cryptomys, are currently considered to be monospecific. The number of species in the genus Cryptomys is uncertain and needs revision; furthermore, the taxa in Cryptomys can be placed in two such genetically distinct groups that they may eventually be assigned to separate genera. Three genera are solitary, one animal inhabiting each burrow system; indeed, this is the usual pattern for subterranean mammals. Bathyergidae are exceptional in that Cryptomys and Heterocephalus are highly social: they live in colonies with a single reproductive female, her consorts, and a number of non-breeding workers (these are closely related to the breeders). Because Heterocephalus diverged from the other Bathyergidae early in their history, this unusual type of sociality probably evolved twice in the family.

Physical characteristics

Mole-rats show anatomical adaptations for life underground. Their limbs are short, their bodies are cylindrical in shape, their necks muscular, but indistinct from the head and body. Their heads are robust, ending in flattened pig-like noses. Their streamlined shape and short limbs enable them to move backwards and forwards with equal ease in the narrow confines of their burrows. The outer edges of their hind feet are fringed with stiff hairs and these, together with a brush of stiff hairs on their short tails (in all except in the naked mole-rat), help hold soil when they move it along the burrows. Their eyes are small and, at best, can only detect light and dark. Their ears lack external pinnae, and their range of hearing is more restricted than that of aboveground rodents. Nevertheless, at least two species of social mole-rats (Heterocephalus and Cryptomys mechowi) have large vocal repertoires, indicating relatively sophisticated levels of communication.

Their hair (except in the naked mole-rat) tends to be thick, short, and easily reversible; this is an asset when moving to and fro in the burrow. Strong muscles lying under a very loose skin enable them to vigorously shake their fur free of adhering soil. The loose skin also facilitates turning round in the narrow confines of the burrow. Because mole-rats live in dark burrows, the sense of touch is important to them. Stiff, tactile hairs scattered over much of the body, and projecting above the level of the rest of the hair, act in much the same way as the whiskers of a cat. Long, stiff sensory hairs abound on the front of the face and are also clustered above the eyes. Although otherwise lacking a coat of hair, all these sensory hairs are present in naked mole-rats.

A notable feature of mole-rats is a pair of large, evergrowing, and forwardly-directed incisors; these lie outside the mouth (extra-buccal). All mole-rats, except Bathyergus, are chisel-tooth diggers, that is, they excavate their burrows by biting at the soil with their incisors. Strongly haired, muscular lips meet behind the incisors to keep soil out of the mouth during digging. In all genera, except Bathyergus, the roots of the upper incisors extend back to originate behind the molars. It has been shown that a disproportionate amount of the brain of naked mole-rats is devoted to picking up sensory signals from these incisors; this is probably also true for the other chisel-tooth diggers in this family. The lower jaws of chisel-tooth diggers can move independently so that their incisors can be splayed apart or brought together, thereby making them a very versatile set of tools. Mole-rats groom lower incisors against the upper ones to sharpen their tips. The skull is dorsally flattened and houses strong jaw muscles (masseter and temporal) that are used in feeding and digging.

Physiological adaptations to subterranean life include a tolerance to high carbon dioxide and low oxygen concentrations and, at least in the naked mole-rat (and possibly others), a blood hemoglobin with a high affinity for oxygen. Mole-rats, like many subterranean mammals, have lower body temperatures and resting metabolic rates than do mammals of similar size living aboveground. The humidity in burrows is high and, consequently, evaporative water loss and convective cooling are of little use if the animal overheats when digging. The naked mole-rat has the poorest thermoregulatory ability of the family; indeed, its body temperature varies with ambient temperature in a similar way to reptiles and, in the wild, it usually lies between 82.4 and 86°F (28–32°C), which is close to the burrow temperature. They regulate their body temperature by moving up and down in the burrows and by huddling together. Microorganisms in the cecum aid in the digestion of fibers in their diet; to fully utilize these digested foods, mole-rats practice coprophagy, a process in which they double up and eat some of the energy-rich feces as they are voided.

Distribution

The family is confined to sub-Saharan Africa. Dune mole-rats (Bathyergus) are endemic to South Africa, occurring in coastal sandy soils of the southwestern and northern Cape and southwestern Namibia. The Cape mole-rat (Georychus) is also endemic to South Africa, mostly occurring in the Fynbos vegetation of the southwestern and eastern Cape, whereas species of the genus Cryptomys occur in western, central, and southern Africa. The silvery mole-rat (Heliophobius) occurs in eastern Africa and the naked mole-rat (Heterocephalus) in the arid regions of the Horn of Africa, namely Kenya, Somalia, and Ethiopia.

Habitat

They occur in a range of habitats, but not in dense forests. Their distribution is usually associated with the occurrence of geophytes (plants with underground storage organs). The solitary genera (Bathyergus, Georychus, and Heliophobius) tend to be restricted to more mesic areas, whereas the social genera (Heterocephalus and some species of Cryptomys) can also occur in arid, semidesert habitats. Soil types range from soft sands (Bathyergus) to soils that become extremely hard when dry (Heterocephalus). Mole-rats live in a burrow system consisting of a network of superficial foraging tunnels, a deeper nest complex with a toilet area, and, often, a food store. Burrows are sealed from the surface, except when disposing of excavated soil on the surface, and, consequently, the mole-rats live in a subterranean micro-habitat that is buffered from the extremes in temperature and humidity. The burrows also offer considerable protection against predators, including snakes, birds of prey, and small carnivores.

Behavior

Mole-rats dig by loosening the soil with strongly developed forefeet (Bathyergus) or by biting the soil with their incisors (the other genera). The loosened soil is pushed under their bodies with their forefeet and then collected and kicked behind them with their hind feet. When a pile of soil has accumulated, they reverse with it up a side-branch and use their hind feet to push it onto the surface. After a mound has been completed, the side branch to the surface is sealed. Naked mole-rats dig cooperatively—one animal works at the earth-face, a chain of animals transports the soil, and another animal kicks the soil out of a hole. The solitary species aggressively defend their burrow system against conspecifics and advertise their presence to their neighbors by drumming on the burrow floor with their hind feet. During the breeding season, different drumming patterns of male and female Georychus advertise their presence to potential mates. The social species are strongly xenophobic towards foreign animals, and naked mole-rats have a unique colony odor that enables them to differentiate between their own and foreign colony members.

Feeding ecology and diet

African mole-rats favor areas in which energy-rich bulbs, corms, and tubers abound. Although these constitute their main food source, they will also eat roots and the aerial parts of plants. These geophytes often occur in clumps and are encountered by the mole-rats as they dig their foraging tunnels, most of which are dug after rain when the soil is moist and easily worked. In arid regions where rainfall is unpredictable and often widely spaced, the mole-rats have to make the most of each digging opportunity in order to find sufficient food to last them until it next rains (sometimes a year later). Clumps of food in these arid regions also tend to be spaced further apart than in mesic regions, or to occur as large isolated tubers. The limited opportunities in which to dig, and the distribution of food in these arid regions, exclude solitary species from these areas. Social species can mobilize their workforce to search a wide area in a short time to locate sufficient food to see them through the dry periods. Colonies of naked and Damaraland mole-rats have been known to dig up to 0.6 mi (1 km) in a single session of digging after a rainfall.

Small-sized geophytes are either immediately consumed or carried to a central store situated near the nest. Large geophytes are left growing and eaten in situ. Sometimes these partly eaten geophytes are abandoned, then they regenerate, and thus provide the mole-rats with a renewable resource. This is important because field studies on the naked mole-rat, Damaraland mole-rat, and the common mole-rat show that colonies remain resident in an area; they do not simply exploit it and then move on. The diet of dune mole-rats includes about 60% aboveground vegetation (forbs, grasses); the mole-rats come up under a plant, loosen its roots, and pull the entire plant into the burrow. They can thus forage with minimal exposure to predators.

Reproductive biology

In solitary species, courtship and mating are brief encounters and the animals then resume their solitary existence. When pups are about two months old, they disperse to establish their own burrow systems. Colonies of all social mole-rats have reproductive division of labor, whereby reproduction is restricted to a single female and a few males. The remaining members are closely related to the breeders, but are reproductively quiescent while in the colony. Colony formation and the mechanisms of suppression differ in the species. Thus, there is strong inhibition to incest in Cryptomys, whose colonies are founded by a female and her unrelated consorts. Initially, their offspring remain as non-breeding helpers, only risking dispersing if environmental conditions are favorable. Colonies break up completely when a breeder dies. Mechanisms of reproductive suppression within this genus range from incest avoidance to physiological mechanisms that inhibit ovulation in the females but not sperm production in the males. Colonies of naked mole-rats, on the other hand, are inbred. The aggressive behavior of the breeding female induces neuroendocrine changes in the pituitary that affect the functioning of the gonads of both non-breeding males and females. When the breeding female dies, some of the oldest females in the colony become sexually active (this can occur within a week of the death of the breeder), and they often fight viciously for the vacated position.

Bathyergus, Georychus, C. hottentotus, and possibly Heliophobius breed seasonally, whereas Heterocephalus, C. damarensis, C.darlingi, and C. mechowi breed throughout the year. Gestation lengths range from about 44 days in Georychus to 100 days in C. amatus. Mean litter sizes are less than four, except Georychus, which have up to six, and Heterocephalus, which can have up to 13. A breeding female Heterocephalus can bear huge litters (up to 28 pups). During her first few pregnancies, her lumbar vertebrae elongate, thereby increasing the size of her abdominal cavity and her capacity to bear these large litters. She thus becomes morphologically distinct from the rest of the colony, a unique feature amongst mammals. This phenomenon, together with having a monopoly of reproduction, large litters, and presiding over colonies of up to 300 animals, makes naked mole-rats the closest mammalian equivalent to social insects such as termites.

Conservation status

The habitat of Bathyergus janetta is threatened by diamond mining. Geographically isolated northern populations of Georychus, which may be a separate species, are small and possibly threatened. Little is known of the status of some species of Cryptomys from central and eastern Africa.

Significance to humans

Mole-rats can be pests in farmlands or in urban developments. The large dune mole-rat (B. suillus) causes the most damage because their tunnels undermine roads and runways of airports, and their large mounds and tunnels can cause damage to farm machinery. Mole-rats also chew through underground cables and irrigation pipes.

Species accounts

Resources

Bennett, Nigel C., and Chris G. Faulkes. African Mole-rats: Ecology and Eusociality. Cambridge: Cambridge University Press, 2000.

Lacey, Eileen A., James L. Patton, and Guy N. Cameron, eds. Life Underground: The Biology of Subterranean Rodents. Chicago: University of Chicago Press, 2000.

Sherman, Paul W., Jennifer U. M. Jarvis, and Richard D. Alexander, eds. The Biology of the Naked Mole-rat. Princeton: Princeton University Press, 1991.

Faulkes, Chris G., and N. C. Bennett. "Family Values: Group Dynamics and Social Control of Reproduction in African Mole-rats." Trends in Ecology and Evolution 16, no. 4 (2001): 184–190.

Jarvis, Jennifer U. M., M. J. O'Riain, N. C. Bennett, and P. W. Sherman. "Mammalian Eusociality: A Family Affair." Trends in Ecology and Evolution 9 (1994): 47–51.

O'Connor, Timothy P. O., A. Lee, J. U. M. Jarvis, and R. Buffenstein. "Prolonged Longevity in Naked Mole-rats: Age-related Changes in Metabolism, Body Composition, and Gastrointestinal Function." Comparative Biochemistry and Physiology A 133 (2002): 835–842.

Spinks, Andrew C., J. U. M. Jarvis, and N. C. Bennett. "Comparative Patterns of Pilopatry and Dispersal in Two Common Mole-rat Populations: Implications for the Evolution of Mole-rat Sociality." Journal of Animal Ecology 69 (2000): 224–234.

[Article by: Jennifer U. M. Jarvis, PhD]