Subfamily Otomyinae
vlei rats and whistling rats

Otomyinae is one of the smaller subfamilies of muroid rodents, with 23 species in 3 genera: Myotomys, Otomys, and Parotomys. This Old World subfamily includes the vlei rats, whistling rats, and karoo rats. ()

Otomyines are native to southern and east central Africa, with an isolated population in Cameroon and Nigeria (Otomys occidentalis). ()

Otomyines live in deserts, veldts, grasslands, savannahs, swamps, marshes, stream borders, alpine meadows, and areas of brushy secondary growth. They are found at elevations up to 4,000 meters. ()

Otomyinae was given status as a subfamily by Thomas in 1896, and since then, there has been disagreement over whether the subfamily belongs in Muridae (Tullberg 1899, Miller and Gidley 1918, Simpson 1945), Cricetidae (Misonne 1971), or Nesomyidae (Chaline et al. 1977), or whether Otomyidae should be recognized as its own family (Roberts 1951). Most recent fossil discoveries and analyses of morphological characteristics link otomyines closely with murines (see summary in Carleton and Musser 1984). In fact, molecular studies based on LCAT and vWF genes (Michaux et al. 2001), IRBP gene sequences (Jansa and Weksler 2004), and GHR, BRCA1, RAG1, and c-myc genes (Steppan et al. 2004) actually show that otomyines fall within the murine clade. For now, however, Musser and Carleton (2005) classify Otomyinae as a separate murid subfamily, in recognition of the unique shared traits of its members.

Another point of contention is just how many genera should be recognized in the subfamily. The number has ranged from five (Roberts 1951) to one (Bohmann 1953), with most authorities recognizing just two, Otomys and Parotomys (Ellerman 1941, Corbert and Hill 1980, Carleton and Musser 1984, Musser and Carleton 1993). However, as Musser and Carleton (2005) note, an analysis of morphological and molecular features suggests that Otomys, as traditionally defined, is polyphyletic, and that two of the species in this genus are actually more similar to Parotomys than to other Otomys, yet they lack certain skull characteristics of Parotomys. Thus, they provisionally place these two species in a separate genus, Myotomys. ()

Otomyines are medium-sized, volelike muroid rodents with stocky bodies, rounded profiles, short hind feet, small eyes, and small ears. Their head and body length ranges from 124 to 217 mm, their tails range from 55 to 150 mm, and they weigh 60 to 255 grams. Otomyines have long, dense, soft fur that varies in color from pale buff to brown to reddish to rusty-yellow with black or brown streaks. The underparts and sides of the body tend to be paler, and are white, grayish white, cream, buff, brown, or gray.

The otomyine dental formula is 1/1, 0/0, 0/0, 3/3 = 16. They have wide, opisthodont incisors. The molars are rooted, and the crowns consist of transverse crests of enamel, with no individually demarcated cusps. The first upper molars each have three of these crests, while the second upper and second and third lower molars each have two. The third upper molars and first lower molars have a variable number of crests. Skull features include a large interparietal bone; somewhat enlarged mastoid bullae; a narrow mesopterygoid fossa that extends between the third molars; long, narrow incisive foramina; a separation of the masticatory foramen and accessory foramen ovale by a strut of the alisphenoid bone; and the presence of an accessory tympanum. Otomyines have single-chambered stomachs and complex ceca. ()

Deomyines live two to four years in captivity; lifespan in the wild is undoubtedly shorter. ()

Otomyines are terrestrial rodents. They are often described as being wary and quick to seek shelter at the slightest hint of danger. Some species are diurnal, and others are nocturnal, crepuscular, or active both day and night. They are active year round.

Some otomyine species construct nests out of sticks and other plant material and place these in burrows, on the ground surface, or in low shrubs above the entrances to burrows. Others do not build nests at all. Burrows may be complex systems of underground tunnels with multiple entrances or simple holes in the ground. Otomyines often make networks of above-ground runways through the grass, which radiate outward from a burrow or surface nest. These runways are sometimes marked by small piles of grass clippings left behind during their construction.

Otomyines are generally cursorial, but they are known to swim and dive to escape predators. Most are sedentary, but those that live near water may migrate during the wet season to avoid being flooded out.

These rodents range in social habits from gregarious, living in pairs or small family groups, to solitary, aggressive, and territorial. They seem to have well-defined home ranges, but these home ranges may overlap with one another. In one study, males were noted to be dominant over females, and heavier animals were dominant over lighter animals (Davis 1972). Also, males and females housed in pairs became mutually tolerant, whereas intrasex pairs always fought. ()

Otomyines perceive their world through vision, hearing, olfaction, taste, and touch. Of these, vision may be less important, whereas hearing and olfaction seem quite keen (Davis 1972). Among Otomys irroratus, Davis (1972) noted that individuals confronted by new objects or environments spent a long time sniffing their unfamiliar surroundings and touching unfamiliar objects with their vibrissae.

Otomyines use visual, auditory, and olfactory signals to communicate with one another. Individuals sniff one another's noses upon meeting, and during social conflicts, they often utter a metallic-sounding chirp. Body posturing also plays a role in social interactions; in one study on O. irroratus, dominant individuals were seen to approach subordinate individuals in a flattened crouch, whereas subordinate individuals adopted an upright posture (Davis 1972). They were also seen to vibrate their tails quickly back and forth as a sign of aggression. Otomyines mark their territories with urine and with secretions from their anal glands. Young otomyines that have been separated from their mothers squeak loudly so that she can locate them, and mothers seem to recognize their offpring by scent. Some species make whistling noises and/or stamp their hind feet when alarmed. ()

Otomyines are primarily herbivorous. They eat grasses, roots, shoots, bark, forbs, seeds, and berries, and sometimes insects. Otomyines have been known to practice coprophagy, especially during times of food shortage. ()

Small mammalian carnivores, snakes, and birds of prey including hawks and owls prey upon otomyines. No doubt their vigilance, agility, and cryptic coloration help them to avoid predation. ()

In general, otomyines are primary consumers. They, in turn, are preyed upon by a number of secondary consumers. In addition, otomyines are parasitized by several flea species, including Dinopsyllus titan, D. longifrons, Ctenopthalmus evidens, and C. cophurus. ()

Otomyines may carry plague in some locations. In addition, these rodents are known to damage trees in nurseries by gnawing the bark. ()

There are no known positive effects of otomyines on humans.

Currently, the IUCN considers three otomyine species to be endangered (Barbour's vlei rats, Otomys barbouri, Mt. Elgon vlei rats, Otomys jacksoni, and Uzungwe vlei rats, Otomys uzungwensis), three to be vulnerable (Burton's vlei rats, Otomys burtoni, Ruwenzori vlei rats, Otomys dartmouthi, and western vlei rats, Otomys occidentalis), and two to be near threatened (Dent's vlei rats, Otomys denti and Tanzanian vlei rats, Otomys lacustris). Another two lack sufficient data to be assessed (Afroalpine vlei rats, Otomys orestes and Dollman's vlei rats, Otomys dollmani). The species that are on the IUCN's list are there because their ranges are very small, highly fragmented, or both. Preserves have been established that may slow the decline of some species, but research is still needed that will allow the creation of comprehensive management plans. ()