The rodents have nine families, 37 genera and 55 species. They can easily be identified through their dental formula and the characteristics of the teeth. Rodents vary in body size, colour, and size of tail and limbs, and hair texture. They occur (often in large numbers) in most habitats.
The characteristics of their teeth are as follows:
1. The two upper and lower incisors are chisel-like and are used for gnawing and biting; they grow and are worn out continuously throughout life.
2. Behind the incisor teeth there is a gap resulting from the absence of canine teeth (diastema).
3. The cheek teeth (3/3 or 4/4 or 5/4) at the back of the mouth are for grinding and chewing and gradually wear away during life so their appearance changes with age.
4. The structure and number of cheek teeth distinguish each family (and some species), and are very useful for identification.
5. The jaw musculature and the form of zygomatic arch are also used to distinguish the three main groups of rodents (Sciuromorpha represented by the squirrels, beavers and marmots; Caviomorpha represented by the Guinea pigs and including porcupines; and Myomorpha, the mice and rats).
Rodents vary in size from the small pigmy mouse (10g) to the large crested porcupine (Hystrix cristata), which weighs from 15 – 20 kg. There is a great variation in external form, colour, size of tail and limbs and hair texture between families and species. They live in most habitats from the rainforest of the South to the semi-arid regions of the extreme North; the majority of the rodents are ground-living, some are arboreal and one species is fossorial (e.g. Giant rat).
None is aquatic although some species prefer damp, swampy habitat and none is capable of true flight although the ‘flying-squirrels’ (family Anomaluridae) can glide from tree to tree. As an order, the rodents are extremely successful.
They are often very numerous but their total biomass is small in relation to that of a few larger herbivores (e.g. a kob that is approximately 60kg has about the same biomass as about 1620 Dalton’s mice weighing 37g each). They consume a variety of seeds, grasses and insects and they form a major part of the diet of many aerial and terrestrial animals (Happold, 1987).
Many species are important to man; some may become pests of crops and food stuffs e.g. Multimammate mouse, the Nile rat, the Savannah Gerbil, Greater Cane rat, some are potential carriers of human diseases (Multimammate mouse, Black rat, Norway rat) and others are important sources of meat in some part of the country (e.g. Gambian giant rat(Cricetomys gambianus) and the Greater cane rat (Thryonomysswinderianus).
Characteristics of Order Rodentia
1. One pair of upper incisors
2. Ears small and held close to head; eyes small, tail long or short with small scales and/or bristles or short hairs, tuft of hairs at the tip of tail in some species.
3. Limbs mostly short in relation to body
4. Movement by scampering, trotting, climbing and jumping; live on ground or in trees; may borrow and rest underground.
Common Rodents Species
Rodents got their name from the Latin word ‘rodens’, meaning gnawing and are, therefore, animals which gnaw, or grate away with the front teeth substances like wood, tree back, nutshells, cassava tubers, etc. Rodents are easily distinguished by the large, chisel stayed incisor teeth and by the absence of canines
1. Flying Squirrel
Flying squirrels are medium-sized arboreal rodents, easily identified by the flap of skin, or patagium, on either side of the body between the fore and hind limbs. When the limbs are extended, the patagium forms a tightly stretched membrane which increases the surface area of the body and enables flying squirrels to glide from tree to tree.
These creatures do not really fly. In fact the flying squirrel glides, and is not capable of upward movement through the air. There are two species of flying squirrel, namely: Anomalurus beecrofti and A. derbianus.
2. Geoffroy’s Ground Squirrel (Xerus erythropus)
The ground squirrel is heavily built with sparse coarse pelage. Dorsal pelage is brownish-yellow to sandy-orange, lightly speckled, hairs brown, usually with black tips and a pale
subapical band. Ventral surface is covered with sparse white hair. There is conspicuous white side-stripe, and white lines above and below each eye.
The tail is bushy and similar to dorsal pelage, each hair with sub-apical black band and long white tip. The ground squirrel is exceptional in that it runs entirely on the ground and lives in burrows. It has very powerful legs for digging.
They are unable to climb trees. They prefer grasslands, especially where there are small clearings, and open bush-scrub habitats in the North of Nigeria. Ground squirrels feed mostly on palm nuts, seeds, bulbs, small insects and sometimes on Agriculture root crops. They often have so many food-stores around their burrows.
Sometimes, the seeds in the food stores germinate instead of being consumed. They make a variety of vocal sounds including alarm call. When alert, the tail is held over the back, and flicked back and forth with the hairs erect; when frightened the squirrel runs with the tail in line with the body. The tail is almost dragged on the ground with the hairs flat when the squirrel is completely at ease.
3. Black Rat (Rattus Rattus)
This is a large dark, fairly slender rat. Dorsal pelage dark-grey to grey-brown, with black elongated gaurd hairs extending beyond pelage, or dull grey. Hairs long and rather coarse. Ears large and hairless. Fore and hind feet relatively large, upper surfaces covered with brownish hairs.
Tail thin and very long, covered with small scale and very small dark bristles. Black rats are introduced species yet extremely prolific, have 5-10 young in a litter and a female may have up to six litters per year.
4. Gambian Giant Rat (Cricetomys Gambianus)
A large, thick-set rodent often called pouched rat, pelage greyish to greyish brown, tending to dark grey in southern localities. Flanks paler merging into whitish ventral pelage. Upper lips, chin, throat and chest whitish. Body hair coarse and rough.
Elongated face, pointed nasal region, very long vibrissae (Whiskers), relatively small eyes, and large fleshy ears. Tail long and thick, basal half brown, terminal half white. It feeds on fruits, vegetables, seeds, yams, maize, oil palm nuts, and other local crops. Litters contain 3-5 young; eyes open at about 20-23 days and weaning occurs at about 4 weeks. Attains adult size in about 5-6 months.
5. Crested Porcupine (Hystrix Cristata)
A large rodent with the dorsal and lateral hairs, especially on posterior part of back, modified into stout smooth quills. Each quill (about 30cm) with alternating wide black and narrow white bands, and white point tip.
Head, neck, and limbs covered with coarse bristles (up to 50mm). Crest of long, wiry hairs (up to 45cm), mostly white with black base, on week and shoulders. Crest and quills erectile. Head rounded and blunt, with small eyes and ears. Limbs relatively short with strong digits and claws.
Tail short, usually not visible, covered with short weak quills, quills at tip of tail elongated, widening to an open ‘cup’ which rattles when tail is shaken. The crested porcupine looks larger than it really is because of its erectile crest and quills. Quills from some localities, are reddish, especially on the white band and tip.
6. Grass Cutter (Thryonomys Swinderianus)
The grass cutter sometimes called greater cane-rat is the second largest species of rodent in Nigeria for instance after the crested porcupine. Dorsal pelage deep brown to rufous brown, flecked with yellow and black.
Dorsal hairs thick and coarse, mostly brown with yellow band at terminal end (usually) black tip. Flanks similar to dorsal pelage, merging into greyish-white ventral pelage. Body thickset, head broad with short flattened muzzle, small eyes and ears. Upper incisor teeth wide and strong, each with three grooves on anterior surface.
Limbs short and powerful, well developed pads and claws on feet. Tail short (about 40% of HB), covered with small hairs. Litter size between 2-6 young (average 4). The young are precocious, fully furred, capable of running, and have their eyes open at birth. Attains sexual maturity at about 5 months.
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