RODENTS

The name Rodentia is derived from the Latin verb rodere (to gnaw), in allusion to the gnawing habits of the group. Among North American mammals, rodents are unique in that the incisors are reduced in number to one on each side above and below, in the absence of canines, and in the presence of never more than two premolars in each jaw above and one below. The dental formula varies from: I 1/1, C 0/0, Pm 0/0, M 3/3 X 2 = 16 to I 1/1, C 0/0, Pm 2/1, M 3/3 X 2 = 22. Most animals assigned to the order are small in size; some, for example the beaver, may exceed 25 kg in weight. Rodents comprise more than one-third of the known kinds of mammals, and individually they are the most abundant mammal in many sections of the world. Sixty-four species of native rodents occupy Texas, making this the most diverse group of mammals in our state.

In habits, members of this order are diverse. Most of them are nocturnal or crepuscular; ground squirrels and tree squirrels are strictly diurnal; others may be active either by day or by night. Considerable adaptive radiation occurs in the group. Some species (pocket gopher) are fossorial; others are aquatic (beaver), arboreal (tree squirrel), volant (flying squirrel), or terrestrial (cotton rat). Most rodents feed on vegetation, but a few species, notably the grasshopper mouse, feed extensively upon animal matter. Most rodents are active throughout the year, but others, notably ground squirrels, may hibernate for several months.

With over 2000 living species placed in about 30 families, rodents are by far the largest order of mammals, at least in terms of number of taxa (well over 40% of mammalian species belong to the order Rodentia!). Rodents range in size from pygmy mice weighing 5 gms to capybaras, the largest of which weigh over 70 kg. They are found around the world except in Antarctica, New Zealand, and on some oceanic islands. Ecologically, they are incredibly diverse. Some species spend their entire lives above the ground in the canopy of rainforests; others seldom emerge from beneath the ground. Some species are highly aquatic, while others are equally specialized for life in deserts. Many are to some degree omnivorous; others are highly specialized, eating, for example, only a few species of invertebrates or fungi.

Despite their morphological and ecological diversity, all rodents share one characteristic: their dentition is highly specialized for gnawing. All rodents have a single pair of upper and a single pair of lower incisors, followed by a gap ( diastema), followed by one or more molars or premolars. No rodent has more than one incisor in each quadrant, and no rodent has canines. Rodent incisors are rootless, growing continuously. Their anterior and lateral surfaces are covered with enamel, but their posterior surface is not. During gnawing, as the incisors grind against each other, they wear away the softer dentine, leaving the enamel edge as the blade of a chisel. This "self sharpening" system is very effective and is one of the keys to the enormous success of rodents.

The condition of a dominant pair of incisors used for gnawing, followed by a long diastema, is not unique to rodents, and in fact rodents are relative latecomers to this condition (even though as a group, they have a very old fossil history, going back to Paleocene times). It is even seen in a group of therapsids (ancestors of mammals), the tritylodonts, which lived during the Jurassic. Multituberculates, a very large and successful but now extinct group of early mammals, had a similar pattern. So do wombats, hyraxes, aye-ayes, and lagomorphs, to give a few examples chosen from modern mammals. Rodents have specialized in gnawing to an extreme, however, seen in few or no other groups of vertebrates.

The main muscle used in chewing by rodents is the masseter, and the rodents can be divided into several groups based on exactly how they use these muscles. These groupings have been used in several ways in the past to classify rodents.

Rodents are found native on all continents except Antarctica. One particular family of rodents, the Muridae, contains over 1100 species: over a quarter of all mammal species are rats, mice, voles, muskrats, lemmings, hamsters, gerbils, and other members of the Muridae. However, rodents show perhaps their greatest diversity of form in South America, which was an isolated continent for much of the Cenozoic. A few of these distinctive South American rodents include mountain viscachas, rabbit-like forms that inhabit dry mountainous regions; Patagonian cavies, very rabbit-like, fast-running forms with elongated ears and short tails; the coypu or nutria, a large marsh-dwelling rodent that has been introduced into North America and is hunted for its fur; and various burrowing forms such as pacas and tuco-tucos. Other South American rodents include guinea pigs, chinchillas, and New World porcupines (one species of which has dispersed into North America). The capybara (shown 

at left), yet another South American species, is the largest living rodent. About the size of a pig, and reaching a maximum weight of 50 kg (110 pounds), the capybara is truly a rodent of unusual size. Capybaras live along rivers in the llanos (plains) of South America, and are often hunted or even ranched for their meat.

Despite their great species diversity, all rodents share common features. Rodents have a single pair of incisors in each jaw, and the incisors grow continually throughout life. The incisors have thick enamel layers on the front but not on the back; this causes them to retain their chisel shape as they are worn down. Behind the incisors is a large gap in the tooth rows, ordiastema; there are no canines, and typically only a few molars at the rear of the jaws. Rodents gnaw with their incisors by pushing the lower jaw forward, and chew with the molars by pulling the lower jaw backwards. In conjunction with these chewing patterns, rodents have large and complex jaw musculature, with modifications to the skull and jaws to accommodate it. Like some other mammal taxa, but unlike rabbits and other lagomorphs, male rodents have a baculum (penis bone). Most rodents are herbivorous, but some are omnivorous, and others prey on insects. Rodents show a wide range of lifestyles, ranging from burrowing forms such as gophers and mole rats to tree-dwelling squirrels and 

gliding "flying" squirrels, from aquatic capybaras and muskrats to desert specialists such as kangaroo rats and jerboas, and from solitary organisms such as porcupines to highly social organisms living in extensive colonies, such as prairie dogs (left) and naked mole rats.

Rodents cost billions of dollars in lost crops each year, and some are carriers of human diseases such as bubonic plague, typhus, and Hanta fever. However, various rodent species are economically important as sources of food or fur in many parts of the world, and others are used extensively in biomedical research.

Shailesh kr Shukla

directoratace@gmail.com