Rats are rodents—that is, members of the order Rodentia (see TAXONOMY). Rodents are further classified into the suborder Simplicidentata (with one pair of incisor teeth), these then being divided into three infraorders: the Sciuromorpha (beavers, chipmunks, wood-chucks and others); Myomorpha (including the genera Mus, mice, and Rattus, rats); and hystricomorpha (guinea pigs, porcupines, capybaras and other large South American rodents). Rabbits and hares are classified separately into the order Lagomorpha duplicidentata (having an extra pair of upper incisors). Rodents are by far the largest order of the class Mammalia, accounting for an estimated 40% or so of all mammals, with rats accounting for over 60% of these. Rodents are classified together by various features, by far the most prominent of which is their dentition. (The name rodent derives from Latin, rodere: to gnaw.) Rodents possess large incisors at the front of the jaw which are edged like chisels: the enamel of the incisors is not present to a significant degree at the back of the tooth, which therefore wears away much faster than the front, allowing the enamelled front edge to become sharpened. The incisors also have no roots and grow continually: all rodents must gnaw on hard objects in order to prevent excessive growth of their incisors. Behind the incisor teeth is a gap (rodents have no canine teeth) filled by a hairy pad (that projects in from the cheek). Behind this are the molar teeth; premolars are also absent.
There are various species of rat: Rattus rattus (the black rat) and Rattus norvegicus (the Norway rat, or common brown rat) are the most common. The name Norwegian rat is something of a misnomer.
The brown rat is thought to come originally from Asia, arriving in Great Britain in about 1730, before it reached Norway. Since its arrival in Great Britain, the brown rat has steadily displaced the indigenous black rat, which is now found only in a few parts of the western British Isles, most notably on some of the Western Isles off the coast of Scotland. Sadly, unlike the disappearance of the red squirrel, under pressure from the non-indigenous grey, no one seems to care about the loss of the British native black rat. In the scientific literature, rats are typically identified by their English names: Lister hooded rats have a white body and black hood around their heads. SPRAGUE-DAWLEY and WISTAR RATS are albinos (white fur, red eyes) much used because their genetic constitution is relatively invariant. There are also a variety of rats with specific genetic mutations: for example, there are spontaneously hypertensive rats and rats with movement disorders; ZUCKER RATS are obese.
Rats have become the animal of choice in most behavioural experiments for a variety of reasons: at a mundane level they are inexpensive to purchase and to maintain; they are of a suitable size for housing in large colonies; and they are an appropriate size for behavioural testing. (Mice, in contrast, are so small that behavioural testing equipment requires too great a degree of miniaturization.) More importantly, rats are behaviourally agile and can accomplish a wide variety of tests (in contrast to pigeons, for example, which do little other than flap their wings and peck); and rats have what might be considered a prototypical mammalian brain. They possess all of the major structures of the brain of PRIMATES, including an extensive development of the CEREBRAL CORTEX, unlike birds, reptiles or amphibia. Similarly, their general physiology replicates closely that of the higher orders of the class Mammalia. These features do not make them identical to primates and humans, but they make rats acceptable experimental substitutes. While one would not leap directly to clinical trials in humans after studying only rats, what one learns from studying a rat’s brain provides a good approximation of what one can expect to find in a primate or human brain.
