Rat (Rattus rattus)

 Introduction

                     In general, orders of mammals are easily recognized and differentiated on the basis of external appearance.  For example, the external anatomy of a rat, a cat, a whale and Biology 1115 student are all very different.  Features such as the appearance of the limbs, the method of locomotion and characteristics of the teeth enable us to differentiate these mammals.  All mammals, though, have two unique external characteristics (as well as other internal characteristics), which distinguish them from all other vertebrate classes:

a)      all mammals have hair at some time during their development.

b)      all female mammals possess mammary glands with external openings, for nourishing their young.

            The major mammalian characteristics can be observed from the study of any mammal.  For this purpose, the white rat is used.  It is an albino strain of the European house rat.  The rat is a rodent (Order Rodentia).  Rodents have enlarged front teeth and other adaptations for a gnawing, herbivorous mode of life.  The rodents are the most numerous mammals in both number of genera and species, and number of individuals.  There are no flying or marine rodents but they do occupy almost every known terrestrial habitat.  Other familiar rodents are squirrels, mice guinea pigs, porcupines and the beaver.

 General Features

            The rat is a tetrapod with digitigrade locomotion, meaning the animal walks on the digits only, with the remainder of the foot elevated.  The principle covering of the body is hair.  It is derived from the epidermis of the skin.  The scales covering the tail in the rat are also epidermal in origin as are the rat’s claws.  The body of the rat is divided into four general regions: head, neck, trunk, and tail , which are discussed in more detail in the following paragraphs.

Head and Neck

The head is large and separated from the trunk by a distinct, moveable neck.  Examine the head.  The mouth is bounded by well developed upper and lower lips.  The upper lip is deeply cleft in the rat.  A pair of large upper incisor teeth and a pair of lower incisor teeth should be visible.  External nares are situated on a more or less flexible nose.  The eyes are protected by upper and lower eyelids.  The ear possesses an external fold called the pinna which directs sound waves into the ear canal (external auditory meatus).  A tympanic membrane lies out of sight at the base of this canal.  The rat, as well as many other mammals, possesses special long sensory hairs called vibrissae (Figure 2 and 3).  Vibrissae grow out from the snout, upper eyelid, cheek and chin of the rat.

Trunk

The trunk is divided into an anterior thorax and a posterior abdomen.  Carefully feel along the ventral surface of the trunk.  The thorax contains ribs but the abdomen does not.  Teats or nipples (the external opening of the mammary glands) can also be located in two rows on the ventral surface of the trunk (Figure 3).  There are usually twelve teats in the rat, evenly distributed between the thoracic and abdominal regions.

Most mammals have separate urogenital and anal openings.  In female rats, the urinary and genital openings are also separate.  In females, the urethral opening is the most anterior with the vaginal orifice just posterior to it.  The anus is located at the base of the tail, posterior to the vaginal orifice (Figure 3).  In males, the urethra is a common urogenital duct and opens at the tip of the penis.  Associated with this, at the base of the tail, is the sac-shaped scrotum containing the testes (Figure 2). 

Tail

The tail of the rat has only a few scattered hairs which are distributed between reptile-like scales of epidermal origin.

        

Figure 2: External Features of Male Rat (Ventral View).

         

Figure 3: External Features of Female Rat (Ventral View).