The mammals represent the highest adaptive level of the vertebrate plan of organization. The ultimate basis for their success as a group is the high degree of efficiency achieved by their homeostatic mechanisms, largely as the result of the culmination of certain morphological, physiological, and behavioral trends initiated at lower levels of vertebrate organization and improved by the lines leading to the mammals.

This exercise will serve as an introduction to the structure of mammals as a background for a better understanding of their classification and biology. It will also afford an excellent opportunity to review many of the general trends that have occurred within the vertebrates.

Obtain a specimen of the albino rat or mouse. The albino rat is a laboratory strain of the Norway or brown rat (Rattus norvegicus), a species that has been widely introduced around the world. The albino mouse is a laboratory strain of the House mouse (Mus musculus) also with worldwide distribution.

Parts of the body. Identify the following major divisions of the body: head, neck, trunk, and tail. The relative proportions of these basic divisions vary in different mammals, often showing a close correlation with habits.

The head is terminated by the fleshy nose or rhinarium bearing the nostrils or external nares. The mouth is relatively small and is founded by well developed upper and lower lips. Note that the upper lip is cleft on the midline and has flaps on each side that extend inward behind the teeth. What is the significance of this arrangement, which is characteristic of rodents and lagomorphs (rabbits and their allies)? A single pair of upper and lower incisor teeth are visible. Note the chisel-like shape of these teeth produced by uneven wear of harder enamel on the front surfaces and softer dentine on the posterior surfaces.

The rather small eyes are located about midway in the head. The portion of the head anterior to the eyes may be termed the facial region and the expanded part to the rear of the eyes, the cranial region. The eyes are provided with well developed upper and lower lids and a nictitating membrane.

The auditory organ of mammals consists of three portions, the inner, middle, and external ear. In most mammals, as in the rat, the external ear is provided with a well-developed fold, the pinna. The opening enclosed by the base of the pinna is the external auditory meatus, which terminates at the tympanum or ear drum. The latter structure marks the beginning of the middle ear. Review the function of the different parts of the mammalian ear and compare its structure with that of lower vertebrates. Consider the significance of any differences that may be apparent. Remember that the origin of the mammalian ear ossicles is connected to the jaw articulation apparatus of reptiles.

The area of the trunk containing the anus and openings of the urogenital system is termed the perineum. The anus is situated at the base of the tail on the ventral side of the body in both sexes. In females, the urogenital papilla is located just in front of the anus. The urethra opens at the apex of the urogenital papilla; and the vulva, the areas surrounding the opening to the vagina, is situated between the base of the UG papilla and the anus. In virgin females or sexually inactive adults, the vaginal opening may be closed by a thin membrane, the introitus vaginae.

The penis of the male lies well forward to the anal aperture and is sheathed by the foreskin or prepuce. In breeding males the enlarged testes occupy an outpocketing of the body wall termed the scrotum. When fully developed the scrotum loses much of its hair, and its skin becomes heavily pigmented and wrinkled. Note - many mammals do NOT posses a scrotum, but have abdominal testes.

In female specimens several pairs of teats or nipples will be found on the ventral aspect of the trunk. These constitute the openings of the mammary or milk glands, which are characteristic of mammals. Determine the number and position of the teats in the rat.

Measurements: Measurements of certain parts of the body are important in the description and identification of mammal specimens. These measurements are taken in the flesh and are usually expressed in millimeters. The standard external measurements taken or mammals are defined in most mammalogy texts and should be reviewed with reference to your specimen.

Body covering: The body is clothed with a dense covering of hair, constituting the pelage. Three general types of hairs can be recognized. The long, stiff bristle-like hairs on the sides of the face are vibrissae. They have a tactile function. The general pelage is composed of two types of hairs, long, stiff guard hairs and shorter, softer, and more numerous underfur or velus.

The hair is an epidermal derivative, apparently derived de novo in mammals. It consists of a column of densely packed, keratinized epidermal cells. The hair develops from a follicle, which is a tiny pit in the skin representing an invagination of the Malpighian hyper of the epidermis. That portion of the hair within the follicle is termed the root, while the exposed part is the shaft. The latter is typically composed of medulla, cortex, and cuticle. Examine demonstrations of the microscopic structure of hair.

Epidermal scales homologous to those of reptiles are also found on the bodies of some mammals. In the rat such scales are preset on the tail and feet.

Skull: The skull of mammals has undergone a reduction in the total number of bones through fusion and loss of elements present in lower vertebrates. A trenchant feature of the mammalian skull is the presence of only a single bone, the dentary, in the lower jaw and the shift from a quadrate-articular to a dentary-squamosal jaw suspension. Other characteristic of the mammal skull include: 2 occipital condyles; development of zygozatic arches; three ossicles in the middle ear, representing the columella (stapes), quadrate (incus), and articular (malleus) bones; secondary palate composed of medial extensions of premaxillaries, maxillaries, and paletine bones; and heterodont dentition.

Dorsal view of dog skull

Ventral view of dog skull

Lateral view of dog skull

Obtain the skull of a larger mammal, such as a bobcat or fox, and identify the following principal bones and other structures by reference to the figures above and below: Premaxillary bones, frontal bone (with postorbital process), occipital bone, parietal bone (fuse at the dorsal aspect to form sagittal crest), squamosal bone, zygomatic process of squamosal, temporal fossa, zygomatic arch, orbit, jugal bone, maxillary bone, nasal bone, incisive foramen, external nares (nostrils), palatine bone, foramen magnum, foramen ovale, occipital condyle, basioccipital bone, tympanic bulla, basisphenoid bone, external auditory meatus, mandibular fossa, pterygoid bone, vomer bone, infraorbital foramen, tympanic (auditory) bulla.

Familiarize yourself with the portions of the typical mammalian dentary bone, as exemplified by the lower jaw of the dog (seen below). You should know the following regions: angular process, condyloid process, and masseteric fossa

Note also the differentiation of the dentition into incisors, canines, and cheek teeth. The latter are further divisible into premolars and molars. How many premolars and molars are differentiated? What bones of the skull bear teeth in mammals?
 

Dental Formulae

Dental formulae are used to indicate the number of each type of tooth for a given species. Because the jaw is bilaterally symetrical, only one half of the jaw is described. The incisors are indicated first, followed by the canine, promolars and molars. The maxillary arcade or upper jaw is listed over the mandibular arcade or lower jaw. For instance, a dog has 3 incisors, 1 canine, 4 premolars and 2 molars on one side of the upper jaw and 3 incisors, 1 canine, 4 premolars and 3 molars on one side of the lower jaw, so the dental formula would be: