9.2: Background Information on Mammals
Mammalian Characteristics
Mammals, comprising about 5,200 species, are vertebrates that possess hair and mammary glands. Several other characteristics are distinctive to mammals, including certain features of the jaw, skeleton, integument, and internal anatomy. Modern mammals belong to three clades: monotremes, marsupials, and eutherians (or placental mammals).
The presence of hair, composed of the protein keratin, is one of the most obvious characteristics of mammals. Although it is not very extensive or obvious on some species (such as whales), hair has many important functions for most mammals. Mammals are endothermic, and hair traps a boundary layer of air close to the body, retaining heat generated by metabolic activity. Along with insulation, hair can serve as a sensory mechanism, which is particularly useful to nocturnal or burrowing mammals. Hair can also provide protective coloration or be part of social signaling, such as when an animal’s hair stands “on end” to warn enemies, or possibly to make the mammal “look bigger” to predators.
Mammals, like birds, possess a four-chambered heart; however, the hearts of birds and mammals are an example of convergent evolution, since mammals clearly arose independently from different groups of tetrapod ancestors. Mammals also have a specialized group of cardiac cells (fibers) located in the walls of their right atrium called the sinoatrial node, or pacemaker, which determines the rate at which the heart beats. Mammalian erythrocytes (red blood cells) do not have nuclei, whereas the erythrocytes of other vertebrates are nucleated.
The kidneys of mammals have a portion of the nephron called the loop of Henle or nephritic loop, which allows mammals to produce urine with a high concentration of solutes—higher than that of the blood. Mammals lack a renal portal system, which is a system of veins that moves blood from the hind or lower limbs and region of the tail to the kidneys. Renal portal systems are present in all other vertebrates except jawless fishes. A urinary bladder is present in all mammals.
Mammalian brains also have certain characteristics that differ from the brains of other vertebrates. In some, but not all mammals, the cerebral cortex, the outermost part of the cerebrum, is highly convoluted and folded, allowing for a greater surface area than is possible with a smooth cortex. The optic lobes, located in the midbrain, are divided into two parts in mammals, while other vertebrates possess a single, undivided lobe. Eutherian mammals also possess a specialized structure, the corpus callosum, which links the two cerebral hemispheres together. The corpus callosum functions to integrate motor, sensory, and cognitive functions between the left and right cerebral cortexes.
Eutherians (placentals) are the most widespread and numerous of the mammals, occurring throughout the world. Eutherian mammals are sometimes called “placental mammals” because all species possess a complex chorioallantoic placenta that connects a fetus to the mother, allowing for gas, fluid, and nutrient exchange. There are about 4,000 species of placental mammals in 18 to 20 orders with various adaptations for burrowing, flying, swimming, hunting, running, and climbing.
Body Plans
Animal body plans follow set patterns related to symmetry. They are asymmetrical, radial, or bilateral in form as illustrated below. Asymmetrical animals are animals with no pattern or symmetry; an example of an asymmetrical animal is a sponge. Radial symmetry describes when an animal has an up-and-down orientation: any plane cut along its longitudinal axis through the organism produces equal halves, but not a definite right or left side. This plan is found mostly in aquatic animals, especially organisms that attach themselves to a base, like a rock or a boat, and extract their food from the surrounding water as it flows around the organism. Bilateral symmetry is illustrated in the same figure by a goat. The goat also has an upper and lower component to it, but a plane cut from front to back separates the animal into definite right and left sides. Additional terms used when describing positions in the body are anterior (front), posterior (rear), dorsal (toward the back), and ventral (toward the stomach). Bilateral symmetry is found in both land-based and aquatic animals; it enables a high level of mobility.
Vertebrate animals have a number of defined body cavities. Two of these are major cavities that contain smaller cavities within them. The dorsal cavity contains the cranial and the vertebral (or spinal) cavities. The ventral cavity contains the thoracic cavity, which in turn contains the pleural cavity around the lungs and the pericardial cavity, which surrounds the heart. The ventral cavity also contains the abdominopelvic cavity, which can be separated into the abdominal and the pelvic cavities.
Within these body cavities you will find the organ systems. Organ systems are composed of various organs and structures with a common function. The chart below summarizes the major systems.
|
Organ Systems |
Function |
|---|---|
|
Digestive |
Processing food involves ingestion (eating), digestion (mechanical and enzymatic breakdown of large molecules), absorption (cellular uptake of nutrients), and elimination (removal of undigested waste as feces). |
|
Nervous |
organism’s control center: it processes sensory information from outside (and inside) the body and controls all behaviors—from eating to sleeping to finding a mate. |
|
Endocrine Musculoskeletal Respiratory |
plays a role in growth, metabolism, and sexual development through the use of hormones provide support to the body and allow for a wide range of movement. facilitate gas exchange by bringing in oxygen and getting rid of carbon dioxide |
|
Circulatory |
supplies the cells, tissues, and organs with oxygen and nutrients, and removes carbon dioxide and waste, |
| Urinary | function to filter blood and maintain the osmolarity of body fluids |
| Immune | diverse array of specialized cells and soluble molecules that coordinate a rapid and flexible defense system capable of providing protection against pathogens |
| Reproduction | Produce gametes and develops offspring |
Fetal Pigs
Fetal pigs are commonly used as dissection specimens to represent mammalian anatomy. Nearly all the anatomical structures are similar between pig and human. Dissection is a powerful tool that provides us with a profound understanding of our own anatomy and physiology as living, breathing creatures and also helps us to develop a stronger understanding of evolutionary relationships between taxonomic groups.
Materials needed:
- Instructions provided by your professor
- Color pencils
- Scissors
- Tape
Activity
For this lab we are going to create a 3D paper based model to study mammalian anatomy and compare it to human anatomy. Your professor will hand out the necessary materials and instructions for you to color and construct your own 3D pig model.
For Review Only, will not be directly linked to in final copy
CLICK HERE ! To view the 3D paper based model activity