Mammalia

Characteristics of Mammals

What is a mammal? How is it recognized from other vertebrate animals? Some body shapes and features of mammals are shared with other groups of animals that have backbones. These shared characteristics are vertebrae, an internal skeleton, and a four-chambered heart.

On the other hand, several characteristics of mammals are unique and help distinguish a mammal from something else. The most familiar of the unique mammal characteristics is that their bodies are covered with hair or fur. Hair is a form of scales and indicates a shared ancestry with reptiles. In fact, scales are still found on some mammals, as on the tails of rats. In some mammals, such as whales, the amount of hair is greatly reduced. This is a secondary characteristic and is considered an evolutionary loss from an ancestor who had hair.

The skin of mammals is unique in that it contains sweat, scent, sebaceous oil, and mammary glands. The skin of all vertebrates is an important aspect of their health and survival, but few have developed such an elaborate variety of specialized glands as the mammals. Sweat glands help to cool the animals. Scent glands help species to recognize one another. Sebaceous glands provide a special type of oily substance for the maintenance of healthy hair and the prevention of bacterial infection. Mammary glands are unique to mammals and are, in fact, the characteristic for which the group is named. The mammary glands in females secrete the nourishing milk that helps the young to grow. There are very few other groups of animals in which so much maternal care is given as is the case with mammals.

The skeletal features of mammals are similar to those of other vertebrates—fish, amphibians, reptiles, and birds—but are easily distinguishable as mammalian. Some of the more obviously distinctive features of mammals are found in the skull. It is difficult to indicate a skeletal feature completely unique to mammals, especially because there are a variety of modifications.

However, the general structural pattern of mammals is easily identified. Their teeth, which many consider to be part of the skeletal system, are what most scientists rely upon to identify mammals. Individual molar teeth have many cusps, or points. Reptiles, amphibians, and fish usually have simple cone-shaped teeth that are often replaceable throughout the life of the animal. Birds have no teeth at all. Mammals have only one set of replacement teeth, and when the second set is worn out, the animal may starve.

The heart of mammals is very efficient because it is four-chambered. Only birds and possibly dinosaurs share this characteristic. There are two atria and two ventricles for increased circulation. The great efficiency of this type of structure is important for high-energy animals, who need a great deal of oxygen to support their high rate of metabolism. Reptiles have a three-chambered heart and, consequently, have less efficient bodies because of the mixing of unoxygenated and oxygenated blood. This increased circulation pressure in mammals is the primary reason they are considered warm-blooded, or homeothermic.

Perhaps the most significant characteristic of mammals is the placental uterus. While many animals keep their eggs inside their bodies until the young hatch, the young are still living off a yolk inside an enclosed egg. The egg may hatch inside the parent, but it receives no nourishment from the parent throughout its development. Mammals, on the other hand, are often simply described as placental animals because of the remarkable temporary organ called the placenta.

Once internal fertilization has occurred within the female, the egg attaches to the placenta, a blood-rich and nourishing lining of the uterus. The egg stays there and continues to develop into a fetus. The young mammal receives all its nourishment from the mother and is completely dependent on her even after it leaves the uterus.

Classifying Mammals

The classification of mammals is complicated and always changing. The natural world does not always fit neatly into schemes of classification. The monotremes are one example. The most famous monotreme is the duckbilled platypus. The spiny anteater, or echidna, is also a monotreme. The monotremes are the only group of mammals that are oviparous, which means they lay eggs that hatch outside the body. Their ancestry is not well-known and no monotreme fossils have been dated before the Pleistocene epoch (1.6 million years ago). Scientists continue to debate whether monotremes should be placed in the mammal group, but an agreement has not been reached.

Marsupials.

Yet there are numerous agreements about classification. For example, one of the many agreed-upon criteria for deciding which groups of mammals are more primitive is based on placental care. The most primitive mammals that are fully accepted into the group are marsupials. Marsupials all have an abdominal pouch, the marsupium, in which they raise their young. The young are initially nourished in the uterus, but only for a short time, and a placenta-like organ is not very well developed.

Perhaps these mammals represent the first type of true mammal. It is believed that the early fossil mammals were marsupials. Fossils with marsupial-like skulls are found in rocks over 100 million years old. Many of today's marsupials are small, rodent-looking mammals like the brown four-eyed opossum or the ashy-mouse opossum. It is easy to imagine that mammals similar to these small creatures lived in the debris of forest floor alongside the mighty dinosaurs. They would have been hard to see and too quick for predators. Insects were in abundance and would, as they do today, compose the major diet of these tiny primitive mammals.

It is believed that marsupials reigned in the mammal world for many millions of years. Fossils of marsupials, and even filmed footage of nowextinct marsupials, like the Tasmanian devil, show that, without competition from the large dinosaurs and reptiles, they grew to very large sizes. Many became large carnivorous marsupials, like the saber-toothed Thylacosmilus. This mighty predator was the size of a jaguar and very dangerous in appearance.

Marsupial-type fossils are found over most of the planet, with the exception of the oceans. Fossil opossums have even been found on Antarctica. They are believed to have survived on this vast expanse of land when the climate there was more comfortable for life. When South America split from the large supercontinent of Pangea, it became a type of continental raft on which species of marsupials evolved in isolation from their cousins in Africa and Europe.

During the Ice Age, when tremendous amounts of water were tied up in ice sheets, the Isthmus of Panama was exposed, enabling marsupials to travel north into North America. Although many have since become extinct, it is still possible to see frequently the hardy opossum, Didelphis, in most regions of Mexico and the United States. In South America many marsupials still live in the forests, although their numbers are dwindling.

The last stronghold for marsupials is the continent of Australia. Because of its isolation from the other continents after the split-up of Pangea, the more evolved and efficient placental mammals never reached Australia. It is as though time was frozen for the marsupials in Australia. Delightful animals, like the kangaroo, koala, wombat, and numbat, still exist in this remote continent. However, as in the rest of the world, they are under threat from the placental mammals. True mammals, like house cats and rabbits, are making it hard for the small marsupials to survive. Rabbits and other grazers are competing for grasslands. The eucalyptus forests necessary for the survival of the koala are threatened. However, many human steps are being taken to help preserve these mammals in the wild and there are still many marsupials found in Australia.

Placental mammals.

In every corner of the world, placental mammals thrive. They exist as two major types of animals, carnivores and herbivores. The story of the herbivores is as complicated as is that of the smaller groups of these mammals.

Rodents are often considered a nuisance, but their success at survival under the most extreme conditions is undisputed. They are hardy and reproduce quickly. Despite the efforts of many farmers, home dwellers, and urban developers, rodents have managed to live side by side with humans and even have secured an entire means of survival from the waste of humans, another highly successful placental mammal. Rodents are at the base of many food chains in the wild and in cities.

Around forty million years ago, an evolutionary adaptation of grasses provided the stimulus for a burst of evolution among the herbivores. Grasses developed a metabolic way to survive in more arid conditions. These new species are called the C4 grasses. These grasses, like crabgrass, are a type of grass that can survive in hostile environments. As these tougher species of grass became available in regions like flat plains and plateaus, the animals that ate the grass ventured into more open space. While this was good for herbivores because it expanded their food resources, it also became easier for predators to see them.

The animals that survived this evolutionary explosion were those with increased running ability. Grazing mammals evolved longer and narrower legs which they used to elude predators. Groups such as the gazelle, antelope, and horse became the fastest long-distance runners on Earth. They still exist and show no signs of slowing down.

In response, the predators also became faster. The large cats are unanimously considered the fleetest predator on land. The amazing speed of the cheetah has been recorded many times. It is heralded as the fastest land animal alive. Before their extermination by humans, many species of large cats roamed over most of the planet. The American lion and saber-toothed cats were only recently, in geologic terms, eliminated from Earth.

Bears, many of whom are actually omnivores, are considered by most to be the mightiest land predator on the planet today. The strength of the grizzly, kodiak, and polar bear is legendary. They are surprisingly fast runners, and very few animals are prepared to withstand combat with them.

Mammals survive not only on land but in the ocean as well. Whales, dolphins, seals, walruses, otters, and other marine mammals are very successful in their ways of life. The killer whale may be the largest predator on Earth. They feed on other marine mammals and are especially fond of seals.

The largest animal on Earth is a mammal. The blue whale is estimated to weigh around 150 tons and is almost 27 meters (89 feet) long as an adult. It feeds on tiny krill, a shrimplike animal that it catches with a the help of a baleen, a sievelike structure in its mouth.

Many species of whales had been hunted to extinction by the 1960s. Fortunately, an international ban was placed on the hunting of these magnificent mammals and many species are increasing in number.

It is surprising to read that the ancestors of whales were land dwellers, a group of mammals called creodonts. Fossil evidence indicates that these animals may have become increasingly adapted to a marine environmment until they evolved to a completely water-based existence. They are still mammals, however, as evidenced by the formation of a placenta, live birth, and young that suckle for breast milk. Baby whales, although quite large, are completely dependent on their mothers for a long time.

One may find a mammal just about anywhere on Earth. They have evolved to fill almost every possible niche and continue to flourish despite harsh environmental and climatic changes. The current loss of mammals, however, continues at an alarming rate and, over the last 20,000 years, the loss of mammal species, like the mammoth, is staggering. It is hoped this trend will not only halt but, in cases of endangered mammals, be reversed.

Ellen Brook Hall

Bibliography

Hickman, C., L. Roberts, and F. Hickman. Integrated Principles of Zoology. St. Louis, MO: Times Mirror/Mosby College Publishing, 1984.

Lillegraven, J. A., Z. Kielan-Jaworowska, and W. Clemens. Mesozoic Mammals: The First Two-Thirds of Mammal History. Berkeley: University of California Press, 1979.

Macdonald, David. The Encyclopedia of Mammals. New York: Facts on File, 1984.

During the Mesozoic era, marsupials were very common in North America, more common, it is thought, than placental mammals. Marsupials persisted in this part of the world until the mid-to late-Tertiary period.