Mammalia (mammals)

Life > Eukaryotes > Opisthokonta > Metazoa (animals) > Bilateria > Deuterostomia > Chordata > Craniata > Vertebrata (vertebrates)  > Gnathostomata (jawed vertebrates) > Teleostomi (teleost fish) > Osteichthyes (bony fish) > Class: Sarcopterygii (lobe-finned fish) > Stegocephalia (terrestrial vertebrates) > Reptiliomorpha > Amniota > Synapsida (mammal-like reptiles) > Therapsida > Theriodontia >  Cynodontia

Aepyceros melampus (Impala) fawn suckling from mother. Production of milk for feeding of infants is one of the defining characteristics of mammals. [photo Arno Meintjes ©]

Long before the evolution of dinosaurs and birds, there existed a group of reptiles that had some mammalian features. These mammal-like reptiles occurred worldwide, but one of the richest sources of their fossils is in the Karoo (South Africa), where fossils have been found showing the sequence of anatomical changes that occurred during the evolution of mammals from reptiles. It is worth paying a visit to the Iziko South African Museum in Cape Town to see these fossils on display.

Particularly important changes occurred during the transition from reptiles to mammals. These include a change from the sprawling posture of reptiles; the loss of all but one paired bone in the lower jaw; the development of grinding and slicing teeth; a palate separating the mouth from the nasal passage (thus making it possible to chew or suckle, and breathe, at the same time); and a larger brain. Many of these features are associated with the need to find and rapidly process food to provide the energy needed to keep body temperatures above ambient, because mammals are "warm-blooded" or endothermic. Other typically mammalian features that cannot be seen in fossils include hair, and milk-producing mammary glands. When they are born young mammals are completely dependent on milk obtained from their mothers, this highly nutritious food enabling them to grow and develop rapidly. During the phase of rapid growth they have one set of teeth (the milk teeth), which are later replaced by adult dentition, the back teeth, or molars, only erupting in this second set.

The earliest mammals, the Prototheria, have young that hatch from a cleidoic egg. Indeed the most primitive living mammals belong to this group of egg-laying mammals but they are also endothermic, have hair, and feed their young on milk. These primitive living mammals are the monotremes: the duck-billed platypus and the spiny anteaters. The group is presently represented by only three species, which are found in Australia and New Guinea.

More advanced living mammals are grouped together as the Theria; all of them have a placenta that links the mother to the foetus developing within her uterus. Within the Theria are two major groups, distinguished from each other mostly by their type of reproduction. The Metatheria, or marsupials (including kangaroos, wallabies, possums, koalas), have a poorly developed placenta and a very short gestation period, giving birth to tiny under-developed young that are (usually) reared in a warm moist pouch in which the mammary glands are situated. About 250 species of marsupials occur today, mostly in Australia and South America. The other group, the Eutheria, or placental mammals, have well developed placentas, longer pregnancies and no pouch. The degree of development at birth varies, some newborn eutherian mammals (cats, dogs and humans, for instance) being helpless, while others (such as the antelopes) are up and running within minutes of birth.

The differences in the reproductive patterns of Metatheria and Eutheria reflect different evolutionary responses to the stresses of the environment. The eutherians produce well developed young with a fair chance of survival, but at a high and prolonged cost to the mother. Female marsupials invest less energy in each embryo and can abandon the pouched babies should environmental conditions become harsh; the females can also rapidly produce more babies once conditions improve. In both groups the period of dependency on the mother provides an opportunity for learning, something particularly important in mammals (such as many carnivores, elephants and primates) with a long childhood, where strong social bonding and exchange of information occurs within the family.

For perhaps a hundred million years, the mammals were small, nocturnal, and rather insignificant creatures. It was only after the extinction of the dinosaurs at the end of the Cretaceous that mammals underwent adaptive radiation, occupying niches left vacant by the demise of the dinosaurs. In time the mammals became the dominant land vertebrates. Today the Eutheria are the most abundant of all mammals, with about 3800 living species. Over a third of these are insectivores and bats.

The major groups of eutherian mammals are classified largely by using features of the skull and teeth. Some have very few living representatives while others are widely distributed throughout the world. Two of the smaller groups that occur in southern Africa are of particular interest - the Hyracoidea (hyraxes or dassies) and the Tubulidentata (aardvark). Dassies display a curious mix of features and, surprisingly,their closest ancestral links seem to be with the Perissodactyla (the horses and rhinos), the Proboscidea (the elephants), and the Sirenia (the manatees and dugongs). The ancestry of aardvarks is even more uncertain. They have simple peg-like teeth constructed quite different from those of other mammals. Aardvarks occur only in Africa south of the Sahara, living exclusively on termites that are eaten with a very long tongue.

Most of the larger orders of mammals have representatives in southern Africa. Within these orders there are often interesting distribution patterns, however. For example, although the Artiodactyla (even-toed ungulates) have a worldwide distribution, the sub-order that includes the deer (the group that sheds its antlers each year) is totally absent from Africa, where numerous species of antelope are found instead. A similar situation is found in the Primates, the monkeys of Africa (Old World monkeys) belonging to a different group from those of the American continent (New World monkeys).

Several groups of marine mammal have land-dwelling ancestors. Among these, the Cetacea (whales and dolphins) are the only mammals that never come onto land. The prolonged and deep diving that the Cetacea undertake calls for extreme anatomical and physiological adaptations. The Cetacea havevery large brains and are among the most "intelligent" of all animals.

Mammals, like the birds (and possibly some of the dinosaurs) show considerable control over their body temperatures: they are endothermic, meaning that body heat is produced within the animal. Other vertebrates are ectothermic, depending on environmental sources of heat to reach a suitable body temperature. Endothermy is a superb way to become relatively independent of many of the stresses of the physical environment, but it is only achieved through the expenditure of a lot of energy. In order to reach a body temperature above ambient, endotherms need to eat a lot more food than ectotherms do.

Many mammals, including humans, maintain an almost constant body temperature. Some mammals, however, allow their body temperature to rise and fall quite considerably. Thus, many small insect-eating mammals (such as many bats) conserve energy by reducing their internal heat production. During the day, when they are not active, body temperature drops and they become torpid. At nightfall they warm up again by metabolising fat, and then go out and hunt for food. In the desert, camels need to conserve both energy and water. To do this they let their temperature drop during the cold desert night until eventually, on very cold nights, the temperature falls as low as 34.5o C. The animal then begins to shiver, using energy to maintain its temperature at that level. After daybreak, the sun warms up the body. (This can take quite a long time because camels are large animals.) If the day is very hot, and the body reaches a temperature of 40.5oC, the camel begins to cool itself by evaporative water loss. By allowing these fluctuations in body temperature, a camel can reduce its total daily water requirements by half. Gemsbok in the Namib desert maintain their body temperatures in much the same way as camels do but, in addition, are able to keep their brains cool and undamaged at a body temperature of 45oC, a temperature that would otherwise be lethal. The information on gemsbok comes from the research of the late Prof Gideon Louw, who was Professor of Zoology here at UCT in the 1980s. Many mammals living in very cold regions rely on a thick insulation of dense fur or a thick layer of fat to reduce heat loss to the environment and thus conserve energy.

Linked to their high energy demands, mammals (and birds) need a rapid delivery of oxygen to the tissues. They have double circulation of the blood, the right side of the heart receiving deoxygenated blood from the body and pumping it to the lungs to be oxygenated. The oxygenated blood is returned to the left side of the heart and the very muscular left ventricle then pumps it at high pressure to the tissues and organs.

The chief nitrogenous excretory product in mammals is urea, a soluble compound that necessitates the loss of considerable amounts of water in the urine. It may at first seem odd that mammals do not excrete dry uric acid as reptiles and birds do. The answer may be as follows. While insoluble uric acid is a suitable excretory product for a cleidoic egg, it is most unsuitable for an embryo developing inside the mother and passing its waste products into her blood stream via the placenta. Instead, the waste substance must be soluble - hence the excretion of urea by foetal mammals. It seems odd that mammals do not switch from producing urea to producing uric acid after birth, but the don’t. Instead, they do have some control over the amount of water they excrete and they have special regions in their kidneys (the loops of Henlé) that enable them to concentrate their urine. Indeed, desert mammals (including the gemsbok) are so good at this that they do not need to drink water at all but obtain metabolic water from their food (often dry grasses or seeds). They have also modified their behaviour so that they are active and feed at night when conditions are most favourable to them.

Mammalian behaviour is fascinating and is often complex. On the whole, and probably because of their relatively large brains, when compared to other vertebrates, mammals use fewer built-in instinctive behaviour patterns and have a greater ability to learn from experience. They are thus able to adapt their behaviour patterns to circumstances in a manner unrivalled by any other group of animals. This ability reaches its peak in the Primates.

What defines a mammal?

Mammals are animals with backbones that have the following unique characteristics:

• hairy - in most species the hair covers the body, thus providing insulation;

• milk producers - mothers provide milk for their young. 

• There have been various modifications of the skull in mammals, including the forming of a secondary pallet, which enables an individual to breath through the nose while eating.

Classification 

Mammals are divided into three main groups:

Prototheria (monotremes, including echidnas and Platypus) The most obvious characteristic of this group is that females lay eggs rather than giving birth to live young like the majority of modern-day mammals. This group contains a single extant order, the Monotremata, which contains two families:  Family: Tachyglossidae (spiny anteaters / echidnas). Found in Australia and New Guinea. Family: Ornithorhynchidae (Platypus). Contains the single species the Platypus (Ornithorhynchus) found in eastern Australia and Tasmania. Extinct orders in the Prototheria include Multituberculata, Triconodonta and Docodonta.
Metatheria (marsupials, including opossums, Koala, kangaroos and others) Give birth to highly undeveloped (altricial) young that are nurtured in a pouch on the female that encloses the mammary glands. This means that they can have mum's milk on tap in a safe and protected environment. Marsupials are only found in the Australian region (including New Guinea) and in the New World (Americas).
Placentalia (placental mammals) [= Eutheria] Give birth to live young. The embryo in the mother is attached to the placenta via an umbilical cord. Most mammals fall within this group and all mammals indigenous to Africa are in this group.

Bibliography

The following works have been used in the production of the mammal pages:

Text by Hamish Robertson