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Gnathostomata (jawed vertebrates) > Teleostomi (teleost
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Sarcopterygii (lobe-finned fish) > Stegocephalia
(terrestrial vertebrates) > Reptiliomorpha > Amniota > Synapsida
(mammal-like reptiles) > Therapsida > Theriodontia >
(Impala) fawn suckling from mother. Production of milk for feeding of infants is
one of the defining characteristics of mammals. [photo
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
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 donít. Instead, they do have some control
over the amount of water they excrete and they have special regions
in their kidneys (the loops of Henlť) 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
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
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.
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)
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.
The following works have been used in the
production of the mammal pages:
& T. 2001 Chris and Tilde Stuartís field guide to the
mammals of southern Africa. Struik Publishers, South Africa
T.E. 1979. Handbook to the orders and families of living
mammals. Mad River Press California, USA
S.J. 1985. The atlas of Africaís principal mammals. Natural
History Books, South Africa.
D.W. (ed). 2006. The encyclopedia of mammals. Oxford
University Press, U.K.
J.D. & Chimimba, C.T.(eds). 2005. The mammals of the
southern African subregion (3rd edition).
Cambridge University Press
Taylor, P.J.. 2000. Bats of Southern
Africa. University of Natal Press, Pietermaritzburg.
Text by Hamish Robertson