Adaptations in the Animal Kingdom

Chapter 1
Temperature Regulation in Animals
There are, broadly speaking, two kinds of animals with regard to body
temperature: exothermic (cold-blooded) and endothermic (warm-blooded)
animals. The exothermic animals, such as reptiles, do not supply body heat
by metabolic conversion of food to heat. Reptiles allow their surroundings
to determine their body temperature. They lie out in the sun to warm their
body. If they are too hot, they seek the shade or even burrow into the ground.
At night they hide from the cold in burrows or squeeze into cracks between
rocks or hide in leaf cover. Reptiles avoid the extremes of temperature. When
reptiles become cool, their movements slow down, and chemical processes
in their bodies, such as digestion, are inhibited. Predators, such as hawks
and eagles, find it easier to prey on lizards and snakes in cooler weather. The
distribution of reptiles is somewhat limited by their exothermic character.
They do not thrive in cold climates1.
What are the advantages and disadvantages in being exothermic? When
the lizard is in a cool environment and cannot find a warmer spot, its body
simply cools to the temperature of the surroundings. It is not necessary for
the exothermic lizard to generate heat to increase its body temperature.
This means that the lizard uses less energy and does not have to eat as
much. As the lizard cools its digestion, breathing rate and heart rate slow,
saving energy. A disadvantage occurs when the cool lizard is attacked by
a predator. If warm, he could run fast and have a much better chance of
1 St. Patrick did not chase the snakes out of Ireland. Ireland was already completely free of
snakes. St. Patrick was instrumental in converting pagans to Christianity. Since the snake was
a symbol used in pagan rituals, St. Patrick was influential in ridding Ireland of the ritual use
of symbolic snakes.
10 Verne A. Simon
evading capture. A warm lizard being chased by a predator can move quite
fast for a short distance, but like other exotherms, lacks endurance and
soon tires. When the exotherm is running fast, its effort is anaerobic, that
is, is not using oxygen, and lactic acid is building up in its body. It soon
tires and is unable to exert itself. It must recover by taking in oxygen to
rid the body of lactic acid. Another disadvantage of exothermic life is that
cold climates are not available as habitat. If there is a sudden climate
change, an exothermic animal wouldn't be able to mount the sustained
effort needed to migrate to a better environment. The exothermic creature
might simply perish.
About 180 million years ago, mammals appeared. Mammals are
endothermic (warm-blooded) and are able to maintain a nearly constant body
temperature regardless of the temperature of their surroundings within wide
limits. Their bodies will not tolerate too high or too low a temperature. If the
surroundings are too hot or cold, causing the body temperature to exceed
allowed limits, the animal will die. Mammals have furry coats to help them
tolerate low temperatures. Sea-dwelling mammals-whales, seals, and
walrus-have thick layers of blubber for insulation. Birds are endothermic and
have feathers to protect them from the cold. Many types of birds and mammals
survive in cold climates. Emperor penguins even live in the Antarctic, in the
coldest climate on earth. Under normal circumstances, mammals and birds
manage to keep this very nearly constant body temperature regardless of
the temperature of their surroundings. Mammals are characterized by having
body hair and suckling their young. This latter behavior gives the class its
name; mammals must have mammary glands. A second advantage is that
endothermic animals are not limited to activity only in daylight hours. In
many locations, it is too cold at night for exotherms to be active. Even very
cold temperatures do not exclude endothermic animals such as mammals
and birds from nocturnal activity. Exothermic animals are not normally found
in cold climates, though there are a few exceptions. Mammals with their hair
can grow warm fur coats as in the polar bear or beaver. Mammals with little or
no hair often have a thick subcutaneous layer of fat for thermal insulation as
in walrus and whales. Birds have feathers for insulation, like the snowy owl,
whose white feathers match the snow.
During long periods of cold weather (winter), some, but not all, mammals
hibernate. Their body temperature drops to a few degrees above freezing, and
their breathing and heart rates almost cease. Other mammals, such as bears,
undergo estivation. Their body temperature may drop by 40oF (22oC). While
in estivation, they live off the fat they have accumulated in preparation for
winter. They may become active for short periods on relatively warm days.
Some animals tough it out in winter, like emperor penguins and moose.
Adaptations in the Animal Kingdom 11
Mammals in cold latitudes develop furry coats to retain heat during cold
winters. Mammals in warm climates may have very sparse hair, for example,
elephants and naked mole rats. Some mammals (sheep, horses, cattle) give
birth to single young with well-developed senses of sight and hearing and with
musculature ready for walking within minutes of birth. Such well-developed
young are said to be precocial.
In contrast to precocial young are altricial young, born in litters, with
closed eyes, closed ear canals, having no capability of locomotion, and
often sparsely covered with hair or feathers. Some examples of endothermic
altricial young are dogs, cats, rats, mice, and polar bears. Human babies
are neither clearly altricial nor precocial. At birth a human baby's eyes are
open and the baby can hear. On the other hand, a human infant is naked and
unable to move about. Humans usually are born singly. Altricial animals are
usually born in litters and are small. Because of their small size they have
a large surface to volume ratio, which makes it more difficult to maintain a
constant body temperature as required by endothermic animals.
How do altricial young cope with temperature fluctuations? Some animals
such as puppies and kittens have numerous litter mates with which they
can huddle, thereby decreasing their exposed surface area. Mothers share
bodily heat with their offspring and provide warm milk as a buffer against
the cold. There is a special organ for heat production in young altricial
animals, that is, brown adipose tissue (BAT). Heat production by BAT is
stimulated by cold temperatures and by norepinephrine2 (noradrenaline).
BAT is present in large amounts in the young, constituting about 5% of
body weight. In adults, BAT constitutes only 1% of body weight and has
traditionally been regarded as unimportant. Adults are able to raise their
body temperature by the muscular activity, including shivering, but infants
seem unable to shiver. Human babies have BAT deposits in the neck and
between the shoulder blades. Heat production by BAT in human babies
can be demonstrated by infrared photography. An unclothed baby lying
prone at room temperature shows hot spots between his shoulder blades
and in the area of his neck in an infrared photo. The distribution of BAT has
important advantages. It is very helpful in keeping the heart and brain from
cooling. Both of these organs show decreased activity if the temperature
drops. The placement of BAT on neck and shoulders aids in keeping the
brain and heart warm.
BAT is brown due to pigmented organelles called mitochondria within
this special tissue. The BAT is very rich in mitochondria. Mitochondria are
2 Epinephrine (adrenaline) is a hormone produced by glands at the top of the kidneys. Epi
means "above" and nephrine refers to the kidney. Norepinephrine is an active metabolite of
epinephrine. Both epinephrine and norepinephrine prepare animals for "fight or flight."
12 Verne A. Simon
present in most b