Hot bodies
Question: Why is the body temperature of mammals 37 掳C? It seems
extraordinarily high when you consider that it is hotter than most environments
on Earth. What purpose does this serve?
Answer: There are important reasons why the body temperature of warm-blooded
or homeothermic animals (mammals and birds) is higher than most Earthly
environments.
A high body temperature speeds the metabolic rate which allows for faster
movement. Reptiles, which are cold-blooded or poikilothermic rather than
homeothermic, were unable to hunt or escape attack until their bodies warmed up,
usually by basking on a sunny rock. The reptile that warmed fastest could be
rewarded with breakfast, or by living to see another sunrise. The first
warm-blooded homeotherms were able to feast all night on virtually comatose
poikilotherms. So, it is likely that rising body temperatures were part of an
evolutionary 鈥渁rms race鈥�.
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To sustain a high metabolic rate you need lots of food鈥攕o an upper
limit on body temperature would have been imposed by the food available. The
temperature at which certain proteins in the body begin to denature sets another
upper limit, while yet another is the temperature at which brain function
becomes disorganised, resulting in confusion or coma.
Because of these limits to temperature, a cooling system is needed. The usual
cooling system consists of water evaporating either as sweat (as in humans and
horses), or from the respiratory tract through panting (as in dogs and sheep).
Obviously, the higher the body temperature can be, the less this system
needs to operate. So animals living in hot climates often operate at
temperatures higher than 37 掳C.
Certain African antelopes maintain very high body temperatures, selectively
cooling only the brain, and thus economising on water. Birds also have a high
body temperature. They probably operate closer to their high limit to save
water, because their flight would be hampered by having to carry water destined
only for cooling. A high body temperature is also advantageous to birds because
it increases their metabolic rate, allowing them to operate their muscles at
high power instead of building more muscle mass. Birds鈥� great mobility probably
allows them to gather the food they need to sustain their high metabolic rate
more effectively than terrestrial mammals.
Peter Bursztyn
Barrie, Canada
Answer: The typical range of normal body temperatures for mammals
is between 36 and 38 掳C.
This range is not fully inclusive, of course. Some mammals, like the
monotremes (platypuses and echidnas) can have a normal body temperature of 31 or
32 掳C; and the gemsbok, a desert antelope, can have a daytime body
temperature as high as 41 掳C. In the case of the gemsbok, the excess heat is
radiated back into the atmosphere at night, lowering its temperature to about 37
掳C. Conversely, mammals in hibernation can have quite low body temperatures,
near ambient in some cases.
The reason for maintaining mammalian body temperature in this range is mainly
concerned with the efficiency of enzymes鈥攑roteins that catalyse chemical
reactions in the body. Enzymes work at maximum efficiency at around 37 掳C,
and this efficiency can decrease by as much as 50 per cent for a 10 掳C drop
in temperature. This explains why cold-blooded animals like reptiles and
amphibians become lethargic at lower temperatures. However, if the temperature
rises much above 40 掳C, enzymes begin to denature and efficiency drops
again. Evolutionary selective pressures have therefore zeroed in on 37 掳C or
thereabouts for mammals as the optimum operating body temperature.
Because of this ability to maintain body temperature mammals are not as
constrained by local conditions as are reptiles or amphibians. Nothing is free
however and, to maintain temperature, mammals need a much higher metabolism than
cold-blooded animals, and need to consume as much as ten times the volume of
oxygen and food.
David Slauenwhite
Dartmouth, Canada
Answer: Even among placental mammals there are differences in body
temperature. For example, humans have a body temperature of approximately 37
掳C and ruminants (cows and sheep, etc) have body temperatures of
approximately 39 掳C. Also, the purpose of a regulated and high body
temperature in mammals is equivocal, with theories ranging from expansion into
colder areas to increased aerobic capacity.
The concept that body temperature is kept constant is false. The body
temperature of endothermic animals may vary by more than 1掳C during the
course of a day. Typically, diurnal animals鈥� body temperatures are higher during
the day than at night, and the opposite is true for nocturnal animals. The
elevation of body temperature during periods when an animal is awake is not
entirely a result of increased activity and feeding鈥攖here is a strong
intrinsic daily body temperature rhythm.
In addition, many mammals go into torpor, allowing their metabolic rate and
body temperature to drop during periods of low food availability or quality. The
most extreme example of torpor in a mammal is the Arctic ground squirrel, which
allows its body temperature to fall to -2 掳C while it is hibernating during
winter.
At the opposite end of the mercury scale, mammalian body temperatures rarely
exceed 41 掳C. Some mammals, such as camels, oryx and eland, reportedly allow
their body temperatures to increase to levels as high as 42 掳C on hot days
and, in so doing, store heat and conserve the water they would otherwise use to
cool the body. They get rid of the stored heat at night by non-evaporative
means鈥攔adiation, convection and conduction. However, recent studies of
free-ranging animals, rather than tame animals under lab conditions, have shown
that body temperature does not fluctuate in this manner.
Peter Kamerman
School of Physiology
University of the Witwatersrand
South Africa
This week鈥檚 questions
Colour blind: If I buy mauve, yellow and white cut freesias, the mauve ones
always die first, then the yellow ones and finally the white ones. This happens
regardless of the time of year they are bought. Why?
Fay Jackson
Birmingham
Marine boy: Many science-fiction books and films feature humans that have
been altered so they can breathe under water. However, none of the mammals that
have returned to the sea have evolved gills. Why is this? Do warm-blooded
creatures such as dolphins burn so much energy that they couldn鈥檛 meet their
oxygen requirements from water using gills?
Leonard Paglia
London