Many organisms have a body temperature that differs little, if
at all, from their environment. A parasitic worm in the gut of
a mammal, a fungal mycelium in the soil and a sponge in the
sea acquire the temperature of the medium in which they live.
Terrestrial organisms, exposed to the sun and the air, are different
because they may acquire heat directly by absorbing solar radiation
or be cooled by the latent heat of evaporation of water. Various fixed
properties may ensure that body temperatures are higher (or lower)
than the ambient temperatures. For example, the reflective,
shiny or silvery leaves of many desert plants reflect radiation that
might otherwise heat the leaves. Organisms that can move have
further control over their body temperature because they can seek
out warmer or cooler environments, as when a lizard chooses to
warm itself by basking on a hot sunlit rock or escapes from the
heat by finding shade.
Amongst insects there are examples of body temperatures raised
by controlled muscular work, as when bumblebees raise their body
temperature by shivering their flight muscles. Social insects such
as bees and termites may combine to control the temperature of
their colonies and regulate them with remarkable thermostatic
precision. Even some plants (e.g. Philodendron) use metabolic heat
to maintain a relatively constant temperature in their flowers;
and, of course, birds and mammals use metabolic heat almost
all of the time to maintain an almost perfectly constant body
temperature.
An important distinction, therefore, is between endotherms
that regulate their temperature by the production of heat within
their own bodies, and ectotherms that rely on external sources of
heat. But this distinction is not entirely clear cut. As we have noted,
apart from birds and mammals, there are also other taxa that use
heat generated in their own bodies to regulate body temperature,
but only for limited periods; and there are some birds and
mammals that relax or suspend their endothermic abilities at the
most extreme temperatures. In particular, many endothermic
animals escape from some of the costs of endothermy by
hibernating during the coldest seasons:
at these times they behave almost like
ectotherms.
Birds and mammals usually maintain
a constant body temperature between
35 and 40°C, and they therefore tend to lose heat in most environments;
but this loss is moderated by insulation in the form of
fur, feathers and fat, and by controlling blood flow near the skin
surface. When it is necessary to increase the rate of heat loss, this
too can be achieved by the control of surface blood flow and
by a number of other mechanisms shared with ectotherms like
panting and the simple choice of an appropriate habitat. Together,
all these mechanisms and properties give endotherms a powerful
(but not perfect) capability for regulating their body temperature,
and the benefit they obtain from this is a constancy of near-optimal
performance. But the price they pay is a large expenditure of energy, and thus a correspondingly large requirement for food
to provide that energy. Over a certain temperature range (the
thermoneutral zone) an endotherm consumes energy at a basal
rate. But at environmental temperatures further and further above
or below that zone, the endotherm consumes more and more
energy in maintaining a constant body temperature. Even in the
thermoneutral zone, though, an endotherm typically consumes
energy many times more rapidly than an ectotherm of comparable
size.
The responses of endotherms and ectotherms to changing temperatures,
then, are not so different as they may at first appear
to be. Both are at risk of being killed by even short exposures to
very low temperatures and by more prolonged exposure to
moderately low temperatures. Both have an optimal environmental
temperature and upper and lower lethal limits. There are also costs
to both when they live at temperatures that are not optimal. For
the ectotherm these may be slower growth and reproduction, slow
movement, failure to escape predators and a sluggish rate of search
for food. But for the endotherm, the maintenance of body temperature
costs energy that might have been used to catch more
prey, produce and nurture more offspring or escape more predators.
There are also costs of insulation (e.g. blubber in whales, fur
in mammals) and even costs of changing the insulation between
seasons. Temperatures only a few degrees higher than the
metabolic optimum are liable to be lethal to endotherms as well
as ectotherms.
It is tempting to think of ectotherms
as ‘primitive’ and endotherms as
having gained ‘advanced’ control over
their environment, but it is difficult to
justify this view. Most environments
on earth are inhabited by mixed communities of endothermic and
ectothermic animals. This includes some of the hottest – e.g. desert
rodents and lizards – and some of the coldest – penguins and whales
together with fish and krill at the edge of the Antarctic ice sheet.
Rather, the contrast, crudely, is between the high cost–high benefit
strategy of endotherms and the low cost–low benefit strategy of
ectotherms. But their coexistence tells us that both strategies, in
their own ways, can ‘work’.
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