All living organisms, including all animals and all plants, get the energy they need to live from their food by a process called respiration. Some organisms can make their own food, for example plants can make their food by a process called photosynthesis, but animals cannot make their own food, they must eat plants or other animals or organisms.
Respiration is a chemical change. In many animals, including humans and other mammals and birds, respiration involves a chemical change such as
If we want our muscles to produce more energy, for example to run faster, we must increase their respiration rate, and this involves not only getting more sugar and oxygen to our muscles but also, just as importantly, removing the extra carbon dioxide and water from them. Our heartbeat gets quicker and we start to breathe faster.
There is a separate
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Respiration is a chemical change, and like all chemical changes the rate at which it takes place depends upon the temperature. So our rate of respiration, and the rate at which our muscles can produce energy, depends not only upon how quickly we can get sugar and oxygen to our muscles and remove the carbon dioxide and water from them, but also on our body temperature.
Animals can be divided into two groups. Mammals (including humans) and birds are warm-blooded, and all other animals are cold-blooded.
Warm-blooded animals (more correctly called homoiothermic animals or homoiotherms, from the Greek for constant body temperature) keep their body temperature almost constant. Warm-blooded animals are also often known as endotherms. All mammals usually have a body temperature of about 36oC and all birds usually have a body temperature of about 42oC. This means that their respiration rate does not depend on the temperature of their surroundings, and they can be as active on a cold winter's night as they are on a hot summer's day.
Cold-blooded animals (more correctly poikilothermic animals or poikilotherms, from the Greek for variable body temperature) do not keep their body temperature constant, so when their body temperature is high, for example on a hot summer's day, they can be very active, but when it is low, for example on a cold winter's night, they can produce energy only slowly and so are torpid (are very inactive and can move only slowly, if at all). Cold-blooded animals are also often called ectotherms.
A mammal with a temperature of 36oC will feel warm to our touch, which is why homoiotherms can quite correctly be called warm-blooded. Usually 36oC is warmer than their surroundings, so to stop their bodies cooling down homoiotherms must produce heat. (There are a few places, for example in the desert during the day, where the air temperature can be much higher than 36oC, but at night in the desert the air temperature is a lot lower, and can even be below 0oC.) Although the majority of metabolic processes produce some heat, mammals can control their respiration and the other metabolic processes taking place in their bodies in a way which allows them to produce sufficient heat to raise their body temperature well above the temperature of their surroundings. Animals which can control their internal heat production in this way are called endotherms: all homoiotherms living today are also endotherms. Mammals need energy to keep their body temperature above that of their surroundings, and this energy must come from their food. So although homoiotherms can be as active in the winter as they are in the summer, they need much more food than poikilotherms. Most mammals have fur, or a thick layer of fat just under the skin, or both, to insulate them when it is cold - this means they do not need as much food as they would if they were not insulated. But this insulation creates another problem: keeping cool when it is hot. A polar bear's insulation of fur and blubber is so effective, equivalent to about twenty duvets, that except in the coldest Arctic winter keeping cool is far more of a problem than keeping warm.
Almost all the various life processes in mammals are very sensitive to temperature, and if our body temperature rises or falls by as little as 1oC these processes may be affected. If our body temperature rises above about 37oC we are ill and say we have a temperature (or more correctly a fever); if it falls below about 35oC we shall soon be suffering from hypothermia and may die.
Today all homoiotherms are endotherms, and visa versa, so some people use the words synonymously, but they do mean different things and it is probable that the earliest endotherms were not homoiotherms.
Birds have a body temperature of about 42oC, higher than that of mammals, so what is true of mammals is even more true of birds. They need even better insulation than mammals so are covered with feathers not fur.
Because of the difference in body temperature, and for other reasons, scientists believe that warm-bloodedness developed first in mammals and then, much later and quite independently, in a group of dinosaurs. Some of these warm-blooded dinosaurs grew feathers for insulation and as a result were better equipped to survive the Great Extinction of 65mya (million years ago) which wiped out all the other dinosaurs. These warm-blooded feathered dinosaurs were the ancestors of birds!
All animals except mammals and birds are poikilotherms. Poikilotherms are very often called cold-blooded animals but this is not really very accurate: the first time they pick up a snake most people are surprised how warm it feels, they expect it to be cold and slimy. (Never pick up a snake unless you are with someone who says it is OK. British snakes are far more afraid of you than you are of them, and will almost always try to slither away, and will certainly never deliberately attack you. But if you pick one up it might bite you in self defence, in the same way that although you would not deliberately pick a fight with a giant you might bite one if it picked you up!)
Most metabolic processes, including respiration, taking place inside animals produce some heat, but most animals are unable to control this heat production in a way which will raise their body temperature significantly - we call such animals ectotherms. Today all ectotherms are poikilotherms, and visa versa, so (as for endotherm and homoiotherm) the terms are often used synonymously. But although ectotherms cannot raise their body temperature to any significant extent by producing heat internally, many have developed ways of heating themselves up in other ways. For example, first thing in the morning a butterfly's body temperature is far too low for it to have enough energy to fly. So as soon as the Sun has risen the butterfly climbs up onto a plant and spreads its wings to face the Sun. On a dull day it will often never get warm enough have enough energy to fly. Using the Sun's heat to raise your body temperature in this way is called basking; many animals bask.
Some poikilotherms have developed very sophisticated mechanisms for reducing heat losses. For example, when a fish is swimming its muscles may become slightly warmer, and to minimise the heat loss to the water flowing through the gills some fishes have developed heat exchangers to take the heat out of the warmer blood entering the gills and putting it back into the colder blood leaving them.
Poikilotherms do not become ill if their body temperature rises or falls, as homoiotherms would, they just become more or less active. The colder it is the less active they are but the less food they need, whereas homoiotherms need more food in cold weather, and it is in cold weather that food is usually most difficult to find. Unfortunately for poikilotherms, one of the easiest meals for a hungry carnivorous homoiotherm to catch on a very cold winter's day is a nice torpid poikilotherm!
© Barry Gray October 2005
