Why do sick animals develop high temperatures?
What purpose, if any, do fevers serve?
Evolutionary biologist Michael Logan from the University of Nevada, Reno, describes this as ‘an enduring enigma’.
Writing in Ideas in Ecology and Evolution, he points out that feverish mammals and birds expend ‘many times the energy needed to maintain high, stable, body temperatures’.
How, he asks, could natural selection ‘have favoured such an energetically costly strategy?’
He suggests that dealing with pathogens, that make us ill, led to its development and that this radically shaped our evolution.
Six hundred million years ago, warm-blooded creatures had yet to appear. Then fish amphibians and reptiles evolved.
Being cold-blooded, they couldn’t regulate their body temperatures internally but relied, as their modern equivalents still do today, on outside sources of heat.
Lizards frequent sunny places to warm up during the day. In the cool of the evening, snakes will venture onto roads still hot from baking in the sun.
Snakes are often crushed beneath the wheels of vehicles.
Trapping the sun’s energy, instead of generating heat internally, is an effective strategy; only modest amounts of food are needed to sustain a cold-blooded animal.
A large crocodile, for example, can go for an entire year without eating.
Warm-bodied creatures, on the other hand, must ‘burn’ much greater amounts of food, if they are to maintain an adequate body temperature.
Only birds and mammals have gone down the self-heating road, although large turtles and sharks use internal warming to some extent.
Failure to meet its energy demands may mean death to a warm-blooded creature. A pigmy shrew, Ireland’s smallest mammal, must eat every few hours or so, or it will starve.
So why did the reptilian ancestors of mammals and birds opt to develop such expensive body-heating systems?
There are several candidate explanations. Being able to generate its own heat, a creature can remain active in poor weather and for longer periods.
Not needing sunlight to keep warm, it can hunt in winter or during the cool of the night.
It’s also easier to raise warm-blooded youngsters.
Once a baby animal can regulate its body heat, it is no longer at risk of hypothermia and doesn’t need to be brooded continually.
Youngsters can be left alone, allowing their parents to go hunting and foraging.
Nesting birds illustrate the point. It takes several days for chicks to achieve temperature control, during which time a parent has to remain at the nest to help them.
Logan believes that the ability to maintain high, and stable, body temperatures evolved as a defence against infection.
He draws ‘on recent work demonstrating that the immune system is highly thermally sensitive’.
‘Individuals that maintain warm body temperatures’, he writes, ‘have enhanced ability to reduce pathogen burdens’.
Warm-blooded creatures ‘maintain body temperature close to the optimum for immune performance at most times’.
They prime their immune systems ‘for a rapid response to infection’.
If he is right, warm blood is the product of ‘an evolutionary arms race’ competing against ‘increasingly virulent pathogens’.
Cold-blooded creatures, Logan notes, ‘often employ the environmentally constrained and costly strategy of behavioural fever’.
Pathogens, it would seem, have played a much more important role in our evolution that we used to think.