Re: Had warm-bloodedness evolve independently in birds and mammals?

Hi Tally,

First of all, let me just define some of the terms. Warm-blooded is a term that refers to animals who have warm blood! A turtle resting on a log could very well have a temperature over 30^oC. A bat hibernating in winter may have a body temperature of only a few degrees. That is certainly very cold-blooded. So although the term 'warm-blooded' is popularly used to describe birds and mammals, it is highly inaccurate.

Even the term "endotherm" cannot really be restricted to birds and mammals. Insects have evolved endothermy in varying degrees. Many dragonflies heat themselves up by wing-whirring. Much like ourselves when we exercise, the beating of their wings generates heat. Bumblebees are even better. They can dislocate their wings and use the muscles far more efficiently for heat generation. They also have some biochemical means to generate heat, but I do not know very much about that. Bumblebees maintain a body temperature between 37^oC and 40^oC. Therefore, I would argue that endothermy had already evolved independently at least once even before the existence of mammals and birds.

The advantage of endothermy is that it allows an animal to forage for food in places and at times where it would be too cold for an exotherm to move. In many places, this gives endotherms a fantastic advantage and it has made both mammals and birds highly successful allowing them to live where other animals cannot survive.

However, endotherms have to expend vast amounts of energy just to maintain their body temperature, so they have to eat enormous amounts of food. A human being may eat well over a thousand kilos of food each year, yet a crocodile can survive on an annual food intake just 1.5 times its own bodyweight. All endotherms have evolved a means to conserve as much energy as possible and they do this through insulation. Mammals use hair and birds use feathers. The principle of double-glazing is used by dragonflies which have air sacs to insulate their flight motor. Bumblebees, which have the most northerly distribution of any bee, use hair. If endothermy in both mammals and birds originated from the same group of animals, then you would expect to see some form of insulation in a line of dinosaurs as well. As far as I know there is no evidence to support this.

The bones of animals which rely on their environment for heat, the exotherms, develop concentric rings like those of a tree. Some species of fossilized birds have these rings strongly indicating that birds were originally cold-blooded. There is some evidence that the ancestors of birds might have been warm-blooded and it has not been established beyond doubt whether endothermy evolved in birds or whether birds evolved from an endotherm. These arguments may be settled if they decide to section the bones of the most primitive bird known, the archeopteryx. However, the evidence we already have makes it highly unlikely that endothermy in mammals and birds evolved from one single event in evolutionary history.

I hope this answers your question.

Richard Kingsley