Couldn’t the food that warm-blooded animals burn to stay toasty be put to better use? (Image: Ted Kinsman / Photolibrary.com)

See a gallery of thermal images of warm- and cold-blooded animals

IF YOU stopped eating today, you wouldn’t survive more than two months. A crocodile, on the other hand, might live for a year or more. Why the difference? You waste most of the food you eat generating heat.

The evolution of warm-bloodedness, or endothermy, is one of life’s great mysteries. Sure, there are some advantages – staying active in the cold, keeping young cosy and warm, and avoiding having to go out into the open to soak up heat from the sun.

The thing is, you could get much the same advantages by turning up the heat only when and where in the body it is needed, as many animals do. So why do most birds and mammals keep the furnaces burning 24/7? Staying warm – which for birds means 40 °C on average – comes at a price. Some warm-blooded animals have to eat as much in one day as similarly sized reptiles do in a month, a dangerous and time-consuming strategy.

Wasteful lifestyle

Biologists have long struggled to understand why we mammals and our feathery cousins are warm-blooded. The standard explanation is that it evolved in small carnivores to enable an active, predatory lifestyle. Last year, however, a radical new idea was put forward: warm blood evolved not in carnivores but in herbivores, as a way of balancing their nutrient requirements. Though it is early days, this idea could explain not only why we have such an apparently wasteful lifestyle, …