What does it mean to be cold blooded?

Listener Jon emailed into the show. Chris Smith put his question to Jason Head from Cambridge Univeristy.

Jason - Along with ‘living fossil’, ‘cold-blooded’ is my least favorite word combination. It is talking about metabolism and whether or not an organism maintains a very high, elevated, or constant metabolic rate; or a lower metabolic rate that is basically dependent on the ambient environment that it lives in - so how much food it eats and how much oxygen it requires. And so there's this classic idea that cold-blooded animals get sluggish when the environment gets colder because the external environment controls their temperature. In the case of amphibians, many amphibian groups - especially things like croakers and peepers - they have all sorts of secondary adaptations to keep themselves warm or to keep their blood from freezing.

They can have high concentrations of glucose in their bloodstream and in their tissues, and that acts as a natural antifreeze. And many groups of what we would call ectothermic or or poikilothermic animals (so these are the classic cold-blooded groups) have actually come up with secondary adaptations to function very efficiently in cooler environments. When we think of cold-blooded animals like lizards and snakes - especially lizards and snakes and their relatives - those animals are really specialized to conserve energy, so their type of cold-bloodedness is an adaptation for minimizing the amount of energy you have to expend in your life. So that allows a high degree of diversity in the tropics and certainly less numbers of species as you go up into the higher latitudes but you still find cold blooded animals toward both poles and they have all these really excellent specializations for surviving just fine.

Chris - So basically, although their body temperature is lower, their tissues are optimised so that despite the lower temperature they can still make their tissues, their muscles, and so on, work just as fast as they need to at those sorts of temperatures. They've just evolved to operate to those low temperatures?

Jason - Yeah. And they've also, some of them have evolved specializations to elevate their body temperatures beyond that of the ambient environment so that they can, instead of being specialized to have the tissues work at low temperatures, they can simply keep the tissues warmer than otherwise you would have based on the temperature around you