Re: CROCODYLOMORPH ENDOTHERMY

["Nicholas R. Longrich" <longrich@phoenix.Princeton.EDU>]

On Thu, 25 Jan 1996, Rob Meyerson wrote:

> A four-chambered heart is a very efficent structure.  Since it has been my
> observation that extreme efficency is an endotherm feature, couldn't we 
> conclude
> that thecodontians were endotherms?

endotherms use about ten times as much energy as an equivalent ectotherm. 
My guess is that one big difference in strategy between ectothermy and 
endothermy is changing from a dependence on temperature to a dependence 
on food. If temperatures are pretty good and allow you to be active a 
lot, then it makes sense to be ectothermic. And if temperatures are bad 
for ectothermy, but food is readily available, then endothermy is an 
effective strategy. The limiting factor on how far north birds can live  
during the winter, 
for example, is not temperature- it's food availability. Endotherms can 
operate somewhat independent of the temperature, because they heat 
themselves, but the cost is that they must eat a lot more. It works a lot 
better in some situations, but not all. 
        I know some authors put therapsids and thecodonts at an 
intermediate level of endothermy, and the predator-prey ratios are also 
supposed to support this.