Re: how can bird testicles tolerate body heat while mammal cannot

Hi Dennis:

It's very hard to answer questions like that, because they involve changes in organisms that happened a very long time ago. I can speculate a bit though!

It is currently thought that mammals arose during the Triassic period, around the same time as the dinosaurs did. Obviously mammals are still around, we're proof of that; it is thought that dinosaurs are also still around, as birds (or more properly, the birds are what's left of the dinosaurs, following a huge extinction event at the end of the Cretaceous).

It has been proposed that external genetalia are the result of the "invention" of warm-bloodedness by mammals. In other words, warm bloodedness created conditions that were suboptimal to internal genetalia, so they "moved out" of the body through natural selection. Sounds good, but there's a small problem with that hypothesis: birds are also warm-blooded!
However, it is possible that warm-bloodedness arose seperately in dinosaurs (and the birds that remain of them). In that case it is possible that birds evolved (again through natural selection) genitals that are able to tolerate the higher heat inside their body. Internal genitalia is also a big help to a bird, because they need to be as aerodynamic as possible.

That's one possibility, but be aware that it's kind of speculative (answers to "why are things different" questions generally are). Perhaps you can come up with some better hypotheses than mine!

Hope that helps,
Rob Campbell, MAD Scientist