 |
| Marianas flying fox, a related species |
It's not the actual temperature of the blood that's relevant, here. The blood of a "cold-blooded" animal, such as a lizard, is, after all, pretty warm when it's basking in the sun. (Which is, in fact, why they bask in the sun in the first place). It's because of that that "warm-blooded" isn't the preferred term among scientists. Back when I was school I was told that the correct word was homoeothermic, but today, when we're talking about mammals and birds, the preferred term is endothermic. The two words don't mean quite the same thing, and it's actually possible for an animal to be one without being the other.
"Homoeothermy" means that the animal is able to keep a constant body temperature, largely regardless of what the environment around it is doing. Get too hot, and you find some way to cool down, and get too cold and you warm up somehow. There are a very small number of non-hibernating mammals that aren't homoeothermic. Naked mole rats are the clearest example; because they live in underground tunnels where the temperature doesn't change much, they've lost the ability to compensate if it ever does change. It's just not a problem they face, and it may be part of the reason why they don't have any fur - they don't need to keep their body heat in.
There are at least two ways for an animal to become homoeothermic. One of them is just to be enormous. Large objects lose heat slowly, so, perhaps with the aid of a few behavioural modifications, a really big animal can keep warm through a cold night simply because the night isn't long enough to have an effect on it. This probably won't work through a long winter, and the animal also takes a long time to heat up (though the latter might be an advantage in the noon-day sun), but, for many purposes, it may well be enough. This sort of homoeothermy isn't really relevant to mammals, but it may well have played a role in the lives of the really big dinosaurs like Diplodocus.
