“It’s a primordial response,” he says. “If you smell a solitary male nearby, chances are he’s hunting or defending his territory.” If you’re in pain, you’re showing weakness.

I don't know about those results, but this explanation sounds preposterous: it doesn't work if you're hunted down by a lioness (which does the hunting while the male lion sleeps); it also doesn't work if you're spotted from the air by an eagle (which is one the main hazards if you're a small rodent).

Why does every behavior has to be explained in terms of survival, we'll never know. Stephen Jay Gould was so good at debunking those easy explanations; how I miss him.