I am no animal expert, but as far as i can tell land mammals (almost) always have testicles hanging out their crotch. I understand that the temperature is too high to develop sperm, but why? And why does it only apply to land mammals, when even whales has internal testicles?
Having your gamet-building-factory outside the body seems like a bad idea in the first place, its very susciptible to damage. Why does the female eggs not have the same problem?
In: Biology
Apparently the benefits of having external testes far outweighs the risks. Considering, like you said, that most male mammals have external testes.
But seriously, from my understanding, at cooler temps the sperm is kind of in a hibernation like state. So, when you ejaculate into a vagina the vaginal warmth sort of activates the sperm to start their swim to the egg.
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