The reason I’ve always heard is that sperm can’t cope with the higher internal body temperature of mammals. But birds are also endothermic and have even higher body temperatures, yet they do just fine with internal testes. Why the difference?
Well, let’s face it - birds would look hilariously funny and be in a lot of pain with external testes flapping around.
When you say endothermic, do you mean warm blooded?
Not being a biologist, I have no idea about birds, but from what I remember about sex-ed, the semen in the male testes only thrive at a degree or two lower than normal body temperature. Which is also why they shrink up to the body in cold water.
Probably it is because birds and mammals developed their warm-bloodedness seperately, and for whatever reason the therapsid line didn’t learn the trick of producing sperm at the higher temperature. Just because an adaption makes sense does not mean that any given lineage will evolve that adaptation.
Oh, and some varieties of mammals do keep their testes internally. Elephants and cetaceans.
Thank goodness! Who would want to see something as big as a pair of elephant or whale balls swinging around? Yikes.
As for the OP, a related question might be, “what kind of evolutionary sense does it make for sperm-developing temps to be lower/different than internal body temps? wouldn’t it have been easier if our male gonads were safely protected inside our bodies (cf. ovaries) and happily producing sperm without fear of getting racked?”
Pablito, the sperm-needs-lower-temps situation may have no advantage at all. Simply nothing came about that was able to outperform it.
I think I read somewhere several years ago about how some marine biologists were trying to find out how dolphins were able to cool their testes. The article said they had found that their blood flows through their tail fins like a radiator before it goes on to cool the testes. Sorry no specific cite.
From Duckster’s link:
Emphasis mine.
So it would appear that elephants and whales (and seals) have traded off higher body temperature with internal testes. For aquatic mammals, the advantage of keeping the family jewels indoors is obvious - streamlining. They must still maintain a body temperature lower than the mammalian average to keep the boys happy. What about the elephant? Well, now that I have a search term, Googling on “testicond mammal” led me to some cites that suggest that the elephant may have originally been aquatic! So their testicond characteristic (and the related lower-than-normal body temperature) may be a vestige from when they lived in the water.