Uh, but most fish eject sperm next to the eggs, rather than inserting into the female. I know there’s a species of shark that carries eggs internally until birth, and also some species of snake. How does the male fertilize those eggs? I assumed that all amphibians did it like fish. But now I realize that not all fish are the same.
Well, there it is. Mammals somehow ended up with external gonads, and they seem to work.
Gonads only refers to the testicles and ovaries.
All sharks and reptiles have hemipenes, which are basically penises, and do internal fertilization. I’m not as familiar with fish, but based on sheer diversity, I’d bet there are some that do internal fertilization as well. Octopuses do.
Internal fertilization has the advantage of making sure the sperm and eggs are close enough together to have successful conception. Once fertilized, the eggs can then be kept internally or put out in the environment.
Could endothermy have something to do with the cause of this development? Do reptile and fish testes work at a greater range of temperatures than mammalian ones?
Well, that part’s easy enough to explain - other phylae are generally not warm-blooded, and therefore have lower internal body temperatures.
My WAG as to why the testes, specifically, need more careful temperature regulation is that they produce cells at a much faster rate than normal which results in more heat energy.
Increased internal body temperatures might make sense in fish and amphibians, but reptiles control their body temperature through behavior, and birds are not only endothermic but have higher running temps than mammals (107 for your typical chicken). If given the opportunity, reptiles will keep their body temperatures comparable to mammals.
I’m not sure if fertility in fish and amphibians varies with temperature as directly as it does in mammals. Maybe someone else on the boards knows.
What do you think of this: Reptiles and other endothermic critters use the same cellular processes as us, and yet they don’t generate enough cellular heat to keep themselves warm. Mammals and birds specifically exploit the ability to decouple our mitochondrial activity in order to generate excess cellular heat so that we can utilize environmental ranges that are unavailable to reptiles.* But, this makes problems for our heat-sensitive sperm. Solution in mammals: Move the testes outside. Solution in birds and reptiles: have heat resistant sperm.
This theory also helps explain the secondary cooling anatomical features of the mammalian scrotum, specifically the excess sweat glands and the pampiniform plexus. Sweat glands on the scrotum are, to my knowledge, universal even in critters that don’t have a lot of sweat glands like bulls and dogs. The pampiniform plexus is a vascular mesh that cools the hot arterial blood traveling to the scrotum with cooler venous blood, insuring that the body doesn’t warm up the testicles via the blood supply.
My theory would be helped if I knew more about the heat sensitivity or lack thereof in birds. Does anyone know? [paging Colibri].
*I know this sentence is riddled with evolutionary fallacies, but I can’t think of a clearer way to phrase it.
Remembered something else:
Marsupials have their penis and testes in the opposite orientation from other mammals: balls closer to the belly button, penis closer to the anus.
Did external testes evolve twice? If so, did the common ancestor of placental mammals and marsupials have internal testes? Is that far enough back that the common ancestor was more reptile than mammal?
Until Colibri shows up, I’ll just throw out that birds are generally more sensitive to heat than, say, us, just because they’re smaller - and because flight requires a higher metabolic rate than walking or swimming. It isn’t a birds-only thing, because bats have the same problem.
Thus migratory birds - rather than adapt to a large range of temperatures, you can just adapt until you’ve got a large range, so to speak. They do also display temperature adaptations like those found in mammals, like retia mirabilia, which IIRC are heat exchangers which work in much the same way as the panpiniform plexus you described above.
Wait, so bird sperm is more sensitive to heat stress than human’s, or birds on the whole are more sensitive to heat stress?
Birds on the whole.
I don’t really stay up on heat sensitivity in avian gametes, to be honest, except as it applies to Scrabble. I’m hardly an expert on avian physiology in general - just sharing the few specifics I do remember.
I suspect, however, that avian sperm is less sensitive to heat variation than mammalian sperm, since birds mostly reproduce by stuffing their cloacae together - which you’d think would result in exposure of the seminal fluid to air during intercourse.
I was thinking that if reptiles’ gonads have the same sensitivity to heat as ours, they could potentially regulate their testes’ temperature with behavior. (I’m imagining an alligator bathing his hindquarters in a cool stream just before going to see his alligatrix love . . .) But warm-bloodedness would make this impossible, so mammals and birds would need to develop other ways of keeping their testes in the right temperature range. The problem with reptiles in particular having heat-resistant sperm is that they would have to have evolved it after the mammals split off from their line, which just raises the question of what they did before that.
We are in serious need of hard facts here. Is there a herpetologist in the house?
I wonder if we’re approaching the problem from the wrong direction. Could it be that mammals evolved with external testicles because they were seen as a sign of fertility and caused certain males to be viewed as more attractive mates?* That’s a total WAG, but I’m sincere when I pose it as a question. Is there a reason why that could be false?
I would think of that argument would apply more in terms of SIZE of the testicles then, and not so much of testicles being on the Outside vs. the Inside.
Perhaps once they were placed outside, then there could be selection for larger and smaller sizes of testicles certainly. But the fact that most land mammals have external testicles regardless of species/mating behavior, seems to suggest that there is an Evolutionary Advantage for having the testicles on the outside for mammals vs. on the inside, otherwise there would be competitive selection between same species mammals with internal testicles and the same species with external testicles. But I don’t think such an example exists, and so that’s why I’d rule out the “more attractive mate quality”, unless there was some crazy mass event that caused all of the internal testicles members of the species to die out
(The Ultimate Extinction Kick to the 'nads of some sort, except vs. those who didn’t have them on the outside… an odd thought. :dubious: )
Have you ever looked at a scrotum and thought, “Yum”?
And, IIRC, even dolphins have elaborate blood vessel systems to cool down their (internal) testicles. Seems that would be discarded if heat was no issue.
The wiki page has quite good details, including on the whole dolphin thing (and by the way, rhinos have internal testicles!). But here’s something particularly useful–a theory on why they’re external:
It’s no fun ruining our harmless groundless speculation with speculation from people who know the subject.
Spoilsport. 
Some interesting information regarding the link between Platapi (Platapuses? Platapussies?) and external ball storage here.
I was behind a guy yesterday who had his hanging on the trailer hitch of his pickup truck.