We have been taught in high school that the reason humans and "all mammals" had external testes was to cool them. But elephants have internal testicles, and, apparently, so do hippos. This seems a much better strategy than having such an important (and sensitive!) organ hanging out at the mercy of predators, foes, or even banal accidents. The evolution explanation for this appears to be lacking.
Evolution is a process of massively parallel multistart hill-climbing where the objective function is "did this creature successfully breed". It doesn't settle on a global optimum, just finds many many local optima that enable creatures to succeed in passing on their DNA.
Why in human males is the prostate such a troublesome thing? Because by the time the prostate becomes a problem, males have generally done any breeding they're going to do, so there is no advantage to natural selection to improving it further. Is it optimal? Definitely not.
Presumably it is (taking the wide view) probably a good thing that evolution doesn't find global optima or there would be far less ecological diversity.
Yeah I totally agree with this. We want to find explanations and justifications for everything, but it's largely possible that the location of testes actually doesn't matter -- internal, external, whatever.
> it's largely possible that the location of testes actually doesn't matter
It's not really that it doesn't matter, just that there are several different options to allow good enough fertility.
If sperm has to be stored/generated at a temperature lower than 36°C, then external testes are a solution to that, but a lower body temperature works as well. Developing enzymes that work good enough at a higher temperature also works (apparently what birds have done). And maybe just accepting a lower fitness of sperm cells works if the animal produces more of them.
Hippopotamuses have a low body temperature of about 35°C, so internal testes work for them.
Males are expendable.
In humans, only about a half of males does reproduce.
More 'experiments' are run on males by the nature, the phenotype variance is higher and includes more of excelent and more of detrimental variations, while females stick to the stable functional baseline.
Having them as an exposed 'weak spot' might accelerate the evolutionary process - those who can effectively protect that weak spot have a better chance of reproducing, those who can't get filtered out of the gene pool?
My guess is, mammals with very large body sizes have slower metabolism, so they don't run as hot as smaller creatures, and can have internal testicles without the downsides.
I learnt recently that primates will actively look to damage them during fights. Not sure if this is general knowledge that I missed but I found it interesting
