I know for a fact that the scrotum temp must be around 34.5°C. Why not lower? Why low temp in the first place and why not high temp?
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Body temperature ruins the sperm, at 37°C the proteins and overall the structure of the sperm decays pretty rapidly for a variety of reason, and since it needs to be stored until use (for up to 2 weeks) that’s not a good thing.
The best temperature isn’t necessarily 34.5°C, it may be lower, some studies found that sperm is well-preserved even at 20°C. But 34.5° is probably the lowest the scrotum can reach, as it’s still near our very hot body.
Why not higher, why not lower? On one level the answer is that evolution is random and it fucks up a lot because there is a large element of path dependency to it. Or it creates problems that it then has to solve, so we end up with very kluge like structures that would never look this way if you were designing from scratch. It’s actually the best argument against intelligent design: no one intelligent would have designed this.
I’ve always assumed the lower temperature incubation was for sperm thermotaxis (navigation by heat) but when I looked up the Wikipedia page on [sperm guidance](https://en.wikipedia.org/wiki/Sperm_guidance) the link isn’t there, so I assume a relationship hasn’t been fully investigated yet. It makes sense these two traits could be selected for together given their temperature relation. Someone else may be aware of more recent work that hasn’t made it into the hallowed pages of Wikipedia. They may not have an evolutionary link, that’s also possible.
The reason why it’s not lower is because for this to be achieved the scrotum would have to hang so low that it would get crushed by our legs when we sit or walk or run and this would be un-beneficial hence the trait did not get passed down to modern day humans.
A lower temp below 34.5 would probably be beneficial for sperm (this is why sperm at the sperm bank is kept in special fridge/freezer and can last very long. It’s just 34.5 is the lowest we can get it without the scrotum and balls being at risk of being crushed which would not only kill sperm, but it would also cause pain for the human attached to the saggy scrotum and this would make it very hard ti hunt for food if every time u ran u felt like u got kicked in the balls.
The other factor preventing a lower scrotal temp would be blood, the scrotum and testes are like any other organ and they need blood to survive, as it brings nutrients and oxygen to them and it takes away waste products. The problem is this blood travels through the rest of the body before it gets to the scrotum and so it’s kept at the same temp as the rest of the body (37c) and this blood being constantly pumped through the scrotum acts like a heat pump trying to warm the balls up, hence 34.5 is the lowest temp they can reach whilst hanging around outside the body
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