[set fair (OP)]

When I was at school they taught us that it was because they needed to be a couple of degrees C cooler that the rest of the body. At the time i thoughy that a load of bollocks and i understand that's how the idea is generally regarded these days. My theory was that they dangled to be exposed to radiation, presumeably for mutations of the sperm. My hypothysis was that there must be an optimal rate of mutation (no evolution without mutation, and no eveolution = getting outcompeted to extinction) and that, since life took hold on earth, the background radiation has fallen significanly - in fact hugely.
   I haven't given up on that idea but covid research has given a new possibility - a different type of radiation. The so called vitamin D paradox - hosptalised patients with low levels of vitamin D fared worse but no amount of vitamin D injection improved matters. The conundrum is claimed to have been resolved by regarding vitamin D levels as a marker of sun exposure and the benefit of sun exposure for covid patients is higher melatonin levels. So perhaps males dangle to get infra red light exposure so they can produce melatonin.

[alancalverd]

In the UK, about half of the ionising radiation received by the squidgy bits inside the abdomen actually arises from the potassium in your food. So moving the testes inside won't make much difference.

There are countries with much higher levels of background radiation, but food grown there will also contain more radionuclides from the soil,so again not much difference.

In any case, nucleic acids are highly labile and undergo survivable "natural" mutations thousands of times more frequently than those attributable to background radiation.

Nevertheless it does seem odd that the manufacture of sperm is optimised at a lower temperature than its storage, thus suggesting that there isn't one Great Designer but three, and the chemist didn't discuss his requirements with the physicist, so as usual the engineer had to build a bodge that kind of works for both even if it looks ridiculous.

 

[evan_au]

it does seem odd that the manufacture of sperm is optimised at a lower temperature than its storage

I'm not sure where you store your sperm, but the epididymis is part of the testes, so a few degrees cooler than body temperature.

I heard a suggestion that a sudden temperature rise of a few degrees is one of the signals that activates sperm (there are chemical signals as well).
https://en.wikipedia.org/wiki/Epididymis

[alancalverd]

But the active part of an ejaculate has been held in the vas deferens and its ampulla prior to discharge.

It's all dreadfully reminiscent of D-day, years of training and preparation, then at the peak of your mental and physical fitness you hang around smoking for a couple of days and get seasick in a landing craft. An Intelligent Designer with infinite resources would have built a direct tunnel, used an airborne assault (pace Woody Allen), or modified the chemistry to operate at a marginally higher temperature.

And no general would plan for 99.9999% casualties - that's politics, not combat.