Why are sperm made below body temperature?

Eva Higginbotham’s been fertile with thought answering this question from listener Jordan…

Eva - I put the question to Bill Colledge, a reproduction expert.

Bill - The first thing to say is that only some mammals have external, otherwise known as pendulous, testes. For example, humans, sheep and dogs have external testes, but elephants have testes located close to their kidneys for example; whereas whales and dolphins have descended testes but they are not ‘hanging out’, which makes sense when you need to swim in a hydrodynamically efficient manner.

Eva - Which is a good thing too, as North Atlantic right whales have the largest testes of the animal kingdom - they can be up to about 900 kilograms!

Bill - In contrast, all female mammals have internal ovaries for making eggs - perhaps because only a small number of eggs are produced at any one time, and so they are more precious and need to be protected better, whereas millions of sperm are produced by males so small disruptions in this process can be tolerated.

Eva - One thing we know for sure is human sperm are fussy - sperm formation for men requires that the testes are around a couple of degrees cooler than body temperature, and raising that temperature can cause problems…

Bill - For example, wearing tight underwear so that the testes are held close to the body and at a higher temperature can reduce sperm numbers and impair fertility, same goes for luxuriating in hot tubs.

Eva - That means no more bubble baths if you’re trying to conceive! But we don’t actually know why sperm formation in species like humans with external, pendulous testes is so temperature-dependent…

Bill - Clearly in some species like the elephant with internal testes, sperm production can take place at body temperature quite effectively. The process of egg formation in women does not seem to be temperature sensitive, so there has been no reason to evolve external ovaries.

Eva - One recent development has been the suggestion that producing sperm at a lower temperature might actually help with the process of fertilisation.

Bill - When sperm that have been produced and stored at the slightly lower temperature associated with external testes are released into the vagina, they encounter a warmer environment and this change actually boosts their swimming activity to increase the chance of fertilising the egg. It’s a bit like they have been moved from a cold to a warm swimming pool and can now swim like a torpedo.

Eva - I should give that a go next time I try and drag myself out for a swim! That was Bill College at the University of Cambridge. Next week, get your toaster and your bread knife ready as we’ll be answering this question from Mervyn

Mervyn - Is sourdough bread a healthy option?