If the testis are on the outside of the body in the scrotum, as opposed to being found internally, so as to keep spermatogenesis efficent at temperatures slightly less than core body temperature, then why are is the scrotum covered in hair, keeping them warm?

MsMathilde

I don't believe the presence of hair on the scrotom has much to do with temperature control. For two reasons. 1. During the embryonic state, the scrotum is formed from the same tissue that would form the labia majora if the fetus were to be female. Since the tissue for the labia majora contain hair folicles so does the skin of the scrotum. 2. The majority of scrotal temperature control is performed by the cremaster muscles. These muscles draw the testis closer to the body for warmth and relax to allow the testis to descend and cool. I'm no doctor or scientist but this makes the most sense to me. Of course I could be totally wrong. May 21 08, 7:53 PM

Baloo55th

The retention of hair there can have similar origins to the hair on the labia. One explanation is that like the hair in the armpits it is there for scent retention. Nowadays we don't use the scent of the body (not counting Chanel No 5 and the stuff they sell at the £1 Shop) for messages about availability and fertility (although according to last week's New Scientist the pheremones indicating female fertility can be detected by certain males but not at a conscious level). Status signals like these are common in most of the above water mammals. Another explanation is the simple one that powers a lot of evolution. The hair is there because there's no advantage in losing it. Many features of animal bodies are there because there is no harm caused by them. Leftovers, but ones that can be reactivated later with different functions when new needs appear. Many so-called vestigial organs have now been found to have functionality quite unrelated to their analogues in other animals. If evolution was a person, one of his/her mottoes might well be 'If it ain't broke, don't fix it'. Source: New Scientist (various dates) et al May 22 08, 1:29 PM