This may relate to the observed tendency for wearers of jockey style undies to have lower sperm motility than for those wearing boxer style. Some "fertility counselors" in fact recommend a switch to the "looser better ventilated" style for those who consult them.

It is well documented that a principal reason for the infertility of caribou - domestic cattle crosses, produced in the attempt to get "beefier" caribou, is the excessive testicular temperature maintained by the thick hair inherited from the caribou. Successful breeding to at least third generation has been reported, but the "researchers" did note that keeping the bulls' balls shaved was a real chore. (They're not exactly house pets...)

John