Some like it cold
In a study of 6,455 semen samples (yup), scientists at Israel's Ben-Gurion University of the Negev found that human sperm were most atheletic — and were found in the highest concentrations — in winter. There was a marked decrease in sperm motility and numbers in spring, summer, and fall
. It's an interesting and logical addendum to the fact that sperm counts and motility decrease in men who subject their testicles to warm conditions; in hot tubs, say, or a pair of overly tight underpants.