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.