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.
— Maggie
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Rob Beschizza at 6:21 am • • 
The latest prospect in male contraception: blasting your reproductive organs with ultrasound to halt sperm production. Experiments on rats using the technique dropped sperm counts to 10m sperm per milliliter; under 15m sperm per milliliter is considered infertile in humans.
The results were published in the journal of Reproductive Biology and Edocrinology. Andrologists warn that much more work is needed to rule out the possibility of damaged, rather than dead sperm. From the BBC:
Lead researcher Dr James Tsuruta said: "Further studies are required to determine how long the contraceptive effect lasts and if it is safe to use multiple times." The team needs to ensure that the ultrasound produces a reversible effect, contraception not sterilisation, as well as investigate whether there would be cumulative damage from repeated doses.
Dr Allan Pacey, senior lecturer in andrology at the University of Sheffield, said: "It's a nice idea, but a lot more work is needed." He said that it was likely that there would be recovery of sperm production, but the "sperm might be damaged and any baby might be damaged" when sperm production resumed.
In the meantime, I understand that blasting Nickelback is also an effective contraceptive.
Testicular zap 'may stop sperm' [BBC]
