PARIS -- Men who enjoy warming their bottom on a heated car seat should beware, for they may also be frying their chances of fatherhood, New Scientist reports in its latest issue.

Sperm production is best when the temperature of the scrotum is one or two degrees Celsius (1.8-3.6 degrees Fahrenheit) below the core body temperature of 37 C (99 F).

Testicle-testing German researchers fitted sensors to the scrotums of 30 healthy men who then sat on a heated car seat for 90 minutes.

After one hour, the average scrotal temperature had risen to 37.3 C (99.5 F), and in one volunteer reached a whopping 39.7 C (103 F).

By comparison, men who sat on unheated car seats reached an average scrotal temperature of only 36.7 C (98 F).

The study, led by Andreas Jung at the University of Giessen, did not verify the volunteer's sperm count or sperm mobility, but the researchers fear that only a slight increase in temperature is enough to damage the sperm-production process, the British weekly says.

Previous work in this field has already found that sitting in a car for more than three hours, even on an unheated seat, can impair a man's ability to conceive.

The report appears in next Saturday's issue of New Scientist. The study appears in full in a specialist journal, Fertility and Sterility.