A lot of attention has been paid to the biological clocks of women recently, particularly when female fertility starts to decline and by how much. Now research published today in the journal Human Reproduction suggests that a man's fertility starts to slide as early as his twenties.

Brenda Eskenazi of the University of California at Berkeley and Andrew J. Wyrobek of the Lawrence Livermore National Laboratory led a team that tested 97 healthy, nonsmoking males between the ages of 22 and 80 and found significant age-related decreases in semen quality, which is a well-known proxy for male fertility. Semen volume waned each year and sperm motility also declined significantly with age. However, Eskenazi notes, "unlike women, there appears to be no evidence of an age threshold, but rather a gradual change over time."

Meanwhile, other research appearing in the same issue links lead exposure to low fertility in men. Susan Benoff of the North Shore-Long Island Jewish Research Institute and her colleagues tested semen collected from 140 men whose partners were undergoing their first in vitro fertilization cycle. The scientists discovered a significant correlation between seminal lead levels and low rates of fertilization. When the researchers subjected healthy sperm to increasing amounts of lead, they determined that the exposure limited the sperm's ability to both bind to the egg and fertilize it. Comments Benoff, "In light of these results, environmental exposure limits for lead might be re-evaluated."