The whiteleg shrimp, which is native to the eastern Pacific Ocean and farmed in South America for food, is the first creature ever identified whose male has a monthly reproductive cycle (or "period.") This startling discovery, by researchers at Ben-Gurion University of the Negev, sheds new light on the widespread belief that males are always ready to mate.

Known in Latin as Litopenaeus vannamei, the creature is not really a shrimp, but a prawn that is caught from Sonora in Mexico to northern Peru. A small amount is also farmed off the waters of Texas.

The males of the species produce two "packets" of sperm that are stuck to the female during intercourse. If the male does not use these packets, they are liable to become so hard that the male is unable to ejaculate them. As a result, the sperm in effect have a "use-by" viability date, after which the male becomes impotent.

BGU researchers in Beersheba headed by Prof. Amir Sagi of the marine biology and biotechnology study center discovered that the male whiteleg have a mechanism that - during their youth - allows them to avoid the stiffening of the sperm packets and resulting impotence.

Pacific Prawn.

Sagi and lab researchers Shmuel Parnas, Shaul Raviv and Assaf Schechter raised 50 males and followed their behavior over a period of eight months. They also documented the cycles of the whitelegs' sperm production.

At the end of every two-month cycle, the sea creatures lose their armor-like shell and grow a new one. Males that have not mated were found to have lost their old sperm packets near the last few hours of their "periods," and new ones appear in their place. This means that male whitelegs renew their sperm packets in a cyclical manner, once every two weeks, in perfect coordination with losing their armor, a process that is accomplished through hormonal supervision.

Parnes explained that this process is very similar to menstrual cycles in human females and primates of reproductive age whose endometrium (uterine lining) breaks down and is eliminated in the form of blood.

"Even if the young whiteleg male does not have success producing progeny during one cycle," Parnes said, he still has another chance with the next cycle without becoming impotent. This finding puts a new light on the common belief that males always want to mate."

The discovery has aroused much interest among scientists and been published in journals such as Experimental Biology, as well as New Scientist and in programs such as BBC Wildlife.