Species That Swing: Is Promiscuity Key to Survival?

(Feb. 25) -- Time to consider an open relationship? Maybe, if you're interested in promoting a thriving human race.

New research out of the universities of Exeter and Liverpool has concluded that promiscuous females could reduce the risk of a species becoming extinct. It helps explain why, among most species, females just can't stop having sex with different partners.

The practice, called "polyandry" among the scientific community, is actually a risky move: It requires extra energy, increases risk of predation and, of course, is so time consuming.

But polyandry also limits the danger of giving birth to all-female offspring, which can happen because of a key chromosome, called sex-ratio distortion (SR), which kills off the Y (male) chromosomes prior to fertilization. The inevitably all-female offspring will then carry SR, and, over several generations, fewer and fewer males will be born.

Males who carry the SR gene produce sperm with only X (female) chromosomes, and also produce half as much sperm as those without the gene. If a female procreates with several partners, that sperm will be out-matched by male partners who have a more robust sperm count.

The research team behind the study tracked populations of fruit flies, giving some the chance to mate naturally and restricting others to a single partner. Over 15 generations, five of the 12 monogamous populations had died out. Among populations where females mated freely, the species continued to thrive.

A multitude of mates inevitably suppresses the propagation of the SR chromosome, leading to a healthy mix of male and female offspring.

"We were surprised by how quickly -- within nine generations -- a population could die out as a result of females only mating with one partner," lead study author Nina Wedell, of the University of Exeter, said in a statement. "Polyandry is such a widespread phenomenon in nature but it remains something of an enigma for scientists."

This is only the latest in a series of studies that suggest we might have something to learn from an overwhelming majority of the animal kingdom. Last year, a study on central African fish concluded that female promiscuity triggered males to produce "stronger, faster sperm" to outdo rival sexual suitors.