Too Many Males: A Fast Lane to Extinction?

Stefan Lovgren
for National Geographic News
November 30, 2005

A new study suggests that having too many males of one species could lead to more sexual aggression toward females and ultimately the extinction of the species.

Researchers studying lizards have shown that an excess of males will cause populations to plummet as the survival and fertility of female lizards drop dramatically.

"[This study] is evidence for a novel and important behavioral factor of population extinction: male aggression toward females exacerbated by male population bias," said lead researcher Jean-François Le Galliard, a biologist at the University of Oslo in Norway.

The lizard data might even apply to human populations. Countries such as China, South Korea, and India have a significant excess of males in the population, mainly due to a preference for male babies.

The study was reported yesterday in the journal Proceedings of the National Academy of Sciences.

Male Domination

Most biologists have argued that sex ratios in a species are stable, because competition among the dominating sex will even out any deviations.

The new research, however, challenges this basic tenet of biology.

To study how animals respond to sex ratio bias in their population, the scientists monitored two groups of common lizards. The reptiles were kept in enclosures covered by nets to stop birds from munching the lizards.

Each lizard population was skewed to either three-quarters male or three-quarters female.

After a year, the group of mostly female lizards had grown from 73 to 118, while the population with excess males had declined to just 35.

The mostly male population became even more skewed toward a male majority, as adult and yearling females in that group died four times more often and produced three or four offspring instead of the usual five.

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