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.

In addition, the females in the male-dominated group had two to three times as many scars and wounds inflicted by the males during mating attempts as females in the female-biased group.

The researchers say the females may have died of stress caused by aggressive male copulation attempts.

Extinction Vortex

The findings suggest that male aggression puts the whole population on a fast lane to extinction.

"This paper uses direct observations and population models to show that an increase in the fraction of adults who are male can actually grow over time," said Shripad Tuljapurkar, a population biology professor at Stanford University in Palo Alto, California.

"As the male fraction continues to increase, females are subject to intense competition that reduces reproductive success, and the population can collapse," said Tuljapurkar, who was not involved with the research.

Some rare species with skewed sex ratios may find it harder to recover if males show aggression to females. Researchers believe male aggression is, for example, a threat to population persistence in the Hawaiian monk seal, one of the most endangered seal species in the world.

Human Decline

A more speculative connection is to sex ratios in humans.

In China planned birth programs and selective abortion have caused rapid changes in the sex ratio at birth. Since the early 1980s, males have far outnumbered females among newborns in China.

"Whether and how these populations will respond to increasingly male-biased sex ratios could depend on their ability to implement rules that protect females efficiently against sexual harassment," Le Galliard, the lead study author, said.

Researchers have argued that the sex ratio will rebalance itself because of so-called competitive feedbacks. A shortage of females will lead to a greater value being placed on daughters, causing male preference to decline.

This study suggests, however, that male aggression and increased competition for available females could make marriage less desirable for females, ultimately contributing to a fertility decline.

"Over the course of human history it is likely that male-biased sex ratios were common across societies," Tuljapurkar, the Stanford population biologist, said. "Perhaps conflict [killing men] provided a stabilizing mechanism against the sort of population trap discussed [in this paper]."

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