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]."
Free E-Mail News Updates
Sign up for our Inside National Geographic newsletter. Every two weeks we'll send you our top stories and pictures (see sample).
|
SOURCES AND RELATED WEB SITES
|
