Spotted hyena cubs are exposed to such high levels of the male hormone androgen in the womb that they are born fighting, and in many cases one newborn cub will actually kill its twin, say researchers at the University of California, Berkeley.
The discovery provides strong new evidence of the link between male hormones and aggression, the researchers report in today's issue of the journal Science.
Females of the species, which is the most common large predator in sub-Saharan Africa, usually give birth to twins, and when both twins are the same sex, one immediately tries to kill the other, the researchers said.
Such behavior has previously been observed in birds, where it is viewed as nature's way of tailoring the size of the brood to the available resources. But the UC Berkeley researchers' report marks the first time that the behavior has ever been observed in mammals.
Although the researchers declined to extrapolate their findings to humans, the neonatal fighting represents an exceptionally pure example of androgen-driven aggression. The studies could also be relevant to certain clinical conditions in humans, such as congenital adrenal hyperplasia, in which females are masculinized in the womb.
The discovery was made by observing and videotaping the animals in a preserve on the Berkeley campus, the only spotted hyena breeding colony in the United States.
"It was the most dramatic thing to see," said psychologist Laurence G. Frank, one of the co-authors with psychologist Stephen E. Glickman and biologist Paul Licht. "We expected to see aggression, but not so severe or so early."
Endocrinologist Pentti Siiteri of UC San Francisco, who has seen videos of the attacks, said, "Nobody in their wildest dreams would have expected to see this kind of aggression expressed within an hour after birth."
The aggression had not been observed in the wild, Mr. Glickman said, because the hyenas normally give birth at the mouth of an abandoned aardvark burrow, and the infants conduct their sibling warfare inside, away from the prying eyes of scientists.
But the Berkeley group has subsequently seen that the same behavior they observed in captivity also occurs among wild animals.
Female spotted hyenas have abnormally high levels of the male sex hormone androgen in their bloodstream and thus are highly masculinized. As a result, the females are larger and more aggressive than males.
"In the clan, the lowest-ranking female is dominant over the highest-ranking male," said Mr. Frank, who has spent many years in Kenya studying the species.