SCIENCE ANIMAL BEHAVIOR : Hyena Study Finds Hormone-Aggression Link
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, UC Berkeley researchers say.
The discovery provides strong new evidence of the link between male hormones and aggressiveness, the researchers report today in 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 are the same sex, one immediately tries to kill the other, the researchers said. That attempt generally succeeds because the animals are born with a full set of teeth and strong muscles.
Such behavior has been observed in birds, where it is viewed as nature’s way of tailoring the size of the brood to the available resources. But today’s 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, which was made by observing the animals in a preserve on the Berkeley campus, the only spotted hyena breeding colony in the United States, caught researchers by surprise.
“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.”
“Nobody in their wildest dreams would have expected to see this kind of aggression expressed within an hour after birth,” said endocrinologist Pentti Siiteri of UC San Francisco, who has seen videos of the attacks.
The aggression had not been observed in the wild, 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 blood stream and thus are highly masculinized.
As a result, the females are larger and more aggressive than males, traits that give them a competitive advantage in searching for scarce food supplies, thereby increasing the likelihood that their offspring will thrive. “In the clan, the lowest-ranking female is dominant over the highest-ranking male,” said Frank, who has spent many years in Kenya studying the species.
When the clan makes a kill, he said, the females feed by rank, the young eat next, and males eat last.
The team studied five litters born at Berkeley, observing them closely from birth through four weeks of age. The twins are usually born about an hour apart, they found, and the first born usually attacked the second within minutes of its birth. In one case, the younger twin was attacked while it was still in its amniotic sac.
The researchers prevented the animals from being harmed, but in the wild the situation is much different, they said. Because the aardvark burrows are so small, one twin can trap its weaker sibling so that it cannot get to the entrance and suckle on the mother, which is too big to enter. Even if the weaker sibling is not killed by its twin, it will starve to death.
When Frank excavated such a den in Kenya, he found that one cub weighed more than four pounds (they weigh two to three pounds at birth) and the second weighed less than half that. The smaller cub also had infected wounds on its shoulders and was emaciated. It soon died.
Much previous research in humans and animals has suggested a link between male hormones and the higher aggressiveness of males, but proving such a link experimentally is extremely difficult, Siiteri said. “People have spent a good part of their lives trying to make such a link, but it is still a fairly open question. That’s why (Frank) went to the trouble of getting the hyenas here.”
Furthermore, Glickman said, “In human beings, and even in hyenas, social context is very important.” Once they get older, people and animals both are able to regulate their behavior despite the presence of high hormone levels.
“It would be unfortunate if people rushed to the conclusion that it (aggressiveness) is all androgen and you can’t do anything about it,” he added.
But, he added, “what is happening normally in hyenas bears some relationship to certain types of birth abnormalities in humans,” in which exposure of female fetuses to high levels of androgen produces masculinization.
The hyena study provides “a striking example of intrauterine virilization and certainly can be expected to lead to new information that will help us understand the process,” Siiteri said.
