By Sean Henahan, Access Excellence

BERKELEY- Aggression pays off for female spotted hyenas -- the most masculinized females in the animal kingdom -- but only if they must survive their first experience with a strange and often fatal birthing process, report researchers at UC Berkeley.

Up to 10 percent of pregnant females may die in the process of delivering their first cubs. For those who survive, however, strong evolutionary benefits go to the females who can dominate other hyenas and the food supply, according to new studies.

Reproduction in the female spotted hyena has sparked intense scientific interest because the animal is extremely masculinized from high levels of male hormones and she gives birth through an elongated clitoris the same size as a penis. Scientists have wondered what evolutionary advantage could offset the dangers of this strange reproductive system.

Researchers at the UC Berkeley Hyena Project, has found that the aggression benefits high-ranking females which raise more than twice as many cubs to maturity as do low-ranking females, apparently because they get most of the food.

The authors calculate that the reproductive success of high-ranking females is 2.5 times greater than low-ranking females, which more than offsets the death rate of mother and cub during first birth.

"We have what appears to be a strong selection for aggressive females. The side effect is that they get male genitals and this ridiculous birth apparatus," said lead researcher Laurence Frank.

So difficult is the birth process that 11 of 18 cubs conceived by new mothers in the Berkeley colony were stillborn. They would get lodged in the long birth canal, which is twice as long as it would be in a "normal" mammal, and suffocate to death. Subsequent births, after the clitoris had been stretched out, were much easier with no excess death.

The benefits of masculinization in these females, however, took longer to understand. Based on 17 years of observations in Kenya with a wild clan of about 70 to 80 hyenas, Frank has found that the aggression helps maintain a rigid hierarchy, wherein the high-ranking females have most of the surviving adult offspring. Cubs of low-ranking females die in youth, probably because they don't have enough food.

"Cubs of lower-ranking females are hungry all the time," he said. "We believe that these hungrier cubs take bigger chances."

Frank's colleagues, Kay Holekamp and Laura Smale, suggest that they may hang around a lion's kill, which is risky because the lion is likely to attack them on sight.

The feeding environments Frank and his associates have witnessed in Kenya are "extraordinarily competitive," he said.,

"When hyenas make a kill, everybody comes out of the woodwork. You see 30 animals arguing over a large kill. A zebra can be reduced to a dark patch on the ground in half an hour," said Frank. "The whole key to high rank is that you get to eat first. These mothers make room for their cubs, but lower-ranking animals get pushed off."

Over the 15 years, Frank estimates that the top ranking matrilinage (female-headed family) has increased its numbers by 50 percent. Middle ranked families have stayed the same and the matrilinages at the bottom are disappearing. The group as a whole has maintained a stable size.

"In 40 years from now (less than 15 generations), all the animals in that clan will be descended from one female," said Frank.

As in female hierarchies among baboons, rank among hyenas is acquired from the mother, so that cubs have the same social position from birth. However, these animals also maintain rank by constant aggression, and some of their fighting, at least, is devoted to teaching cubs their place in the hierarchy.

Lest anyone wish to draw comparisons with human aggression, Frank emphasized that there are "few parallels. "He said one major difference is that "cooperation in food sharing was critical in human evolution, whereas hyenas don't share."

Frank noted that early humans, who were scavengers, would have been in competition with hyenas for killed meat, and rather than get in close with clubs, humans probably developed "the ability to stand back and pitch a rock with power and accuracy."

The new hyena research appeared in Nature,10/18/95.

Related information on the Internet

Hyena Homepage