FEMALE HYENAS AND MALE HORMONES, A STRANGE
By Sean Henahan, Access Excellence
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
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
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
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.
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