Are young hyenas just misunderstood? 

The spotted hyena is one of the few carnivores born with functional teeth and the strength to use them in attacking a sibling. (University of Pennsylvania African Studies Web site) Two research teams are taking a new look-perhaps a kinder one-at some of the most vicious sibling rivals among mammals.

 Young spotted hyenas don't kill same-sex twins as a matter of routine, reports Kay E. Holekamp of Michigan State University in East Lansing. It's tough times that promote siblicide, she argues after monitoring hyenas in Kenya.

 Those tough times, when mothers must commute long distances to hunt, lead to extra aggression in young hyenas in Tanzania, confirm Waltraud Golla of the Max Planck Institute for Behavioral Physiology in Seewiesen, Germany, and her colleagues. Their report in the October Animal Behaviour follows one published there in September by Holekamp, Laura Smale of Michigan State, and Paula White of the University of California, Berkeley.

"There's no evidence for obligate siblicide," Holekamp maintains. "Suddenly, hyenas fit into the bigger picture: It just doesn't make sense to routinely kill your sibling."

 It does for some birds, like the masked booby, however. A mother booby routinely has an extra chick, which its sibling kills. Although newborn piglets sometimes kill their littermates, siblicide among mammals is rare.

 Holekamp traces much of the interest in hyena siblicide to studies in the early 1990s led by Laurence Frank of University of California, Berkeley. In the wild, researchers saw mixed-sex twins but few male, and no female, pairs. Observing violence between siblings, the researchers proposed that one newborn, same-sex twin starves the other by keeping it from nursing.

 The notion of hyena infants as programmed killers took on a life of its own, according to Holekamp. "People love to go, 'Gee, golly, whiz, what a weird animal it is,'" she grumbles.

 The team Holekamp worked with observed a split in a hyena clan. Afterward, each animal that remained had more food than before. "Within one reproductive cycle-boom-they started producing girls like mad," Holekamp says. That burst of female-female twin sets dashed the idea of automatic sister killers, the team argues.

 Golla monitored aggression between siblings, including same-sex sets, during the year or more that their mothers nurse them. She observed siblicide when the mothers commuted long distances to hunt scarce prey. In fat years, when most cubs grew fast, she saw less aggression than in lean years.

 Frank acknowledges that hyenas don't automatically kill same-sex siblings. He adds that he never used the term obligate to describe hyena siblicide. "That's setting up a straw man," he objects.

 However, a difference of opinion remains on the mechanism for gender-ratio shifts. Holekamp speculates that some aspect of the female reproductive tract could effect the switch.

"There's no evidence for a prenatal mechanism," Frank argues. It is at least as plausible, he says, that mothers control the sex ratio after birth by intervening in siblicidal battles.