Really big guys restrain youth violence.

Biologists say the way to stop killing sprees by male juvenile delinquents is to bring in older males, at least if you're dealing with elephants.

Until recently, the orphan male elephants in Pilanesberg National Park in South Africa lived in an unnatural, predominantly adolescent world, explains Rob Slotow of the University of Natal in Durban. Starting in 1992, the young males began rampaging, and in 5 years, they had killed more than 40 white rhinos.

Introducing six full-grown bull African elephants in 1998 settled down the youngsters, Slotow says. In the Nov. 30 NATURE, he and his colleagues attribute the turnaround to a shortening of the time a young male spends in a testosterone-crazed state called musth.

The study should help conservationists who transplanted orphans to restore elephant herds, Slotow says. Such relocations had skewed Pilanesberg's population. Slotow predicts that other sites will have a problem with out-of-control juvenile elephants.

The project provided an unusual chance to see if social structure controls testosterone rushes, Slotow adds. As male elephants grow up, testosterone surges lasting days to months make them irritable, violent, and possessive of the herd's receptive females. The males dribble urine, and temple glands ooze what smells like coal tar. At the height of musth, an algal layer builds up on the elephant's penis, creating a green sheen.

In mixed-age groups, males first enter musth for several days or weeks between the ages of 25 and 30. As males age, musth lasts longer. A male in his 40s typically stays in musth for 2 to 4 months. Coauthor Joyce Poole of Nairobi, Kenya, had noted that young males lose the obvious signs of musth within hours or even minutes of being menaced by a higher-ranking male in musth. Biologists hypothesized that youngsters were physically capable of sustained musth but that run-ins with older males suppressed the youngsters' hormones.

Park ecologist Gus van Dyke first proposed importing older males to see if they'd inhibit musth, which was lasting up to 5 months, in the rampaging youngsters. After extensive detective work, he'd fingered the orphans as rhino slayers.

Kruger National Park could spare some big bulls, but the scientists needed a special low-slung vehicle that would hold standing elephants but fit under bridges on the roads between the parks.

Kruger's game-capture team pioneered a protocol for the logistical nightmare of moving adult bulls. The team injected each elephant with a dart of tranquilizer, winched the immobilized animal to the edge of the truck, and then woke it up only enough to walk into the vehicle. During the day's 500-kilometer-plus drive, the vet kept a pair of tranquilized bulls just alert enough to stand on their own.

Over the next 20 months, at least one of the six older adults was in musth during three 4-month intervals. During those spells, youngsters showed signs of musth for only brief periods and no elephant killed a rhino.

The study "provides the first field evidence in an experimentally altered situation that indeed musth in older male African elephants does affect and reduce musth in younger males, comments L.E.L. Rasmussen of the Oregon Graduate Institute in Beaverton, who studies elephant pheromones. She calls the work "a highly significant study."