Zoology: Overbreeding Down Under

“I can’t keep it down with trapping. It has eaten my beans and peas and has stripped the bark and branches off 50 young trees. It can stand up on its hind feet and reach more than two feet into the air to snap off small limbs.” The voracious creature that stirred the Australian orchardist to complain to the Maitland Pastures Protection Board seemed fearsome indeed. But it was easily identified. After having been nearly down and out Down Under, the wild rabbit is staging an ominous and increasingly destructive comeback.

Australia’s perennial problems with rabbits seemed all but over in 1950, when scientists deliberately introduced myxomatosis, a mosquito-carried viral disease that quickly killed off as much as 90% of the nation’s 500 million furry, long-eared pests. Since the great epizootic, though, both the virus and the rabbits have undergone mutations; many of the new strains of the myxomatosis virus are relatively harmless, and new generations of animals have developed immunity to even the more virulent strains. As a result, Australia’s rabbit population has doubled in the oast several years. It is now estimated at around 100 million.

False Chins. Haunted by memories of the pre-1950 hordes that stripped much of Australia of its vegetation and caused sheepherders and farmers an annual loss of half a billion dollars, Australian scientists are now desperately attempting to forestall another population explosion by rabbits—a pair of which, under moderate conditions, can be responsible for 9,000,000 descendants in three years.

Tons of diced carrots containing a newly developed lethal poison have been dropped from planes onto rabbit-infested areas of New South Wales, a technique that has effected a 90% kill in some areas. The trouble is that most of Australia’s rabbits have proved unwilling to leave their immediate territory to seek out poisoned bait. And since the poison-spreading planes can cover only limited areas, this rabbit reticence has severely limited the effectiveness of the poison campaign.

Painstaking prying into rabbits’ habits has revealed that their territorial boundaries are staked out by the dominant male in each group. In a process called “chinning,” he brushes his chin over twigs and stones on land that he claims; a gland in his chin deposits a distinctive scent that makes his own group feel at home but warns away outsiders. By simulating these scents, the Australians hope to ring pasture lands with odors that will keep marauding rabbits out. False chin trails may lead the unsuspecting animals directly to poisoned bait.

Dead Fleas. Most Australian scientists are convinced that myxomatosis is still the best weapon against rabbits. Geneticist William Sobey, for one, is attempting to breed a more potent type of virus by combining existing strains. He has already produced some “interesting results” but needs lengthy field tests.

Suspecting that Australian mosquitoes now carry a relatively harmless strain of myxomatosis virus that imparts immunity to more deadly strains in the rabbits it infects, the geneticist has been seeking-another virus carrier. But his search has been plagued by misfortune. Impressed by the European flea, which has successfully spread myxomatosis over the Continent, he had some 12,000 fleas shipped to Australia. Only nine of the little pests withstood the trip.

By carefully breeding those survivors, though, Sobey managed to raise his flea population to 100—only to have them wiped out when someone inadvertently sprayed insecticide into the building where they were housed. Now he has ordered fresh shipments, hopeful that the next batch will soon be ready to go forth to spread a more virulent myxomatosis throughout the land.