National Geographic : 1977 Jul
have helped develop two effective new sub stitutes. I'm confident science can stay ahead of the rats' ability to acquire resistance. "Our best source of new toxins," he added, "is the pharmaceutical companies. Sometimes a prospective new drug kills the rats it's tested on. This may be disconcerting to the drug company, but it interests us. That's one way we come upon our new poisons." Anticoagulant poisons, which cause fatal internal bleeding, have formed the first line of defense against rats since their develop ment at the University of Wisconsin nearly three decades ago. Investigating why cattle that eat spoiled hay often hemorrhage and die, scientists finally isolated a chemical called dicoumarol. From this came the syn thetic compound warfarin, deadly against rats but relatively safe with other animals. In their search for safe new rodenticides, scientists must contend with the rat's frus trating bait shyness, an almost supernatural ability to detect harmful food-as little as one part in a million. And once shy, a rat will starve rather than eat food it distrusts. Trapping presents similar problems. Rats exhibit an almost paranoid suspicion of the new and different. Set out a trap-or even a harmless brick-and rats may vanish for a night or two. This wariness gives rise to much of their reputation for cleverness. To overcome these defenses, scientists ex periment with a host of other potential weap ons: chemicals and radiation devices for sterilizing rats, high-frequency-sound genera tors to create sonic barriers, gluey surfaces to mire down trespassing rats. A rambunctious device already on the market grabs the victim in steel jaws, electrocutes it, then dumps the body into a plastic bag. In light of our age-old hostility toward rats, it is strange that we have known of their most destructive role-as a carrier of disease-only since the turn of the century. Certainly Italian merchants felt no alarm in 1347, when ships laden with spices from Shocked but alive, a rat decides it's had enough of a new sublethal electric fence developed by the U. S . Fish and Wildlife Service in Denver for use in farm areas. A mild shock deters rats and spares livestock and other animals.