RICK SHINE can’t stand the smell of whisky. His aversion stems from a youthful excess of the spirit that left him puking. That was more than five decades ago. Now the evolutionary biologist at the University of Sydney is trying to trigger the same sickness-induced repulsion in Australian predators. It’s not that they have a drinking problem. But they do need to learn to avoid another noxious substance: cane toads.
These large amphibians are a massive nuisance. Some 80 years ago, a few were transported from Hawaii to Queensland in north-eastern Australia to gobble up pests that were damaging fields of sugar cane. Today, perhaps as many as 1.5 billion of them cover northern Australia. It’s like a cheap all-you-can-eat buffet for native carnivores – one that comes with guaranteed food poisoning. When threatened, the toads secrete venom from glands on their shoulders that is strong enough to kill many predators on the spot. As a result, cane toads are decimating populations of native species including lizards, freshwater crocodiles and the northern quoll, a now endangered marsupial. Shine hopes he can save the predators’ lives by putting them off cane toads in much the same way he acquired his loathing of whisky.
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It is not the first time conservationists have tried this approach, formally known as conditioned taste aversion. In 2011, wildlife biologist Bill Given had the idea of trying it out on lions. Their drastic decline is partly down to a taste for livestock, which leads farmers to shoot them. So Given began feeding beef laced with a nauseating toxin to lions that had killed cattle and been brought to a safari lodge in Botswana. Pinning down the right dosage has been tricky. And the lions’ robust constitution hinders learning. “Their systems are quite forgiving when they get ill,” he says. Nevertheless, he has successfully triggered aversion in one lioness, and is hopeful that he is finally closing in on the best approach.
Also in 2011, the US Fish & Wildlife Service began a trial involving endangered Mexican wolves, the rarest subspecies of grey wolf in North America. Like lions, these wolves have a taste for livestock that incurs the ire of farmers. This time there was a different problem. “In the captive facility, we were able to generate an aversion for wolves to livestock bait,” says Colby Gardner of the Mexican wolf recovery programme. “But once we let them go, we couldn’t say whether or not they were killing livestock.” Without hard evidence that the experiment was working, the initiative eventually fell by the wayside.
Nevertheless, a recent project leaves no doubt that conditioned taste aversion has great potential in conservation. Researchers in Richard Golightly’s lab at Humboldt State University, California, were looking for a way to stop Steller’s jays eating the eggs of the endangered marbled murrelet. Pia Gabriel had a crazy idea of training the clever corvids to dislike the eggs, says Golightly. “Then we researched it a bit and realised maybe it wasn’t so crazy.”
“The animals dropping dead were gorging on the plumpest, most toxic toads”
So they painted small chicken eggs to mimic the green-blue hue of murrelet eggs and injected them with a nausea-inducing chemical. Sure enough, , jays learned to avoid the eggs. The team then began scattering fake murrelet eggs throughout California’s Santa Cruz mountains, where the murrelets nest. The results were dramatic. Once aversion kicked in, it seemed to persist – jays even appeared to train their offspring to keep away from murrelet eggs. In 2017, after four years, the project ended because the murrelet population had recovered from near-collapse.
Shine is hopeful he can emulate this kind of success. Ten years ago, when cane toads first reached his study site in Darwin, the capital of the Northern Territory, he realised that the predators dropping dead were those gorging on the plumpest, most poison-packed toads at the invasion front. He suspected that if they received a lower dose of the toxin, it wouldn’t kill them, but it might give them an educational bellyache.
Since then, experiments by Shine and others have shown that conditioned taste aversion can indeed deter native predators from tucking into toxic toads. The trick is to pick the right bait for a given species. Northern quolls, which often scavenge prey, have successfully learned to abstain after being fed small, dead cane toads or . For lizards and crocodiles, however, may be the best deterrent. Before this research, it was assumed that the only way to protect endangered native fauna was to go after the invader. “This really turns it on its head,” says Shine. “It says that you can actually modify the outcome of the interaction between the native species and the invader.”
The fact that Australia’s predators excel at learning to avoid toxins is no great surprise to Alison Greggor at San Diego Zoo’s Institute for Conservation Research. They have evolved on a continent with venomous creatures galore, she points out. “Other species that don’t have that evolutionary history may be slower to learn that a foul taste or a sickness can be associated with a certain prey item.”
In fact, some may never learn. For example, although insect- and fruit-eating bats quickly developed a distaste for foods accompanied by a nausea-inducing dose of lithium chloride, didn’t seem to get the memo. Their one-track diet has left these blood sippers with little capacity to recognise bad food. The vampire bats’ stubborn persistence with doctored blood, even after getting sick, illustrates a crucial prerequisite for conditioned taste aversion to work: your target animal must eat a variety of foods.
Nevertheless, Greggor believes the technique could have wide conservation value. “It’s a very exciting area of research because there’s a lot of potential,” she says. “We just need to figure out exactly how to implement it.”
When it comes to cane toads, that has already been done. Shine is now set to target the invasion front, which is rapidly sweeping westward through the Kimberley region in the north-west of the country. Part of the plan involves releasing live baby toads so predators meet these non-lethal snacks before they encounter the real deal. “When I first suggested releasing live toads in advance of the front, there was an outcry,” says Shine. “Community groups thought it was the world’s stupidest idea that this idiot wanted to release more toads.” What a difference some good science can make. Now the doubters are enthusiastically on board.
This article appeared in print under the headline “Alien vs Predator”
