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Endangered animals like northern quolls can thrive on isolated islands.

Jonathan Webb

This endangered Australian marsupial was set to make a comeback—until it stopped fearing wild dogs

It was supposed to be a rare success story in the bitter battle to save an endangered species: Over the past 80 years, the northern quoll, a cat-size marsupial with a pointed snout and long, bushy tail, had vanished from much of Australia, the victim of an invasion of toxic cane toads. To ensure the species’s survival, conservationists relocated a handful of the animals to a pair of island sanctuaries free from predators, where they flourished. But when people tried to reintroduce them to the mainland just 13 years later, the quolls were quickly eaten by dingoes—a threat their forbearers had more easily avoided.

Now, a new study explains why the quolls were so quick to fall victim to wild dogs: Idyllic island living can rapidly shape the genetics and behavior of endangered populations, leaving them ill-prepared for the hazards they face back in their natural habitats.

It’s “fascinating and highly significant,” says ecologist Adrian Manning of the Australian National University in Canberra, who was not involved with the work. The new results, he says, “provide insights into … how quickly predator savviness can be lost.”

Northern quolls once lived across a wide swath of northern Australia. After the arrival of Europeans, new predators like cats began to make a dent in the quoll population. But things went downhill fast after cane toads were introduced in the 1930s. Northern quolls are ravenous predators that feed on insects, small mammals, and reptiles, and they found in the cane toads an abundant food source. The only problem—the toads are highly poisonous. Today, northern quolls are endangered, and they exist only in small pockets on the Australian mainland.

However, on Astell and Pobassoo islands, a few kilometers off Australia’s northern coast, the animals are flourishing, thanks to a lack of both predators and toads. Since conservationists relocated 64 animals there in 2003, their numbers have swelled into the thousands.

In 2016, Christopher Jolly, a Ph.D. student at The University of Melbourne in Australia, and his colleagues attempted to reintroduce 29 quolls from Astell Island into Kakadu National Park, a nearby mainland reserve where the quoll population had previously been decimated by cane toads. To give them the best shot at life, the researchers even trained some of the quolls not to eat toads. But then an unexpected threat came along: dingoes. The feral dogs wiped out almost the entire population within 21 weeks. “You solve one problem and you discover another,” Jolly says.

To find out why, the team compared how quolls from Astell and those from mainland Australia respond to the scents of predators. They set up three boxes containing mealworms, which the quolls could eat by reaching their snouts through a hole in each box. The holes were lined with collars containing dingo fur, cat fur, or no fur at all.

Quolls from the mainland, which normally live alongside predators, were particularly cautious when it came to the predator scents. They ate fewer mealworms from the dingo and cat fur–lined boxes, and spent more time investigating them. But the Astell Island quolls behaved the same regardless of the box, showing no aversion to the scents of the predators, the researchers report today in Biology Letters. Intriguingly, offspring from the two populations—which had been raised in captivity—showed the same patterns of behavior as their parents, suggesting there was a genetic basis to the animals’ responses.

The rapid loss of antipredator behavior in the Astell Island quolls “surprised all of us,” Jolly says. The quolls had been on the island for just 13 generations, and most researchers have assumed that loss of these kinds of traits would only emerge after hundreds or thousands of years of evolution. The findings suggest that the very behavior that helped certain quolls avoid predators on the mainland may have doomed them on Astell Island, the researchers say. For example, a particularly cautious quoll might survive predatory dingoes, but that wariness could have prevented it from venturing out and finding enough food on a predator-free island, leaving only the more daring animals to survive and pass on their genes.

Jolly says it may be common for animals to quickly lose their aversion to predators when they are kept in isolation, which could also raise concerns for other conservation programs.

Daniel Blumstein, a behavioral ecologist and conservation biologist at the University of California, Los Angeles, who was not involved with the work, agrees. “One of the main conservation strategies now is building predator-proof fences,” he says. “What this paper shows is that if you’re doing that … you can lose antipredator behavior pretty quickly.”

One solution? Keep predators around. Some conservation groups are including small numbers of predators in protected areas to ensure that that endangered species don’t lose their defenses. Blumstein, for example, is involved in a project in South Australia called Arid Recovery, in which two other marsupials, burrowing bettongs and bilbies, are conserved alongside a small number of feral cats; they’ve found some evidence that these animals are more wary of predators than those that live in predator-free zones.

“Unless the endangered species have some way of dealing with [predators],” Jolly says, “they’re going to be relegated to these islands or enclosures for time immemorial.”