Purge by Snacks: Saving Australian crocodiles by yucking their yum

Jack Tamisiea

When Dr. Seuss compared the Grinch to the “tender sweetness of a seasick crocodile,” he may have been more right than he knew. When nauseous, these toothy reptiles don’t cough up whatever they just ate; instead, they’re largely lethargic and prefer to lay about instead of swimming after another meal.

Researchers in Australia believe being a bit nauseous could help save crocodiles from a poisonous pest. That’s why ecologists recently set traps baited with toad carcasses in gorges where freshwater crocodiles like to hunt. Instead of the deadly toxins the toads usually carry, these had been injected with a nauseating chemical. The results of the experiment, published Wednesday in the journal Proceedings of the Royal Society B, reveal that these tainted toads may save crocodile lives by changing their eating habits.

Cane toads were brought to Australia in 1935 to eat pests that feed on sugar cane crops. As with many foreign species in Australia, the toads quickly became pests themselves. Capable of growing as big as chihuahuas, they look like a tasty treat to native predators. But the amphibians secrete lethal toxins from glands near their heads when threatened. Even top predators like freshwater crocodiles are susceptible to a deadly dose of toad. In some areas the toads have already reached, crocodile populations have plummeted by more than 70 percent.

According to Georgia Ward-Fear, a conservation ecologist at Macquarie University in Australia, eradication efforts have failed and cane toads “are here to stay.” So Dr. Ward-Fear and her colleagues have taken a different approach to the toadpocalypse: training native wildlife to live alongside these invaders. To prevent predators from eating the toads, the team utilises a strategy called conditioned taste aversion. Dr. Ward-Fear compares the tactic to a beneficial bout of food poisoning, reasoning that conservationists could “train animals not to eat cane toads if we gave them nonlethal experiences with cane toad baits.”

To test this strategy with crocodiles, the team focused on Australia’s remote Kimberley region. There, rivers carve the rugged terrain into gorges that are suited for freshwater crocodiles. This area is also the frontline of the toad invasion. The researchers conducted necropsies of deceased crocodiles and found cane toads in several of their stomachs.

To alter the local crocodiles’ eating habits, the team worked with Indigenous rangers to set bait along the riverbanks of four gorges (another river was left un-baited and used as a control). Each of the bait stations contained a cane toad carcass and a chicken neck dangling over the water. The team removed the toads’ poisonous glands and injected a nauseating lithium chlorine solution into the toads’ back legs. They monitored the crocodiles’ feeding activity with remotely triggered wildlife cameras.

Initially, the toad carcasses were a big hit with local crocs, who scarfed down equal amounts of toad carcasses and chicken necks on the first day. But, as the baiting period continued, the crocodiles consumed fewer toads. It wasn’t that they had lost their appetite — they still snapped up the chicken necks — but their tastes had shifted away from the tainted toads.

In the aftermath of the baiting experiment, the researchers observed a lower mortality rate among crocodiles at the four test gorges compared with the crocodile population at the un-baited control site.

Dr. Ward-Fear and her team believe that a taste aversion strategy can help make cane toads unpalatable to other predators. But she stresses that each predator needs an aversion strategy that is fine-tuned to its hunting style.