Researchers use tainted toads to save Australian crocodiles

Researchers in Australia believe being a bit nauseous could help save crocodiles from a poisonous pest. Photo / 123RF

The aquatic reptiles cannot resist eating invasive toads that are toxic, so scientists gave the crocodiles a dose of nonlethal food poisoning to adjust their behaviour.

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 reached, crocodile populations have plummeted by more than 70%.

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 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. 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 front line 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 unbaited 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.

After the baiting experiment, the researchers observed a lower mortality rate among crocodiles at the four test gorges compared with the crocodile population at the unbaited control site.

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.

For example, the team has also worked with yellow-spotted monitors. These large lizards actively pursue live prey (the team originally tried cane toad sausages, but the aversion didn’t stick). So the team utilised young, nonlethal “trainer toads”. In another paper, the team concluded that yellow-spotted monitors who scarfed down juvenile cane toads largely avoided eating adult toads.

According to Steve Johnson, a wildlife biologist at the University of Florida who was not involved in the new paper, getting ahead of the toad invasion with these taste aversion tests will likely help prevent an initial mass mortality event among freshwater crocodile populations. But it remains to be seen how long these reptiles will retain this knowledge.

“It’s not a one-and-done thing,” Johnson said. “Even if it’s wildly successful, you have to maintain it.”

At least freshwater crocodiles seem to be willing to change their snacking habits.

“Thankfully, it looks like crocodiles might be a bit better than us at learning from their mistakes with invasive species,” Johnson said.

This article originally appeared in The New York Times.

Written by: Jack Tamisiea

©2024 THE NEW YORK TIMES