A freshwater crocodile with an open mouth sits in running water

Australia's freshwater crocodiles have been badly affected by the invasion of cane toads. Image © Bildagentur Zoonar GmbH/Shutterstock

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Crocodiles prefer aquatic meals after toxic toad invasion

Crocodiles are more likely to eat their meals in the water after toxic toads invade their habitat.

Scientists believe that the behavioural change may help the reptiles wash away poison from the cane toad, an amphibian which has decimated Australian biodiversity. 

To avoid eating their final meal, Australian crocodiles may wash their mouths out and try a different diet instead.

Along with many other animals of northern Australia, the reptiles' habitats are being overrun by toxic cane toads, an invasive species that was introduced in the 1930s. Eating just one of these amphibians is fatal to the crocodiles.

To try and minimise this possibility, crocodiles from toad-filled areas seem to have adapted to eat their prey in the water, where they can flush out the toad's poison if they start to eat the amphibians by accident.

They also become seemingly choosier about their food, with crocodiles which are used to the toads being more likely to reject them in favour of another meal. 

The findings of the study were published in Scientific Reports

A cane toad sits in leaf litter

Cane toads are spreading rapidly across northern Australia, changing the country's ecosystems. Image © Peter Yeeles/Shutterstock

What's the trouble with toads?

Currently ranked at 16th out of 100 by the International Union for the Conservation of Nature's (IUCN) Invasive Species Group, cane toads are one of the world's worst invasive species. They are naturally found in Central and South America, where a range of natural predators keeps their populations under control.  

The toads have a voracious appetite, eating almost anything that will fit in their mouths. They mostly eat invertebrates and small vertebrates but have also been found to eat plants and even pet food.

Their appetite was recognised by twentieth century scientists as being potentially useful to controlling insect pests that were decimating crops. As a result, the toads were initially introduced to islands such as Puerto Rico and Hawaii to protect farms.

Encouraged by these early experiments and despite some early warnings, in 1935 the toads were introduced into Queensland, Australia, in an attempt to control pests on sugar cane plantations.

Since being introduced to the northeast of Australia, the toads have spread in all directions. As the amphibians are toxic at all stages in their lifecycle and with no natural predators, they reached Brisbane within a decade and have since spread across much of northern Australia. 

Despite efforts to destroy their eggs, amid suggestions of using viruses, walls and even handbags to stop them, the cane toads continue to advance across the island. The Australian government estimates that their range is expanding by as many as 60 kilometres a year as they adapt to life right across the country.

As they've spread, the toads have had a devastating impact on Australia's wildlife. Their poison has killed many native predators, leaving their prey to proliferate. Anything the amphibians don't poison they try to eat, with birds such as the rainbow bee-eater particularly hard hit as the toads eat their chicks.

However, Australian species are fighting back. Some crows have learnt to flip the toad onto its back so they can eat its soft, non-toxic underbelly. Some snakes are adapting to the threat by becoming more resistant to the toad's toxins, while others have become smaller so that their mouths are no longer big enough to swallow the amphibian.

Now numbering among these species are the freshwater crocodiles, who have been observed to change their behaviour in response to the cane toads. Following anecdotal evidence, a group of Australian scientists decided to investigate the claims more closely.

A crocodile prepares to bite a carcass in the experiment

Crocodiles were allowed to choose between chickens and de-toxified toads.  Image © Aiyer et al., licensed under CC BY 4.0 via Nature.

Smiles for crocodiles?

The researchers set up two testing areas in the Kimberley region of Australia, encompassing one area where the toads already live and another where they do not. In each area they then hung dead toads which had their toxic organs removed and chickens alongside each other along water courses.

These bait areas differed by whether they were over land, water or on the water's edge. Freshwater crocodiles were then observed choosing a food item and their subsequent response to it.

The scientists found that crocodiles in toad-invaded areas were much more likely to pick a dead chicken over a toad than those from areas where the amphibians have not yet been recorded. Even if they did pick up a toad, the crocodiles already acquainted with them were more likely to discard them.

The most dramatic differences, however, were between where the crocodiles were feeding. Crocodiles from toad-infested areas took food from over the water in 83% of cases and just 4% from on land, compared to 39% and 25% in toad-naïve crocodiles.

Videos taken of the crocodiles seem to indicate that when toads were taken, they were more likely to be handled for longer in the water, often before being discarded. The researchers suggest that this 'washing' behaviour is similar to that seen in species like the slender loris, which covers toxic prey in bodily fluids before consuming it.

However, the study had many more crocodiles take bait in the toad-free areas than the toad-occupied ones. This may be a result of crocodiles having been killed off by the toads, leaving only more cautious crocodiles alive who are more naturally wary of the experimental equipment.

The research was also conducted over a small area of the country, with larger studies needed to assess whether the behavioural changes were just the result of an unusual set of crocodiles.

For now, the cane toad continues its rampant march across Australia with little chance of an end in sight.