From Invaders To Cannibals

Cane toads in Australia have undergone a startling transformation in the 86 years since their introduction to the continent, the noxious pests so numerous they have developed a taste for one another. These toxic and invasive amphibians have become highly cannibalistic, exhibiting behaviour not observed in their South American counterparts. Researchers are intrigued by this rapid and extreme evolutionary shift in behaviour, as it usually takes place over a much longer timescale.

Cane toads (Rhinella marina) were initially brought to Australia in 1935, with the intention of controlling cane beetles that were damaging sugarcane plantations. However, these giant toads failed to reduce the beetle population but succeeded in reproducing prolifically. Their highly poisonous skin, coated in bufotoxins, made them free of natural predators, and they spread throughout the northern and eastern regions of the country.

While adult cane toads grow up to 25 centimetres in length and are formidable creatures, it is their tadpoles that exhibit cannibalistic behaviour. Groups of tadpoles can consume more than 99% of the hatchlings from a single clutch. This behaviour prompted scientists to investigate whether it was widespread among all cane toads or specific to the Australian population due to their invasive nature.

To understand this phenomenon, researchers collected cane toads from Australia and French Guiana, breeding them to produce hatchlings and older tadpoles. They conducted experiments exposing a single tadpole to 10 hatchlings from its group, originating either from Australia or South America. The results showed that invasive Australian tadpoles were 2.6 times more likely to cannibalize hatchlings than their South American counterparts.

The research also unveiled that Australian tadpoles are attracted to hatchlings due to the scent of the younger animals' toxic skin. This was evident in experiments, with Australian tadpoles displaying a strong preference for swimming towards traps containing hatchlings. In contrast, South American tadpoles showed no such preference.

What is most striking about this evolutionary shift is the speed with which it occurred. Even more surprising is the rapid development of a defence mechanism against cannibalism by the toads. Researchers observed that invasive Australian hatchlings sharing a tank with older tadpoles from the same group had a shorter developmental period compared to South American hatchlings. This adaptation may limit the time spent in a vulnerable stage and prevent cannibalism, even if it eventually affects the toads' growth.

Scientists believe that the differences in behaviour between invasive Australian toads and their native South American counterparts are likely genetically based. This fascinating shift raises many questions that researchers are eager to explore.

One thing is certain: the cane toads currently found in Australia are significantly different from those first introduced to the continent. This rapid transformation in their behaviour provides valuable insights into the adaptability of invasive species and the mechanisms behind such evolutionary changes.

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