Without Predators, Cannibalistic Cane Toads Eat Their Young—and It’s Rapidly Accelerating the Species’ Evolution

Hatchlings in Australia have halved their vulnerable growth stage to avoid becoming their pal’s next meal

An image of a cane toad. The amphibian is a light burnt sienna color and has warts on its skin.
Cane toad tadpoles are observed eating other tadpoles in South America, their native habitat. However, the cannibalistic behavior occurs more often in Australia. Froggydarb via Wikicommons under CC BY 3.0

Australian sugarcane farmers began using cane toads (Bufo marinus) as a form of pest control in their fields in 1935—but soon enough, they became a pest themselves. The warty amphibian with thick ridges above their eyes and highly poisonous olive-brown skin devours anything it can fit in its mouth from tiny rodents to birds. When the toads became established in the country, the species had no natural predators. The invasive species has since expanded its reach across large areas of Northern and Eastern Australia with more that 200 million cane toads hopping around the country, reports Nature's Max Kozlov.

Without predation to keep population numbers low, cane toad tadpoles began to eat their peers, reports Ars Technica's John Timmer. The cannibalistic behavior appears to be an evolutionary response to the toad not having another competing species, causing the toads to turn on the only species competing for resources: themselves.

However, researchers at the University of Sydney have found that tadpoles are evolving ways to avoid becoming a snack, such as developing at faster rates to reduce the amount of time they are vulnerable to other ravenous tadpoles, reports Laura Geggel for Live Science. The study was published this week in the peer-reviewed journal Proceedings of the National Academy of Sciences.

In the species' native South American habitat, cane toad tadpoles have previously been observed eating their peers. However, cannibalistic behavior occurs more often in Australia. To see if the behavior results in differences between native and invasive species populations, researchers gathered toads from South America and Australia and bred them. Then, they introduced one large tadpole to a group of ten tiny hatchlings just emerged from their eggs. The team discovered Australian tadpoles were 2.6 times more likely to cannibalize hatchlings than South American tadpoles, Nature reports. But once hatchlings were too big to be cannibalized, the older tadpoles left them alone. Similarly, older tadpoles did not have an appetite for other tadpoles their size and age.

While South American toads spent five days at the hatchling stage, Australian toads only spend three days in this stage, suggesting that the pressures from being cannibalized cut their development time by half, per Ars Technica.

"We found that cane toad clutches from Australia developed more quickly; they reached the invulnerable tadpole stage in about four days, whereas native range clutches took about five days," says study author Jayna Devore, a University of Sydney herpetologist, to Live Science.

The find may give researchers an understanding of how competition within a species ignites an evolutionary arms race and drives rapid evolution.

"The good news is that cannibalism can control population growth," DeVore tells Live Science. "So, although cane toads are unlikely to drive themselves extinct, these cannibalistic behaviors may help to regulate their abundance post-invasion."

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