Cane toads are invasive species that are native to South America. They were introduced to many other countries in hopes that they could stop agricultural pests. Unfortunately, this species turned out to be a pest itself, especially in Australia.
According to Wired, cane toads have no predators and parasites since their poison glands are also hazardous for most species that try to eat them. Their relentless invasion of species where they have been introduced has made them turn toward cannibalism.
Cane Toads Eat Almost Anything
National Geographic reported that cane toads secrete a milky poison from their parotoid glands located behind their shoulders. This toxin is called bufotoxin that contains harmful chemicals, such as bufagin and bufotenine.
Sugarcane plantation owners and the Australian government decided to release about 2,400 cane toads into north Queensland in 1935 as a biological intervention against cane beetles that eat sugarcane roots. They took advantage of the frogs' ability to eat almost anything and reproduce easily to control the population of pests in their lands.
Today, the population of cane toads in Australia has boomed into millions, and they continue to expand further in thousands of square miles in Australia.
In addition, they were also introduced to other countries and areas, like the Philippines, Hawaii, Florida, the Caribbean islands, Papua New Guinea, Guam, the western Pacific islands, and elsewhere.
Cane toads breed any time of the year and lay about 8,000-32,000 eggs at a time in freshwater bodies. Even their eggs and tadpoles are also poisonous like the adults. They are very adaptive to their environments that they can even be found in both urban and agricultural areas, dunes, coastal grasslands, edges of rainforests, and mangrove swamps.
Unfortunately, they outcompete the native species for food resources and breeding habitat, making them the poster animal for invasive species.
Cane Toads Turn Toward Cannibalism
It seems that natural selection has favored cannibalistic cane toad tadpoles, according to Science Alert. In the study titled "The Evolution of Targeted Cannibalism and Cannibal-Induced Defenses in Invasive Populations of Cane Toads," published in PNAS, researchers wrote that the boom of cane toads population in Australia might have pushed them to become cannibals.
Jayna DeVore, an ecologist from the University of Sydney, and her colleagues compared the cane toad tadpoles from South America and Australia.
The team found that tadpoles from Australia were 30% more likely to enter a container with the hatchling than tadpoles from South America, Science Alert reported. Moreover, Australian tadpoles were 2.5 times more likely to gobble up these tadpoles than their Soth American counterparts.
"Cannibalism therefore shifts from an opportunistic behavior in the native range to a targeted response in Australia, whereby tadpoles cease their normal foraging activities upon detecting hatchling cues in order to locate and consume [them]," the researchers explained in their paper.
Furthermore, they discovered that this behavior has led to evolved strategies in Australian tadpoles, developing slower at their later stages of development than tadpoles from South America. However, they grow rapidly during the pre-feeding stage of their development.
The team said that the cannibalistic characteristics of cane toads might also be explained by a previous study that shows they travel six times faster than their migrant ancestors, allowing them to colonize new, cannibal-free habitats. All these changes happened within 86 years, displaying the power of evolutionary pressures.