Nothing can eat Australian toads, so they eat each other

The cane toad can be the poster animal for invasive species. Native to South America, it has been introduced to many other ecosystems in the hope that it will reduce agricultural pests. Instead, the toad itself has become a pest, especially in Australia. Free from the predators and parasites of its natural range, the toad’s poisonous glands have been shown to be a danger to most species that attempt to eat it where it has been introduced.

But that does not mean that it is completely free from the risk of predation. Australian tadpoles have been observed feeding on their fellows. This cannibalism appears to be an evolutionary response to the lack of competing species in its invasive range, forcing the toads to turn against their remaining competition: against each other. And the toad has already turned to an additional evolutionary response in an attempt to limit the danger of cannibalism.

Only compete with themselves

From an evolutionary standpoint, cannibalism can make sense as a means of limiting the competition posed by other members of your species. But the University of Sydney research team that has hunted down the cannibalism of the toad suggests that the species’ successful invasion in Australia has increased this evolutionary pressure, which can also occur with other invasive predators. One of the marks of a invasive species is its abundance in its new range, in which case competition for limited resources becomes more likely. Cannibalism not only limits this competition, but also provides nutritional resources.

With the Australian population reaching around 10 times the population density within the toad’s natural range, there are plenty of opportunities for competition between toads. And this competition has been documented in the early stages of toad development. Newly hatched toads spend several days turning into tadpoles, and during this time are often eaten by older, more mature tadpoles. In a densely populated body of water, clutches of eggs laid after the presence of mature tadpoles can be completely wiped out before they can live past the hatchling stage.

Tadpoles eating tadpoles can occur in South America. But it happens much more often in Australia. So the researchers decided to see if cannibalism produced biological differences between native and invasive populations.

To do this, they obtained toads from native and invasive populations and followed the behavior of the offspring. To begin with, the researchers simply placed fertilized eggs in a container with a single tadpole. This showed that Australian toads had become aggressive cannibals, as eggs placed with them were more than 2.5 times more likely to be cannibalized before producing a tadpole.

While many changes can make this kind of a difference, researchers have shown that Australian tadpoles are more likely to look for recently hatched toads. When given the choice of moving around in empty containers and a container of toad hatchlings, the invasive Australian toads were almost 30 times more likely to enter the container with hatchlings.

By the time newborns reach the tadpole stage and are too big to eat, their peers lose interest. There are indications that the early attraction is based on toxins introduced into the fertilized egg by the mother.

The best defense

High levels of predation tend to produce evolutionary responses to limit vulnerability, and cannibalism is no different. Researchers found that Australian toads simply spent less development time in the vulnerable stage of hatchlings in order to avoid some of the impact of cannibalism.

This happened through two different mechanisms. One of them depended specifically on the presence of tadpoles. In other words, when the threat was present, development accelerated. But a distinct acceleration was present whether or not tadpoles were present. While South American toads spent a total of about five days in the hatching stage, Australian populations only spent three days. Thus, the pressure of cannibalism had reduced the development time of newborns by almost half.

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