Cane toads (Rhinella Marina) have been quickly eating their way across Australia's northern half since their 1935 introduction. Their hunger for food, which is unchecked and unaffected by parasites or predators, has caused their numbers to explode.
The number of cane toads is so high that natural selection favors cannibalistic cane tadpoles. A new experimental study just discovered this.
The original 102 toads were introduced to Australia to control local cane beetles (Dermolepida albohirtum). They are now estimated to number in the 200 million range, with densities that are 10 times higher than their native South American habitats.
Cane toads can be toxic for both adults as well as tadpoles. This has created huge problems for Australian predators such as the cute northern quolls (Dasyurus Hallucatus). These marsupial predators are now rare and have nearly died trying to eat the cane toads.
Tadpoles of cane toad cane toads eat decomposing organic matter and algae. Although they are known to occasionally be cannibalistic in their home lands, and have been seen munching on one another even as adults in their native habitats, researchers suspect that their cannibalistic tendencies may have been exacerbated by the high number of cane toads in Australia.
Jayna DeVore, University of Sydney Ecologist, and her colleagues conducted over 500 tests to test this hypothesis. They compared native South American tadpoles against Australia's wild population.
The tadpoles were offered an empty container or a container that contained a hatchling. The hatchlings were more popular with Australian tadpoles than those from South America. They were nearly 30 percent more likely to be enticed into the container by Australian tadpoles, while the South American ones did not prefer it. Australian tadpoles were 2.5 times more likely than their native habitat to take in hatchlings.
The team explained that "Cannibalism shifts from an opportunity behavior in the native range, to a targeted response [in Australia], whereby tadpoles stop their normal foraging activities when they detect hatchling cues to locate them and consume them," in their paper.
The researchers measured the tadpole's development to determine if it had influenced other evolutionary changes.
They found that the tadpoles of Australia had developed strategies to decrease the time they spent in the vulnerable hatchling stage. The pre-feeding stage is when the tadpoles grow rapidly.
However, this comes at a price. Researchers explained that the development of Australian tadpoles was slower than in South America's later stages. This indicates "an evolutionary arms race" between the cannibalistic stage of tadpole development and the vulnerable egg- and hatchling stages in invaded habitats.
The increased cannibalism of Australia's cane toads could be explained by previous research that found changes in how adult toads move. They travel six times faster than their migrant ancestors and move in straighter lines. This is the first time that path straightness has ever been recognized as a hereditary trait in animals.
DeVore explained that this increased dispersal capability accelerates colonization of new cannibal-free environments.
This type of predator-prey-driven natural Selection is well documented in many species. However, it has not been demonstrated as clearly within one species.
These changes took place in just 86 years. Although they reached sexual maturity quickly, it still leaves a lot of toads. This really shows the power of evolutionary pressures in shaping all life forms.
This research was published by PNAS.
