Cane toads that travel in straight lines are the vanguard of those taking over Australia. This discovery could help plan for invasive species, while further expanding our ideas of how evolution occurs.
More than 150 years after the publication of “On the Origin of Species,” Professor Rick Shine of the University of Sydney uncovered a previously unrecognized way in which evolution can operate. Now he’s put a further tweak to his ideas.
Cane toads (Bufo marinus) were imported into Australia in 1935 in the hope they would control the cane beetle. Instead, they became one of the most disastrous invasive species, destroying wildlife through coastal Queensland and becoming a prime nuisance in urban areas. Gradually they began spreading across Australia’s north. Wildlife researchers were taken by surprise, however, when the speed with which the toads invaded new territory accelerated dramatically.
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Estimates of when the poisonous amphibians would arrive in vulnerable wetlands were repeatedly proved wrong as the toads moved faster and faster. Shine explained the phenomenon through the concept of “spatial sorting,” obvious in hindsight, but never before proposed.
Shine noted that some toads move faster than others. As the toads invaded new territory, the faster movers inevitably were the first to arrive. These individuals, equipped with particularly long legs, bred with each other (since there were no short-legged slow-coaches around to mate with) and produced even speedier offspring.
Generation after generation the toads sped up, until Shine came across toads who had taken things as far as they could go, developing spinal arthritis from moving too much.
However, Shine wasn’t finished with what he could learn from Bufo marinus. In the latest Proceedings of the Royal Society B, Shine and co-author Dr. Gregory Brown reveal that it is not just speed that distinguishes the frontline toads, but a tendency to move one way without backtracking.
“The extent to which you move in a straight line – and therefore travel the greatest distance with the least amount of effort – is important,” says Brown. “Taking a straight trajectory can massively increase the distance you travel per day so the evolution of straighter paths might be a particularly effective way to achieve rapid dispersal.”
Toads arrived at the Adelaide River in 2005. Shine’s team, already intensively studying the ecology of the area, tracked the toad’s movements. “The offspring of parents on the invasion front moved in straighter lines than those of toads from long-colonised areas. We can conclude from this that the trait of traveling in a straight line can be inherited,” says Shine.
As Shine notes, “These findings are worrying. They confirm that invasive animals can evolve quickly to move faster, which makes it even more difficult to manage the threat they pose to our native wildlife.”
Shine has led highly promising research on ways to control the toads by turning their cannibalistic behavior against them. However, even if this proves effective against cane toads, other invasive species may show the same evolutionary pattern without the same weakness.