TOAD CONQUEST: The cane toad has been able to methodically conquer Australia thanks to a set of traits derived from a ancient toad ancestor, including the relatively large size displayed by "Big Bertha" here. Image: Peter Gibney
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Cane toads are seemingly innocuous enough. First imported to Australia to control a beetle pest of sugarcane fields, they are now frog-marching their way across the island continent, wreaking havoc on in situ flora and fauna. The key to their domination has been protection from would-be predators and an ability to breed fast. But how were cane toads gifted with those traits in the first place?
A new study published February 5 in Science aims to answer that question. Biologist Ines Van Bocxlaer of Vrije University Brussels and her colleagues analyzed the kinds of traits that allow various toad species to thrive under many conditions and thereby expand their ranges: independence from constant access to water and humidity; glands that produce poison as protection from predators (which double as water storage); and an ability to lay large amounts of fast-hatching eggs in temporary waters, among others. Perhaps most surprisingly, at least in the case of toads, bigger body size is better. Unsurprisingly, the cane toad—and many of its 500 Bufonidae family brethren—shares most of these traits, including a propensity for quick adaptation and blitzkrieg-like range expansion.
The toad family originated in the tropics of South America before colonizing the rest of the globe. That initial colonization was set off, according to this new analysis, by the development of this set of traits, which has subsequently allowed most conquests of new territories, such as the expansion of toads from tropical niches in India to more diverse, drier habitats. In fact, this may explain why toads that are only distantly related genetically often share so many of the same traits: Conditions cause the various species to converge back on the traits of the same ancestral range-extending type of toad.
Those toads that do not share these traits, such as the harlequin, are not doing as well. Climate change is making life more difficult for specialized amphibians of all kinds, and amphibian chytrid fungus, an infection that is helping to wipe out populations around the globe, afflicts as much as 50 percent of extant amphibians.
That said, the cane toad may just be living up to its genetic legacy. "The origin of this range-expansion ability," Bocxlaer wrote, "appears to be rooted deep in the evolutionary tree of toads and may be a remnant of when toads colonized the world."
Slide Show: How Toads Conquered the World