Testicularly Diminutive Toads Dominate Forefront Of Invasion Across Australia

This toad's grumpy expression may reflect being told his balls are small, or just because they're a fundamentally unfriendly species. David Nelson

It doesn’t take big balls to be a pioneer. Quite the opposite in fact. The idea that larger testicles correlates with the courage to break new grounds is not just sexist, it’s the reverse of the truth, at least for Australia’s cane toad pests.

Cane toads were introduced into Australia in 1935 and quickly became perhaps its most damaging invasive species. Professor Rick Shine of the University of Sydney has sought to find their weaknesses and has made some fascinating discoveries in the process.

His latest is the announcement in Biology Letters that toads in newly infested areas known as the frontier, have testicles 30 percent smaller than those in long-populated areas.

“It turns out that male cane toads are more interested in dispersal than sex at the invasion front,” Shine said in a statement

Frontier population density is low. “So, when it comes time to breed, [a male pioneer] will probably be the only male in the pond when a female comes along,” Shine added. “Because his sperm won’t have to compete with those of other males, the pioneer male can afford to invest a bit less in making sperm, and a bit more into traveling faster and farther, to stay at the invasion front.”

First author Dr Chris Friesen of the University of Wollongong told IFLScience this is the first example of testicular shriveling the team are aware of, but it didn’t come as a complete surprise. “We know the size of testes correlates with sperm production and is usually greater where there is more sperm competition,” he said.

Still, the change has been remarkably fast, with the toads only having been in Australia for 85 years, and the current speed of their spread being an even more recent phenomenon. However, the finding fits with Shine’s previous discovery of an entirely new form of evolution, where frontier toads keep evolving longer legs and a greater tendency to keep going in the same direction. This is a product of sorting, since only toads with these traits get to the frontier, and then breed with those like them.

Friesen said the toad’s distinctive dispersal is likely to have enhanced the effect, but he suspects other invasive species would show similar traits. It might be drawing too long a bow to speculate on human endowment, however.

No method has been proposed to use this knowledge to fight the toad’s spread, but, Friesen said, “The more we know about the toads the more likely we are to be able to control them at a local level.” Traps that arise directly out of Shine’s previous work have been highly effective in controlling toad numbers in some locations.

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