Toads are an evolutionary success story. In a relatively short span of time, they diversified into around 500 species and spread to every continent except Antarctica. Now, Ines van Bocxlaer from Vrije University has uncovered the secrets of their success. By comparing the most home-bound toads with the most invasive ones, she has outlined seven qualities that enabled these amphibians to conquer the world. In a common ancestor, these seven traits came together to create an eighth – a pioneer’s skill are colonising new habitats.
Some, like the harlequin toads, are restricted to such narrow tracts of land that they are vulnerable to extinction. Others, like the infamous cane toads, are highly invasive and notoriously resistant to extinction despite the best efforts of Australians and their sporting equipment. This diversity of lifestyles allowed Bocxlaer to search for characteristics shared by the most pioneering of toad species.
She compared over 228 species, representing just under half of all the known toads, and constructed a family tree that charts their relationships. She showed, as others before have suggested, that the family’s fortunes kicked off in South America, around 35-40 million years ago. This was the start of their global invasion.
Seven qualities make for wide-ranging toads. For a start, the adults don’t have the typical amphibian dependency on constant water or humidity. They have skins that can cope with the drier side of life, giving them a chance to seek out new habitats away from the safety net of moist environments. Secondly, they tend to have fat deposits near their groin, which act as a back-up energy source when food is scarce. Thirdly, they tend to be larger (meaning at least 5 centimetres in length), which also helps to conserve water. Larger animals have larger bladders so they retain more water, and they lose less of it because they have small surface areas for their size.