How poisonous glands helped modern toads conquer the world
Paul Arnold
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Gaby Clark
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Robert Egan
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Modern toads (Bufonidae) are among the most successful amphibians on the planet, a diverse group of more than 600 species that are found on every continent except Antarctica. But just how did they conquer the world? An international team of researchers set out to find the answer and discovered the toads' global success was due to their toxic glands and geological timing.
Modern toads are a type of frog with a stout, squat body, relatively short legs, toothless mouths and a thick, dry, warty skin. One of their most distinctive features is a large gland behind each eye that secretes a poison to deter predators. They originated in South America and are found in diverse habitats like deserts and rainforests.
To find out how they got from South America to almost every other continent, the scientists analyzed fresh DNA samples from 124 species from Africa, Asia, Europe, South America, North America and Oceania. They combined this with existing genetic data from hundreds of other species. Using powerful computer models to process the genetic information, they traced the geological spread of toads over millions of years, identifying when survival features like their poisonous glands evolved and when they branched out to form new species.
The work is in the journal Proceedings of the Royal Society B: Biological Sciences.
Crossing continents
The team confirmed that modern toads originated in South America around 61 million years ago, which is only a few million years after the dinosaurs died out. From there, they most likely crossed the Atlantic Ocean into Africa. This was surprising as they're not strong swimmers and can't drink saltwater. And prevailing theories suggest they would have taken an easier route and traveled north through Central and North America to get to Asia. It's not known how they traversed the ocean, but it could have been on floating mats of vegetation.
The research also revealed that shortly after dispersing out of South America, there was a dramatic rise in the number of new toad species. This success was driven by a key evolutionary feature. The team found that the development of the poisonous parotoid glands occurred at about the same time as they were leaving the continent. This powerful defense against any would-be predators was a key reason for their global success, demonstrating how animals with biological limits can overcome seemingly huge obstacles.
"Our results highlight the need to seriously consider both oceanic and Antarctic corridors in reconstructing amphibian biogeography and demonstrate that, under favorable climatic and geological conditions, even physiologically constrained groups like amphibians can overcome formidable geographic barriers," wrote the scientists in their paper.
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More information: Dongyi Wu et al, Earth history and trait innovation drive the global radiation of modern toads, Proceedings of the Royal Society B: Biological Sciences (2025).
Journal information: Proceedings of the Royal Society B
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