That knobbly bump behind a toad’s eyes turns out to be far more than a weird facial feature. A new study suggests this single organ — called the parotoid gland — was the game-changer that led toads to fan out from South America to almost every corner of the planet.
“The parotoid gland is that big bump you see just behind a toad’s eyes,” says Wei Xu at Institut de Recherche pour le Développement (IRD), France. “It gave toads a built-in survival toolkit, protection, hydration, and resilience, all in one structure.”
Those two functions — deterring hungry predators and helping the animal manage water and salt — made toads tougher and more versatile than most amphibians, which usually need moist, gentle environments to thrive.
What’s new here is the scope and timing. By building a giant DNA family tree for 124 species spread across six continents, Xu and his colleagues traced modern toads back to South America about 61 million years ago. Only after that did they surge into Africa and Asia — right when parotoid glands appear in the lineage. In other words, the gland shows up just as toads begin their global road trip.
The researchers also checked other possible “make-or-break” traits. Body size, for instance, often matters in biology. But here, it wasn’t the driver of new species forming. Toads did get bigger in some places, yet size didn’t map onto bursts of diversification the way toxicity from the parotoid gland did. In plain terms: bulk wasn’t the breakthrough — biochemistry was.
Xu says the gland didn’t merely harden the toads against danger — it opened doors. “The gland didn’t just help them stay alive, it helped them move farther and diversify faster.” That helps explain why toads managed feats that are rare for amphibians, popping up across continents that most of their frog cousins never reached.
So how did a salt-sensitive animal line cross oceans at all? The timeline points to two workable paths out of South America. One is a straight shot to Africa on storm-tossed rafts of vegetation; the other is a stepwise trek via a then-milder, greener Antarctica with intermittent land or island links.
The study’s analyses favor an out-of-South-America jump to Africa around 29 million years ago, followed by a spread into Eurasia — rather than a northern detour through Beringia. In the Americas, some toads went directly to North America while others stepped through Central America, but there’s no sign of backtracking into South America afterward.
“Either way, it’s an extraordinary story. A salt-intolerant amphibian group managing to cross oceans and colonize new continents,” says Xu.
Journal Reference: Proceedings of the Royal Society B: Biological Sciences DOI: 10.1098/rspb.2025.1928
