No one is sure exactly when or how the invasion began. However, in 2014, when Asian common toads (Duttaphrynus melanostictus) were officially spotted in Madagascar’s largest seaport, scientists immediately sounded the alarm. These invasive amphibians secrete a toxic slime, stored in specialized glands behind their eyes, that could spell death for any native predators that try to eat them — a major threat to Madagascar’s unique biodiversity. Cane toads (Bufo marinus), which can remain toxic long after they are dead, created a similar problem when they were introduced to Australia in the 1930s.
However, the poisonous sacs, known as parotoid glands, do not just allow toads to wreak havoc as invasive species. They may also have helped toads spread across large parts of the planet millions of years ago, according to research published in mid-October 2025 in the Proceedings of the Royal Society B. The findings “shed new light on how toads came to dominate much of the globe,” says Daniel Paluh, an Evolutionary Biologist at the University of Dayton who was not involved in the new study. The research also raises “new, exciting questions” about the routes toads took between continents, he adds, and demonstrates how Earth’s geological history and the evolution of new traits can work together to shape modern biodiversity.
With their warts, toothless mouths, and toxic slime, modern toads — which include all species in the family Bufonidae — are a “particularly weird” group of frogs, says Molly Womack, an Evolutionary Biologist at Cornell University who did not participate in the new work. They are also one of the most successful groups, with hundreds of species thriving in a variety of terrestrial, arboreal, and burrowing habitats on every continent except Antarctica.
The group’s evolutionary history, however, is somewhat mysterious. Evidence suggests toads got their start in South America but it is unclear exactly when and how they spread to other continents. Some scientists believe the amphibians first expanded into Central and North America, then crossed into Asia via the Bering land bridge. However, this scenario is subject to speculation.
To find out more about how toads conquered the world, scientists analyzed DNA from 124 species across six (6) continents, representing all major Bufonidae clades. Their reconstructed evolutionary history confirmed that toads originated in South America approximately 61 million years ago; however, it also uncovered something unexpected: Instead of dispersing into Asia from North America via the land bridge, early toads appear to have crossed directly from South America to Africa.
They could have made this journey in two (2) ways, explains study co-author Christopher Raxworthy, a Herpetologist at the American Museum of Natural History.
- The amphibians could have migrated in a stepwise fashion through Antarctica, which once had a much warmer climate than it does today and may have been intermittently connected to South America until about 30 million years ago. The discovery of frog fossils in Antarctica in 2020 would seem to support this theory; and
- It is also possible that toads sailed across the Atlantic Ocean on floating mats of vegetation — a type of dispersal known as rafting. Although the concept of seafaring toads may seem fantastical, the same theory has previously been proposed for some island-dwelling species. Reed frogs on islands in the Gulf of Guinea, for example, appear to have traveled overseas from mainland Africa. Toads are notoriously resilient, Raxworthy notes, and may have been able to endure long journeys with little access to food and water. Both scenarios are “really interesting,” Womack says.
The team also discovered a “peculiar” and rapid rise in the number of new species shortly after toads began to disperse out of South America, Raxworthy explains, which coincided with a historic transition known at the Eocene-Oligocene extinction event, about 33.5 million years ago. During that same period, toads evolved their parotoid glands, which ward off predators by secreting milky white alkaloid substances called bufotoxins. “That was almost like a eureka moment for me,” Raxworthy says. “Once you evolve this antipredator strategy, you can imagine, it gives you a tremendous edge over other amphibians.”
“The parotoid gland was the real game changer,” agrees study co-author Wei Xu, an Evolutionary Biologist at the Research and Development Institute, Montpellier.
The team also investigated whether other biological traits could have helped toads spread across the globe and become more diverse. The development of so-called inguinal fat bodies, which help toads hang on to extra energy reserves during times of hibernation or food scarcity, appear to have played a prominent role. Body size, by contrast, had relatively little impact.
Some of this deep evolutionary history appears to be repeating itself with modern invasive species, Raxworthy notes. Although toads have spread around the world, “there were a few barriers that were too much for them.” Toads never made it on their own to Australia, for example, or crossed the Mozambique Channel from Africa to Madagascar and the nearby Mascarene Islands. Now, with a little help from humans, they have conquered these final frontiers.
REFERENCE: Science; 15 OCT 2025; Phie Jacobs