guttural toad

Toads on tropical islands are shrinking rapidly as they evolve

By Adam Vaughan

A guttural toad (Sclerophrys gutturalis) in Tanzania

Shutterstock / Chedko

Toads that invaded two tropical islands have shrunk by a third in less than a century, a remarkably short amount of time on evolutionary time scales.

The guttural toad (Sclerophrys gutturalis) is native to large parts of Africa. A population from Durban in South Africa was brought to the islands of Mauritius and Réunion in the Indian Ocean in 1922 and 1927, respectively.

Now researchers have found that the female toads in Mauritius are up to 33.9 percent smaller than the original population in Durban and the female toads on Réunion are up to 25.9 percent smaller. The males shrank on Mauritius, but not on Réunion. Such shrinkage in amphibians on islands typically takes thousands or millions of years.


Islands have long been known as unique test beds for seeing how animals adapt and develop – from dwarfism to the gigantism demonstrated by giant tortoises in the Galapagos Islands – though it can be difficult to tell how long the changes will take . Recent human introductions, intentional or unintentional, make it easier to track these shifts.

It’s not yet clear how and why the island’s guttural toads have shrunk, says James Baxter-Gilbert of Stellenbosch University in South Africa. Between June 2019 and March 2020, his team caught and measured 158 toads in Mauritius, 186 in Réunion and 151 in Durban.

The mechanism could be natural selection. Alternatively, the species might already have possessed the ability to shrink – phenotypic plasticity – if the right changes were made in its environment. “However, if this is a product of natural selection and functional adaptation, this is a little more surprising,” says Baxter-Gilbert, given the speed with which the change was made.

One possible driving force is that the frogs breed on the islands year-round, while elsewhere they breed seasonally. Unless women need to build and store more energy in a short period of time to produce lots of eggs, they may not need to get that big.

“The next big step is to find out whether these island-specific reductions in body size and shape are due to adaptations through natural selection or phenotypic plasticity, or to an interplay between them,” says Baxter-Gilbert.

Journal reference: Biology Letters, DOI: 10.1098 / rsbl.2020.0651

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