Toads on tropical islands are dwindling rapidly as they evolve

Of Adam Vaughan

A guttural toad (Sclerophrys gutturalis) in Tanzania

Shutterstock / Chedko

The toads that invaded two tropical islands dwindled by a third in less than a century, a remarkably short amount of time on evolutionary timescales.

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

Now researchers have found that female toads in Mauritius are up to 33.9% smaller than the native population of Durban, and female Reunion toads are up to 25.9% smaller. Males dwindled in Mauritius but not in Réunion. Such shrinkage in amphibians on the islands typically takes thousands or millions of years.

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

It’s still unclear how and why the island’s guttural toads dwindled, 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 may already possess 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 frogs breed on islands all year round, while elsewhere they breed seasonally. Unless women need to build up and store more energy over a short period of time to produce a lot 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 an interaction between them,” says Baxter-Gilbert.

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

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