Guttural toads shrank by a third after just 100 years on two islands

On two islands in the Indian Ocean, the toads just aren’t what they used to be. Less than a century after their introduction by humans, the islands’ toads have shrunk in size by about a third.

The finding, reported online November 18 in Biology Letters, potentially illustrates that “island dwarfism,” where animals evolve to be much smaller after settling on an island, can occur over very short timescales.

“When you imagine insular dwarfism, you imagine this happening over thousands or millions of years,” says James Baxter-Gilbert, an invasion biologist at Stellenbosch University in South Africa. “But this [research] has the potential to show it happening in four orders of magnitude shorter timeframe, which is super exciting.”

Guttural toads (Sclerophrys gutturalis) are native to much of the eastern half of sub-Saharan Africa. Humans brought the toads to Mauritius — an island east of Madagascar, roughly 2,000 kilometers from mainland Africa — in 1922 to devour cane beetles. Just five years later, toads from Mauritius were brought to the nearby island of Réunion to control mosquito populations.

Baxter-Gilbert was fascinated by how those deliberate introductions might have influenced the toads’ evolution and biology. Thanks to previous work on the toads’ genetics, it was clear that the island toads’ likely progenitors were from Durban, South Africa.

“We know the [genetic] blueprint that they came with,” he says. “We can kind of see how they have changed from that original blueprint.”

From June 2019 to March 2020, Baxter-Gilbert and colleagues caught nearly 500 toads from Mauritius, Réunion and Durban. The team noted the sex of each toad and measured body length plus different dimensions of the jaw, legs and feet.

Island toads were quite a bit smaller than the roughly 7-centimeter-long toads in Durban. Female toads measured on average about 34 percent shorter on Mauritius and 26 percent shorter on Réunion. Male Mauritius toads were about 22 percent shorter, while male Réunion toads were about the same size as their mainland counterparts. Both sexes of island toads had legs and feet that not only were shorter than Durban toads’, but also disproportionately stumpy for their already reduced body size.

left: guttural toad in Durban. Right: guttural toads in Mauritius
Adult female guttural toads in Durban (left) are substantially larger than their island relatives on Mauritius (right, the male has a yellow throat patch and the female has a pale throat).From left: J. Baxter-Gilbert; John Measey

Both the fossil record and recent natural history are full of examples of island dwarfs, ranging from Ice Age mammoths to hippos on the island of Crete to dinosaurs (SN: 11/10/04). Such shrinking may be the result of less available food on an island, making smaller bodies more advantageous. The reduction could also be an evolutionary response to a lack of predators.

“In the case of [dwarf] elephants, they have the luxury to evolve miniature size when no lions and tigers are on the hunt for their calves,” says Alexandra van der Geer, a paleontologist at the Naturalis Biodiversity Center in Leiden, Netherlands, who was not involved with this research. “Why would this not apply to frogs?” she says. On the mainland, being larger may be more useful in dissuading the toads’ predators, she says. Those predators can include snakes and birds.

The new study’s findings provide a fresh look at some of the early stages of this downsizing and how quickly it occurs. Major body size changes to other island dwarfs might have also occurred soon after the animals arrived on islands, and just persisted over millennia, Baxter-Gilbert says.

The island toads’ short legs may have arisen because there’s little pressure on tiny islands to move into new territory quickly to beat competing toads to an area of rich resources, unlike what cane toads invading Australia are experiencing. Those dinner plate–sized toads are evolving into forms that can travel efficiently over long distances across Australia’s giant landmass (SN: 10/14/14).

“If you’re a toad on an island and there’s nowhere to disperse to, then you don’t need to invest energy into getting these long gangly legs that let you jump straight for days on end, because there’s nowhere to go,” Baxter-Gilbert says.

Baxter-Gilbert acknowledges that the shrinking may not be due to natural selection and could instead be a consequence of the toads having the developmental flexibility to respond to a quirk of their island environment. Another possibility is the toads are smaller due to a dietary deficiency, or maybe the climate isn’t quite right for the toads to reach full size.

He wants to raise some island toads on the African mainland with their Durban counterparts in a “common garden” experiment. This could help determine if diet, temperature or some other environmental condition on the two islands, rather than rapid evolution, could be behind the toads’ diminutive size.