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On two islands in the Indian Ocean, frogs are not what they used to be. Less than a century after its introduction by humans, the islands' frogs have reduced in size by about a third.
The finding, published online Nov. 18 in Biology Letters, potentially illustrates that "island dwarfism," where animals evolve to be much smaller after settling on an island, can occur in very short periods of time.
“When you imagine an island dwarfism, you imagine this happening over thousands or millions of years,” says James Baxter-Gilbert, invasion biologist at Stellenbosch University in South Africa. "But this (research) has the potential to show that a shorter period of time occurs in four orders of magnitude, which is super exciting."
Guttural toads (Sclerophrys gutturalis) are native to much of the eastern half of sub-Saharan Africa. Humans brought frogs to Mauritius – an island east of Madagascar, about 2,000 kilometers from mainland Africa – in 1922 to devour cane beetles. Just five years later, Mauritius frogs were taken to the nearby island of Reunion to control mosquito populations.
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Baxter-Gilbert was fascinated by how these deliberate introductions may have influenced the evolution and biology of frogs. Thanks to previous work on frog genetics, it was clear that the likely progenitors of the island’s frogs were from Durban, South Africa.
“We know the (genetic) plan they came up with,” he says. "We can see how they have changed from that original plan."
From June 2019 to March 2020, Baxter-Gilbert and his colleagues captured nearly 500 toads from Mauritius, Reunion and Durban. The team observed the sex of each toad and measured the body length plus different dimensions of the jaw, legs and feet.
The toads on the islands were much smaller than the approximately 7-centimeter-long toads in Durban. Female frogs measured on average about 34 percent less in Mauritius and 26 percent less in Reunion. Mauritius male toads were about 22 percent shorter, while Reunion male toads were about the same size as their mainland counterparts. Both sexes of the island's toads had legs and feet that were not only shorter than the Durban toads, but also disproportionately lumpy due to their already reduced body size.
Adult guttural frogs in Durban (left) are substantially larger than their island relatives in Mauritius (on the 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). This shrinkage may be the result of fewer foods available on an island, which makes smaller bodies more advantageous. The reduction could also be an evolutionary response to the lack of predators.
“In the case of elephants (dwarfs), they have the luxury of evolving into miniatures when there are no lions or tigers looking for their calves,” says Alexandra van der Geer, a paleontologist at the Naturalis Biodiversity Center in Leiden, the Netherlands. who did not participate in this research. "Why wouldn't this apply to frogs?" she says. On land, being larger may be more helpful in deterring frog predators, she says. These predators may include snakes and birds.
The findings of the new study offer a new look at some of the early stages of this reduction and how quickly it occurs. Baxter-Gilbert says that significant changes in body size could also occur to other dwarfs on the islands shortly after the animals reached the islands and only persisted for millennia.
The short legs of the island toads may arise because there is little pressure on the small islands to move quickly to new territory to beat the competing toads in a resource-rich area, contrary to what cane frogs experiencing Australia experience. . These dish-sized frogs are evolving into shapes that can travel efficiently over long distances across Australia’s giant land mass (SN: 14/10/14).
“If you’re a frog on an island and there’s nowhere to disperse, you don’t need to invest energy in getting those long ganglion legs that allow you to jump straight for days after, because there’s nowhere to go,” Baxter-Gilbert says.
Baxter-Gilbert recognizes that shrinkage may not be due to natural selection and could be a consequence of frogs having the flexibility to develop to respond to a stranger on their island. Another possibility is that frogs are smaller due to a food deficiency or perhaps the weather is not right for the frogs to reach full size.
He wants to raise some frogs from the island on the African continent with his Durban counterparts in a “common garden” experiment. This could help determine if the diet, temperature, or some other environmental condition on the two islands, rather than rapid evolution, could be behind the small size of the frogs.
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