A new study comparing invasive cane toads in Japan and Australia has found substantial changes in body size and shape have developed much more rapidly than suggested by long-held ideas of the pace of evolution.
Researchers measured and weighed wild-caught cane toads (Rhinella marina) on subtropical Ishigaki Island in southern Japan and compared them to toads measured in Australia, Hawai'i and South America.
The most striking difference was in absolute body size – adult toads from Ishagaki weighed an average 190g compared to 135g for toads from Australia, and their average length was 122mm compared to 111mm.
Ishigaki toads also had wider heads, shorter arms and longer legs than toads from other locations, the researchers reported in the journal Royal Society Open Science .
Cane toads have been translocated to more than 40 countries worldwide from their ancestral habitat in north-eastern South America, initially to Puerto Rico and thence to Hawai'i and from there to Australia in the 1930s. The toads of Ishigaki were introduced from Hawai'i (via Taiwan and the Daito Islands) in 1978.
"Given these populations of toads in Japan and Australia shared a common history in Hawai'i until the 1930s, these differences in size and body shape have developed in less than 100 years," said senior researcher Professor Rick Shine AM, an evolutionary biologist and ecologist at Macquarie University in Sydney.
"The idea that evolutionary change happens at a glacially slow pace is being challenged by recent evidence showing rapid changes in species confronted with novel challenges, like being translocated to a different habitat," said Professor Shine.
The study didn't collect sufficient data to allow researchers – from Macquarie University, the University of Sydney and Kyoto University – to test alternative hypotheses about what might be driving the changes in body size.
"We don't have a clear idea of the evolutionary forces that might be involved, so we can't say why body mass and shape has changed among the toads in the Japanese system," said Professor Shine.
However, the researchers speculated that the larger body sizes of Ishigaki toads could reflect favourable climatic conditions especially year-round rainfall or the impact of lower predation pressure on the predator-free island.
