When cane toads were introduced to Japan’s Ishigaki Island in 1978, the crested serpent eagles that lived there were already well prepared. Research in BMC Ecology and Evolution suggests that the critically endangered raptor already carried a genetic mutation that enabled it to eat the toxic critter with relative impunity.
In 1935, cane toads, which are native to South and Central America, were introduced to Australia to control the beetles damaging sugarcane crops, but the plan went wrong. The beetles were ignored. The toads thrived, and because they are toxic, began poisoning the native animals that started to dine on them.
So, when cane toads were introduced to Ishigaki Island for the same reason, eyes were on the local wildlife. Conservationists witnessed native Ryukyu snakes die after eating the toads but then noted that the resident crested serpent eagles seemed to be ok.
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The crested serpent eagle is common across Asia but critically endangered in Japan where there are about 200 individuals.
“I was deeply fascinated by how the crested serpent eagle has managed to adapt to a toxic invasive species,” says Alisa Tobe, from Kyoto University, who studied the birds.
It’s known that elsewhere several species that successfully prey on toxin-secreting animals carry a mutated version of a key gene that protects them. So, Tobe and colleagues decided to see if crested serpent eagles possess something similar.
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Clues in the DNA
The researchers studied DNA sequences from three populations of crested serpent eagles – from Ishigaki and Iriomote Islands in Japan, and Simeulue Island in Indonesia – and compared them with DNA from eight other raptor species.
None of the other raptors carried a resistance gene, but crested serpent-eagles from the three different populations did.
Crested serpent eagles on Ishigaki and Iriomote Islands don’t mix, and there are no cane toads on Iriomote Island. This suggests that the birds already had the helpful mutation before the cane toads arrived, but further research is needed to confirm this.
"It is fascinating to consider how its ancestors might have acquired this mutation in the first place," says Tobe.
Unanswered questions, however, remain. No one knows how often crested serpent eagles eat cane toads, nor if the amphibians affect their long term health. Equally unclear is how the cane toads fit into the islands’ food web, as well as its impact on other native species. More research is planned.
Find out more about the study: Evolutionary insights into Na+/K+-ATPase-mediated toxin resistance in the Crested Serpent-eagle preying on introduced cane toads in Okinawa, Japan
Top image: crested serpent eagle. Credit: Getty
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