Crested Serpent-eagles in Japan show genetic resistance to cane toad toxins
Scientists uncover a surprising genetic trait in the Crested Serpent-eagle that helps it survive a deadly invader
A Predator That Outsmarts Its Prey
On the islands of Ishigaki and Iriomote in southern Japan, an endangered bird of prey has learned to feast on a deadly invader. The Crested Serpent-eagle (Spilornis cheela perplexus) has been seen eating poisonous Cane Toads (Rhinella marina) – a species known for killing predators with its toxic skin. But unlike other animals, these eagles suffer no ill effects.
A new study has now revealed the secret: a tiny change in one of the eagle’s genes gives it a special resistance to the toads’ poison. This change may have existed long before the toads were ever brought to Japan, allowing the eagle to take advantage of an easy meal while other species avoid it or die trying.
The discovery not only explains the eagle’s unusual diet, but also sheds light on how animals evolve to survive new threats – sometimes with lucky advantages inherited from the past.
The Science Behind the Superpower
Cane Toads, introduced to Ishigaki Island in the 1970s to control crop pests, release a powerful toxin from their skin. This toxin attacks a crucial part of animal cells – a protein that helps control salt and water balance. For most animals, eating a toad is fatal.
But the Crested Serpent-eagle carries a mutation in the gene for that very protein – known as ATP1A1 – that makes it harder for the toxin to attach. The key change, known as Q111E, has also been found in other animals that eat poisonous prey, such as certain snakes, birds, and rodents. Scientists call this kind of repeated, independent evolution “convergent evolution.”
What’s most surprising is that this mutation is found in eagle populations on both Ishigaki and nearby Iriomote Island – even though Iriomote doesn’t have Cane Toads at all. That suggests the mutation didn’t arise because of the toads, but was already present in the eagle’s genetic toolkit.
Separate Populations, Shared Resistance
The researchers analysed the DNA of eagles from both islands and confirmed they are genetically separate populations – even though the islands are only 20 km apart. The eagles don’t seem to move between islands, and their genes have diverged over time.
Despite this, both groups share the same mutation linked to toxin resistance. A third group of Crested Serpent-eagles from Simeulue Island in Indonesia also has the same version of the gene. This points to a deeper evolutionary origin – a trait that may have helped the eagle or its ancestors deal with toxic prey elsewhere in Asia long ago.
A Shared Story With Other Snake-eating Eagles
When the team compared the eagle’s gene with those of other birds of prey, one close match stood out: the Black-chested Snake Eagle from Africa. Like the Crested Serpent-eagle, it eats toads and shows similar changes in the same gene. Both belong to the same subfamily of raptors, Circaetinae, known for feeding on snakes and other dangerous prey.
This suggests that the ability to resist toxins might run in the family – a shared adaptation that allows these birds to survive where others would not. In some cases, resistance may involve more than one change in the protein, or even other ways of neutralising the poison entirely.
Why It Matters
The Crested Serpent-eagle is a protected species in Japan, with only around 100 individuals on each island. This new genetic discovery helps conservationists understand how the species has adapted to changing environments and invasive species like the Cane Toad.
It also shows how evolution can quietly equip animals with powerful defences – sometimes before they even need them. As the researchers put it, the eagle’s ability to resist the toad’s toxin may have less to do with recent changes, and more to do with ancient evolutionary history.
And while we now know more about how this eagle handles poison, many questions remain. Does the mutation carry any hidden costs? Could it offer other benefits? And how many other animals carry similar secret weapons waiting to be discovered?
August 2025
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