Crested Serpent Eagle vs. Cane Toad Venom
A Crested Serpent Eagle was eating a toxic Cane Toad. Below I reproduce the article I wrote on Crested Serpent Eagle venom resistance, published in the newspaper on 19 July.
No Symptoms
Cane Toads were introduced to Ishigaki Island around 1978 to control sugarcane pests. They secrete a potent cardiotoxic steroid, bufadienolide, from their skin — a toxin that can stop the heart. There are confirmed cases of the native Sakishima Rat Snake dying after eating one. The Crested Serpent Eagle, on the other hand, shows no ill effects even after consuming the toad's highly toxic skin. Local birdwatchers had long noticed this, but no scientific explanation had been established.
A Gene Mutation?
The research team analysed tissue samples from 24 Crested Serpent Eagles that had died from accidents or illness between 2020 and 2023. Focusing on an enzyme responsible for ion transport across cell membranes — the target of the toxin — they discovered an amino-acid substitution in a key region associated with venom resistance, known as the Q111E mutation. This mutation appears to prevent the toad's toxin from binding to the enzyme, neutralising its effects. Similar mutations have been found in snakes that prey on toads and in birds that eat toxic butterflies, and the finding is regarded as an example of convergent evolution — the process by which unrelated lineages independently arrive at similar traits through different evolutionary paths in different places.
An Ancient Defence
Strikingly, Crested Serpent Eagles on Iriomote Island — where Cane Toads have not established — and even a subspecies of Crested Serpent Eagle on the distant Indonesian island of Simeulue carry the same resistance gene. Genetic analysis suggests the trait has been preserved since around the time the Crested Serpent Eagle diverged as a species, long before any alien species was introduced. Tobe of Kyoto University's Wildlife Research Center explains: "The venom resistance of the Crested Serpent Eagle is not a recent adaptation to environmental change, but an ancient trait inherited from its ancestors — one that has, by coincidence, also helped to mitigate the physiological impact of an invasive species. It is a rare example of that."
Other Raptors Too
The study also confirmed similar resistance gene mutations in snake eagles closely related to the Crested Serpent Eagle. There are reports that the Black-chested Snake Eagle in South Africa showed no health problems after being fed toxic toads in captivity for 12 months, suggesting this ability may be characteristic of the group as a whole. Meanwhile, on Japan's Ogasawara Islands, Eastern Buzzards have been recorded preying on the same Cane Toad, but no clear venom-resistance mutation was found in their genes, pointing to the possibility that they cope through a different mechanism.
A New Perspective for Conservation
The study also found that the Crested Serpent Eagle populations on Ishigaki and Iriomote islands are genetically distinct. Despite a separation of only about 20 kilometres, movement of individuals between the two islands appears to be almost nonexistent — a finding the team says "suggests that the Iriomote and Ishigaki populations should be considered independent conservation units." The research demonstrates that an organism's "hidden abilities" may play an important role in responding to environmental change. While the findings offer a significant new perspective for biodiversity conservation, Tobe also sounds a note of caution: "The fact that the eagle is making use of the invasive species does not mean the invasive species problem can be ignored. Once the Crested Serpent Eagle is preying on them, the toads are being incorporated into the ecosystem, and there is a risk of further disrupting the overall balance." As the global impact of invasive species on ecosystems becomes an ever greater concern, this discovery — shining a light on the unexpected adaptive capacity of native species — underscores both the depth of evolutionary processes and the complexity of conservation strategy.
A Crested Serpent Eagle eating a Cane Toad. As the research papers suggest, the toxin seems to pose no problem for it.
A Crested Serpent Eagle targeting something in an irrigation channel.
A Brown Shrike was nearby as well.
A flock of Common Snipe in the rice fields.