- Researchers had previously blamed Guam’s devastating brown tree snake invasion on the snake’s uniquely toxic venom.
- This venom, irditoxin, is actually present in all snakes within the cat-eye family, a new study shows, leading researchers to re-evaluate the brown tree snake’s success.
- The snake has killed most of the island’s native bird species, threatening the ecological future of this tropical island.
The highly toxic brown tree snake, which has killed off most of the native birds on the Pacific island of Guam, was once thought to have its own distinctive venom. However, scientists have now proven that other snakes are armed with the same poison.
In a recent study in the Journal of Molecular Evolution, biologists compared the venom of brown tree snakes (Boiga irregularis) to that of five other types of cat-eye snakes. The team extracted the molecular blueprint of each venom and analyzed the genetic components. The entire cat-eye snake family shares a particularly potent toxin called irditoxin, the researchers found.
“This toxin that was thought to be unique to the brown tree snake is actually much more widely made,” said Bryan Fry, a toxicologist at the University of Queensland in Australia and the study’s senior author. “From there, that allowed us to infer that this is what allowed the cat-eye snakes to be such an explosive group.”
The cat-eye snake evolved in Africa and underwent a rapid explosion in diversity. Several species now flourish across Southeast Asia, India and Australia, including the dog-toothed cat snake, the gold-ringed cat snake, the black mangrove snake, the black-headed cat snake and the common cat snake. This success is attributable to the snakes’ irditoxin, the researchers believe.
Irditoxin is classified as a “three-finger” toxin (3FTx) protein, which originated over 100 million years ago and is one of the most widespread varieties of snake toxins. 3FTx refers to the shared chemical structure: three protein loops, or fingers, attached to a single core made up of the amino acid cysteine.
Among the 3FTx toxins, irditoxin is by far the most potent. The venom is highly poisonous to birds and reptiles, but it is about 1,000 times less toxic to mammals.
With such a specialized toxin, the bird-rich tropical island of Guam was a perfect hunting ground for any of the cat-eye snakes. But after World War II, only the brown tree snake hitched a ride from Australia to Guam on troop carriers. With no natural predators and abundant prey, this snake has since wiped out 10 of the island’s 12 native forest bird species.
The resulting ecological harm has been profound. About 60 to 70 percent of tropical trees on Guam rely on birds to eat their fruit and spread their seeds. When birds digest the seeds and defecate, the seeds disperse to areas far from the parent tree where the competition for light, water and nutrients is less intense.
Without these feathered friends, around 95 percent of seeds from two common tree species, the Ahgao and the Aploghating, fall directly beneath their parent tree. This lowers their chances to germinate and threatens the survival of this tropical forest.
This new understanding of the brown tree snake and irditoxin can greatly aid research on invasive species and future conservation efforts, said Richard Shine, an evolutionary biologist from the University of Sydney, Australia, who was not involved in the study.
“It’s exciting to see our knowledge of venoms move to this very sophisticated level and begin to incorporate lineages that were largely ignored in previous years,” Shine said. “[There has been] a tendency for people to focus almost entirely on large snakes that could kill people.”
Today, ecologists, biologists and government agencies are trying to stop the brown tree snake and any other cat-eye snakes from hopping a ride to other tropical islands. For places such as Hawaii, these invaders could ruin both the state’s forests and its ecotourism, a huge part of Hawaii’s economy.
“It’s very difficult to control invasive species once they arrive somewhere and get going,” said Shine. “With increased vigilance and careful surveillance, it may be possible to stop them from getting started in the first place.”
- Dashevsky, D., Debono, J., Rokyta, D., Nouwens, A., Josh, P., Fry, B.G. (2018). Three-finger toxin diversification in the venoms of cat-eye snakes (Clolubridae: Boiga). Journal of Molecular Evolution, 86, 531-545
Helen Santoro (@helenwsantoro) is a graduate student in the Science Communication Program at the University of California, Santa Cruz. Other Mongabay stories produced by UCSC students can be found here.