Black-spotted pond frog attempting to prey on a giant hornet. (Photo courtesy of Shinji Sugiura, Professor at Kobe University)
With their potent venom and aggression toward external threats, hornets pose a serious danger to other organisms. In Japan, incidents of people being stung continue to occur, sometimes resulting in injury or death.
Yet the black-spotted pond frog, which lives in rice paddies and ponds, preys on these formidable hornets with apparent calm.
How can these frogs do so without fear of their venomous stings?
Researchers at Kobe University have recently uncovered the answer to this mystery.
A Sting That Can Kill
Hornets are notorious for deterring predators with their powerful venomous stingers. Their stings can cause intense pain and lead to severe symptoms, including breathing difficulties or even loss of consciousness.
According to statistics from the Ministry of Health, Labor and Welfare, Japan has reported approximately 20 deaths annually from bee stings in recent years. Most of these fatalities are believed to result from hornet attacks, making them one of the most lethal creatures in the country's natural environment.
There have also been numerous reports of people being pursued by large numbers of worker hornets after disturbing their nests.
As the population of worker hornets grows from summer into fall, their aggressiveness intensifies. Careful vigilance, therefore, remains essential for anyone hiking in the mountains or working in forestry.
A Frog's Dangerous Meal
Despite the danger, field surveys have repeatedly found dead hornets in the stomachs of black-spotted pond frogs. It was believed that the frogs targeted hornets that came near the water, but their behavior remained a mystery.
Frogs, which instinctively leap at moving objects, face a high risk of being stung during such attacks. And yet, they somehow manage to avoid fatal consequences, successfully capturing and swallowing the hornets.
Are the frogs skillfully dodging the stingers, or do they have a natural resistance to the hornet's venom? Several theories have emerged. However, no definitive answer existed — not until a research team at Kobe University decided to tackle the mystery head-on.
The team conducted laboratory tests in which black-spotted pond frogs and three species of hornets — the yellow hornet, the yellow-vented hornet, and the Asian giant hornet — were placed together to observe predation behavior.
A total of 45 frogs and 45 hornets (15 from each species) were used. All the hornets were equipped with venomous stingers, and each individual extended its stinger in a threatening display when provoked.
Surviving the Hornets
One black-spotted pond frog and one hornet species were placed together in the same enclosure to observe whether the frog would attack and successfully capture its prey.
Most of the frogs pounced on the hornet without hesitation, attempting to grab it in their mouths and swallow it. The frog's tongue struck with astonishing speed, allowing it to snatch prey almost instantaneously.
However, many instances were also observed in which the hornet took advantage of that brief moment to counterattack, stinging the frog's face or the inside of its mouth.
The result was striking. Over 90% of yellow hornets and nearly 90% of yellow-vented hornets were consumed. Even Asian giant hornets, the largest and most venomous species, saw about 80% of their numbers eaten.
More remarkably, not a single frog showed any immediate signs of weakness after being stung, nor did they exhibit any decline in condition over time.
Evidence of Venom Tolerance
Even when stung, none of the frogs suffered fatal effects. This outcome cannot be explained solely by the tree frog avoiding the hornets' venomous stingers.
Given that most frogs were able to prey on the hornets despite repeated counterattacks, the team concluded that the frogs possess a high tolerance to hornet venom.
"Although previous research had suggested that black-spotted pond frogs prey on hornets, it was astonishing to discover that they could easily swallow even the highly dangerous Asian giant hornet while being stung," said Professor Shinji Sugiura, an ecologist at Kobe University.
In the wild, hornets are commonly found in the stomachs of frogs living near water. The latest experimental results are consistent with field observations.
Hornets have also been detected in the stomach contents of other larger frogs, such as bullfrogs and wrinkled frogs, suggesting that these species may share a similar tolerance.
A Mystery with Human Implications
Frogs are not the only natural enemies of hornets, however. Birds swoop down to prey on them, bears are known to raid their nests, and spiders sometimes capture flying worker hornets.
Among these predators, some species may possess mechanisms that allow them to withstand venomous stings. In the natural world, certain animals swallow venomous insects whole or crush their hard exoskeletons.
Within this ongoing contest of offensive and defensive adaptations, tolerance to stings has likely evolved as yet another survival strategy.
Still, the mechanism by which the black-spotted pond frog neutralizes the pain and toxin remains unknown.
Are its mucous membranes and muscles less susceptible to the venom? Does it possess an internal detoxification mechanism? Or does it have unique neural characteristics that make it less sensitive to pain? For now, the mystery remains unsolved.
If this mystery can be solved, it may pave the way for the development of new treatments for people stung by hornets. The toxin resistance possessed by this tiny frog could ultimately bring unexpected benefits to humans as well.
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Author: Juichiro Ito, The Sankei Shimbun
(Read this in Japanese)
