Toads eat beetles. Sometimes, beetles make them regret their meal choice.
Posted February 6, 2018 10:57 p.m. EST
Updated June 7, 2018 11:16 a.m. EDT
For most prey, the game is over once they have been swallowed. But one species of beetle can escape from a toad’s stomach nearly two hours after being eaten, according to a new study.
Found in wooded areas on nearly every continent, bombardier beetles — a group that consists of more than 500 species — get their name from their signature defense mechanism: When threatened, they shoot a hot chemical spray from their rear end. In Japan, the insects have long been known as, “the farting bug.”
Toads have been observed vomiting bombardier beetles after eating them, but no one knew exactly why, or how long the beetles survived after their brush with digestion.
To better understand the beetle’s defenses, two biologists from Kobe University fed a species of bombardier beetle to two different species of toad collected from forests in central Japan. One toad species shared its natural habitat with that particular species of beetle, while the other was unlikely to encounter it in the wild.
After the beetles were swallowed, a small explosion could be heard inside each toad, indicating that the insects were firing their defenses. Overall, 43 percent of the toads vomited the beetles, taking anywhere from 12 to 107 minutes.
Most important (at least to the insects): Despite being covered in mucus, meaning they had entered the toads’ digestive system, every evicted beetle was still alive, and all but one survived for another two weeks. No toads died on account of eating the beetles.
“It surprised us that the beetles vomited by toads were still alive and active,” said Shinji Sugiura, an author of the study, which was published Tuesday in the journal Biology Letters.
But some toads were better able to digest the beetles than others. Only 35 percent of the toads that shared habitat with the beetles coughed them up, compared to 57 percent of the toads with no common habitat. The findings suggest that regular exposure to bombardier beetles has allowed some toads to evolve a tolerance to their poison.
Size mattered, too. Large beetles escaped more frequently than small beetles, and small toads were more likely to vomit beetles than large toads. The reason, presumably, is that large beetles are able to produce more poison, and smaller toads are less able to tolerate it, said Sugiura.
To be sure that the beetles were using their spray to escape, the researchers also fed the toads beetles that had been forced to expel their poison just before being consumed. Nearly all of those beetles were successfully digested.
The authors say it is still not clear whether the beetles have evolved a resistance to a toad’s digestive fluids and enzymes, which may have helped them survive their ordeal inside a toad’s belly. That may be the case, but it is also possible that their poison spray prevented the toads from producing enough digestive juices to harm them.
Bombardier beetles are not the only creatures that can shoot toxic liquids. Fire ants, spitting cobras and even some species of birds are known to disable their prey with harmful sprays.
Based on the current study, Sugiura predicted that other spraying species may also be able to escape from a predator’s stomach. “Well-defended species that force some predators to vomit will be found in other animal groups,” he said.