Bombardier beetles are known for their feisty response to predators. When confronted, the insects blast their adversary with a boiling-hot secretion of noxious chemicals.
If they get swallowed anyway, they have a back-up plan: Blast their way out from the inside.
In a messy experiment, scientists watched as a Japanese stream toad readily swallowed an Asian bombardier beetle. But 44 minutes later, the toad barfed up the contents of its stomach, depositing the six-legged test subject to the floor of its enclosure. Covered in mucus, the insect scuttled away, physically unharmed.
“The vomited beetle was alive and active,” study authors Shinji Sugiura and Takuya Sato of Kobe University in Japan reported Tuesday in the journal Biology Letters.
What, the researchers wondered, makes bombardier beetles such adept escape artists?
To find out, they needed more of them, and they needed to feed them to more toads.
Along the forest edges of central Japan, scientists gathered 15 species of ground beetles, including the bombardier beetle. They also collected Japanese common toads, which are natural bombardier beetle predators, and Japanese stream toads, whose habitat does not overlap with the insect.
Sugiura and Sato hypothesized that over years of of exposure, the common toad species evolved a greater tolerance to the bombardier beetle’s cocktail of toxic chemicals than the stream toads had.
The bombardier beetles were divided into two groups. Some were poked with forceps, which prompted them to discharge all their toxic chemical spray. Other beetles were left alone.
Then they were fed to the toads.
Each toad flung its sticky tongue at its prey, nabbed and swallowed.
The amphibians that swallowed a fully loaded bombardier beetle were in for a surprise, the researchers soon observed.
“An explosion was audible inside each toad, which indicates that [the bombardier beetle] ejected a chemical spray after being swallowed,” the authors wrote.
Then they watched and waited.
The common toads vomited their prey 35% of the time. The stream toads ejected their meals 57% of the time.
That pattern provided clear support for their hypothesis about the amphibians’ evolutionary adaptation.
But back to the beetles: All 16 of the vomited insects were “alive and active” 20 minutes later, the researchers reported.
And not only did the beetles survive, but 15 out of 16 lived at least another two weeks. One escapee survived for 562 days after its sojourn in the toad stomach.
Those were the lucky ones. Almost all the beetles that were poked into ejecting their defensive chemicals before meeting the toads were “successfully digested,” according to the study.
The fact that the depleted beetles were so readily digested told the researchers that the beetles’ boiling chemical spray was indeed their ticket to freedom.
That left one remaining question: How do the beetles survive inside the toads’ digestive systems?
The 16 vomited bombardier beetles spent between 12 and 107 minutes stewing in the digestive fluids and enzymes of the toads’ stomachs.
In additional tests, other ground beetle species were fed to toads. Fewer of these beetles got barfed out by the amphibians, and if they did, fewer were able to walk away.
And so it appears that in addition to evolving their offensive defense mechanism, bombardier beetles also evolved a high tolerance for digestive juices, making their survival strategy complete. Perhaps it’s a two-fer — the beetles’ toxic brew may neutralize the toad’s stomach fluids and enzymes while also causing them to vomit, the scientists wrote.
No toads were seriously harmed, and they were released back into the wild when the experiments were done, the authors noted. The same couldn’t be said for all the beetles.