Water beetles can live on after being eaten and excreted by a frog

For most insects, the sticky, slingshot ride straight into a frog’s mouth spells the end. But not for one stubborn water beetle.

Instead of succumbing to the frog’s digestive juices, an eaten Regimbartia attenuata traverses the amphibian’s throat, swims through the stomach, slides along the intestines and climbs out the frog’s butt, alive and well.

“This is legitimately the first article in a while that made me say, ‘Huh! How weird!’” says Crystal Maier, an entomologist at Harvard University’s Museum of Comparative Zoology. “There are still a lot of truly bizarre habits of insects that still wait to be discovered,” she says.

Surviving digestion-by-predator is rare, but not unheard of in the animal kingdom. Some snails survive the trip through fish and birds by sealing their shells and waiting it out. But research published August 3 in Current Biology is the first to document prey actively escaping through the backside of a predator.

Feeding beetles to predators to see what happens is a regular activity for Shinji Sugiura, an ecologist at Kobe University in Japan. In 2018, he discovered that bombardier beetles can force toads to vomit the insects back up by releasing a mix of hot, noxious chemicals from their rear ends (SN: 2/6/18). 

On a hunch that R. attenuata might have evolved its own interesting evasive behaviors, Sugiura paired a beetle with a frog that the insect often encounters while swimming through Japanese rice paddies. In his laboratory, he watched.

The frog made easy prey of the unsuspecting beetle. While the amphibians lack teeth that could kill prey with a crunch, a trip through the acidic, oxygen-poor digestive system should be sufficient to neutralize the insect. But as Sugiura monitored the frog, he saw the shiny black beetle slip out from the frog’s behind and scurry away, seemingly unharmed.

“I was very surprised,” he says. “I was expecting that the frogs might just spit out the beetles or something.”

After more than 30 additional beetle-frog pairings, Sugiura found that over 90 percent of beetles survived being eaten, greatly outshining other animals known to survive digestion-by-predator. Those creatures typically survive less than 20 percent of the time. On average, it took six hours for the beetles to escape, though one intrepid individual completed the journey in just six minutes. 

Sugiura confirmed that the beetles were actively escaping from the frog’s digestive tract by using sticky wax to fix some beetles’ legs together. None of these immobilized beetles survived, and their carcasses took a day or longer to pass through the frogs.

R. attenuata’s aquatic lifestyle likely prepared the beetle to survive digestion, Sugiura says. Its streamlined, but sturdy, exoskeleton may shield the insect from digestive juices. And its ability to breathe underwater via air pockets tucked under its hardened wings likely prevents suffocation.

Sugiura plans to test the limits of R. attenuata’s abilities by pairing the insect with larger frogs, toads and even fish. “I’m looking forward to finding unimaginable types of antipredator defense,” he says.