These water beetles walk right out of a frog’s butt

Published: August 31, 2020 · Updated: September 1, 2020

Pelophylax_nigromaculatus (Pelophylax_nigromaculatus) Courtesy Shinji Sugiura

In the Biblical tale of Jonah and the whale, a full-grown man is swallowed alive by a giant aquatic mammal. As the story goes, Jonah travelled around for three days in the whale’s stomach before the creature spits him out whole. Thankful for God’s protection during his ordeal, Jonah goes on to become a prophet. Rather than leaving their fate to an insect-overseeing deity, water beetles caught in a similar predicament take matters into their own hands. Or, rather, their many feet.

A biologist at Kobe University in Japan published a report in Current Biology announcing his discovery that if pond frogs eat a particular species of water beetle (Regimbartia attenuata), the beetles can not only survive being swallowed whole and entering the amphibian’s digestive tract, they’ll literally walk right out of the frog’s rear end.

Earlier, the ecologist behind the research, Shinji Sugiura, had discovered that bombardier beetles can escape ingestion by releasing noxious chemicals that force the frogs to vomit them out.

This time he set out to see if anything could survive by escaping from the other end. To do so, he set up a lab experiment where he fed the beetles to frogs to observe what happened. After feeding 30 beetles to the frogs, he waited and watched while 90 percent of them literally walked out the amphibian’s rear end and carried on with their day. On average, it took about six hours for the beetles to escape. But one beetle made its way out in a mere six minutes.

“I was very surprised. I was expecting that the frogs might just spit out the beetles,” Sugiura told Science News.

Frog and water beetle demonstrate active escape of prey from predator vent via the digestive tract
Courtesy Sugiura, S. (2020) Active escape of prey from predator vent via the digestive tract. Current Biology 30:R867–R867 (https://doi.org/10.1016/j.cub.2020.06.026)

To confirm that this wasn’t merely a passive passing, he then used wax to stick the beetles’ legs together and repeated the experiment. None of those beetles survived. Sugiura speculates that the beetles’ hard exoskeleton protects them from the frogs’ digestive juices and that they used pockets of air under their wings to breathe.

Perhaps surprisingly, water beetles aren’t the only creatures that can survive travelling through another animal’s digestive system. Researchers in Europe have shown that carp eggs can survive being eaten by mallards and emerge from their feces as viable eggs.

There are also species of snails that are known seal up their shells when consumed by fish or birds to ride out the journey out of the animal’s rear end.

More commonly, a variety of plant species rely on their seeds being eaten by birds and mammals and later deposited elsewhere as a means of dispersal, a process known appropriately enough as endozoochory.

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