Inside Job

Watch: This animal uses "predator defecation" as a way to escape death

These beetles get devoured whole — then live to tell the tale.

When a beetle gets swallowed up whole by a predatory frog, it's not necessarily the end of the insect's time on Earth.

New research delves into how some aquatic beetles can survive being devoured by, essentially, prompting the frog to defecate. Their journey brings a whole new meaning to the phrase, what goes in, must come out.

Researcher Shinji Sugiura fed five frog species water-dwelling beetles called Regimbartia attenuata. In lab experiments, 93 percent of the beetles were able to make their way from the frog's mouth to its vent — the hole small animals use to pass waste matter out of their bodies. In this case, the beetles accompanied the waste matter as they reemerged, unscathed.

Sugiura is the first to document this remarkable escape and describes the process in a letter published Monday in the journal Current Biology.

Many frog species, like Pelophylax nigromaculatus, or the black-spotted frog, used in the experiments, don't have teeth. It swallows prey whole, pushing it through its esophagus, stomach, small intestine, and large intestine. Whatever doesn't get digested is excreted through the frog's rear vent.

That includes food that's still alive.

When a living beetle makes its way through a frog, it's a faster-than-usual process. The quickest journey took just six minutes, the researchers report.

An accompanying video shows the beetle breaking free from a black-spotted frog:

The "vent" is a very polite way of putting it.

This research suggests that beetles actively plot their getaway from within. First, they seem to crawl their way through the digestive system using their legs. Then, somehow, they are able to get the frog's sphincter, which controls the vent, to open up.

The beetles were always excreted head first, the researchers report, suggesting that a beetle "stimulates the hindgut, urging the frog to defecate."

Active adaptations — This is the first example of prey stimulating an animal's excretion process in order to escape.

Previously, researchers thought that getting pooped out was more of a passive survival tactic. However, the speed of the beetle indigestion suggests a more active process.

"This study is the first to document active prey escape from the vent of a predator and to show that prey may promote predator defecation to hasten escape from inside the predator’s body," Sugiura writes.

The researchers believe that the beetles have adapted their bodies and behavior over time to survive after almost becoming a frog meal. The beetles respire using a small air pocket beneath their wings, so they could feasibly survive the oxygen-free conditions of a frog digestive tract. Their hard exoskeleton may also offer protection against acidic digestive juices.

Plus, the beetles are already used to watery environments — and their streamlined shape may help them navigate the "long tubular structure" that leads to the frog's butt.

The bizarre beetle evolution suggests that perhaps other aquatic insects, like these water beetles, have escapist adaptations of their own.

Summary: Predation pressures can lead to the evolution of escape behavior in prey animals [ 1 , 2 ]. Most previous studies investigated how prey can escape from predators before contact [ 1 , 2 ], whereas recent studies have focused on the post-contact escape of prey [ 2 ]. Predators can damage prey by biting or chewing, and the predator’s digestive system ultimately kills almost all prey after swallowing. However, several species can survive passage through a predator’s gut and are ultimately excreted with feces [ 3 , 4 , 5 ]. Such escapes from the predator’s vent (cloaca or anus) are considered passive [ 3 , 4 , 5 ]. Survival in the extreme pH and anaerobic conditions of the predator digestive tract depends on the speed of passage and activity of the prey. This lethal environment may impose selective pressures on the evolution of quick and active escape behavior in swallowed prey species. Here, I report active escape of the aquatic beetle Regimbartia attenuata (Coleoptera: Hydrophilidae) from the vents of five frog species via the digestive tract. Although adult beetles were easily eaten by frogs, 90% of swallowed beetles were excreted within 6 h (0.1–6.0 h) after being eaten and, surprisingly, were still alive. When beetle legs were experimentally fixed with wax, all of the treated beetles were killed in the frogs’ digestive system and finally excreted >24 h (38.3–150.3 h) after consumption. Therefore, swallowed beetles likely used their legs to move through the digestive tract toward the frog vent, hastening their escape. This study is the first to document active prey escape from the vent of a predator and to show that prey may promote predator defecation to hasten escape from inside the predator’s body.
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