Usually being eaten means death for a prey – not so for a water beetle native to Japan. Because if the beetle Regimbartia attenuata is swallowed by a frog, it stays alive. Neither digestive juices nor other processes inside the frog seem to harm it. And not only that: the beetle crawls through the intestinal tract of the frog to its anus and emerges there around six hours after being eaten, as scientists have observed. Seemingly unimpressed by its passage through the frog, the water beetle crawls away.
Some plant seeds and parasites are already known to survive the intestinal passage through an animal. Usually, however, it means certain death for an animal if it is devoured by a predator. For one thing, the robber’s teeth and jaws often cause fatal injuries. If they survive this – for example, because some birds or frogs swallow their prey whole due to a lack of teeth – the digestive tract awaits them with hostile conditions. “The predator’s digestive system kills almost every prey after swallowing,” explains Shinji Sugiura from Kobe University. Because caustic digestive juices, decomposing enzymes and lack of oxygen make longer survival almost impossible.
An unexpected reunion
The only way to avoid the deadly consequences of this milieu is to shield yourself from the environment, as some plant seeds do, or to pass through the intestine as quickly as possible – for example, by actively accelerating your intestinal passage. Sugiura has now discovered and observed such a case. For his study, he tested whether and how well different types of particularly resistant insects survive being eaten by a frog. In one of the experiments, he fed the frog Pelophylax nigromaculatus, which occurs in Japanese rice fields, with Regimbartia attenuata, a four to five millimeter water beetle that also lives in the rice fields. As expected, the frogs gobbled up the water beetles offered to them with a snap.
But after feeding, the biologist observed something surprising: Within the first six hours afterwards, the eaten water beetle suddenly reappeared – from the frog’s anus. “Around 90 percent of the swallowed water beetles came out this way and were surprisingly still alive,” reports Sugiura. In order to find out how the beetles managed to escape from the intestine, the researcher next fed some of these water beetles, to which he had glued the legs with wax. As a result, they were no longer able to actively crawl. It turned out: “All beetles treated in this way were killed in the frog’s digestive tract and only excreted more than 24 hours after they were swallowed,” says Sugiura. “Accordingly, the swallowed beetles must have used their legs to crawl through the digestive tract in the direction of the anus and thus accelerate their flight.”
Manipulated bowel movements
This is the first documented case of such an active escape of a prey from the intestine of its predator, as the researcher explains. The water beetle not only seems to actively crawl through the intestine, but also manipulates the bowel movements of its “host”. Because normally the muscle ring on the frog’s anus ensures that its anus remains tightly closed. The beetle could not overcome this barrier on its own. In order to get outside, he therefore stimulates the frog to deliver droppings, possibly through tactile stimuli. As soon as the anal ring opens, the little beetle crawls outside. This trick does Regimbartia attenuata not only with this one type of frog, but also with four others, as Sugiura found out through feeding tests.
(Video: naturalist2008)
Source: Shinji Sugiura (Kobe University), Current Biology, doi: 10.1016 / j.cub.2020.06.026