The nice thing about being a frog is that you don’t have to chew your food—just gulp, and down the hatch. The problematic thing about being a frog is that you don’t have to chew your food, which means that if you’ve happened to nab the aquatic beetle Regimbartia attenuata, your food might come out the other end in an undesirable fashion: alive and literally kicking.
Writing today in the journal Current Biology, Kobe University ecologist Shinji Sugiura describes how the beetle, locked behind the frog’s jaws, turns around and scrambles through its digestive tract. In carefully designed lab experiments, Sugiura found that 93 percent of the beetles he fed to the frog Pelophylax nigromaculatus escaped the predator’s “vent”—aka anus—within four hours, “frequently entangled in fecal pellets,” he writes. The quickest run from mouth to anus was just six minutes. The beetles then went about their day as if they hadn’t just spelunked through a digestive system, and even swam effectively.
Apparently understanding their unique predicament, the R. attenuata beetles seem to have clambered through the intestines of the frogs. Sugiura showed as much by immobilizing some of the beetles’ legs with wax—this time, none of them emerged from the anus alive, but as feces, over 24 hours later. This all came as some surprise to Sugiura himself. Given that the predator and prey share habitat in Japan’s rice paddy fields, he hypothesized that the beetle could have evolved some sort of anti-frog defense. “However, I did not predict that R. attenuata can escape from the frog vent,” Sugiura writes in an email to WIRED. “I simply provided the beetle to the frogs, expecting that the frogs spat them out in response to the beetles’ behavior or something.”
Serendipitously, it may be that the adaptations the beetle had already evolved for the life aquatic prepared it for the great journey through a frog’s digestive system. For one, these insects swim quite effectively by kicking their legs, so perhaps they’re in effect swimming through the waste in the frog’s intestines. Also, insects breathe through holes in their hard shells, or exoskeletons. So to breathe underwater, this particular species of beetle traps a small pocket of air under its wing covers, which are known as elytra. (Think of the polka-dotted flaps that a ladybug opens to take off.)
Perhaps it does the same while finding its way through a frog’s innards. “I would imagine that an air bubble would help the beetle breathe, and may provide a little jacket to keep stomach acid at bay while an escape is made,” says Christopher Grinter, collections manager of entomology at the California Academy of Sciences, who wasn’t involved in the research.
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