Stick bugs are ingeniously adapted for survival. Not only do they look exactly like a twig, but their ability to completely freeze makes them essentially invisible. Despite these tricks, though, they still end up being a regular meal for many types of birds.
Now, a new study out of Japan suggests that stick insects might be even better built for surviving than we realized. When pregnant females get eaten, their hard, seed-like eggs can survive the trip through a bird's digestive system, and, incredibly, some still might hatch. That's per a paper published in Ecology last week.
This finding challenges the long-held assumption that insect eggs have zero chance at survival once they’re predated, Kenji Suetsugu, lead author on the study and biologist at Kobe University, writes in an email. It might also help explain why stick insects are so prolific across every continent except Antarctica.
After all, poop is a pretty effective delivery system. When a bird eats a berry or other piece of fruit, it can fly for many miles before depositing a viable, undigested seed. But most insect eggs are not nearly as tough as plant seeds, making it difficult for them to stay intact when passing through a bird.
Stick insects are an exception; their eggs are coated in calcium oxalate crystals, the same substance that makes up kidney stones in mammals. To test the durability of a common stick insect's eggs, Suetsugu and his colleagues fed 70 eggs to Brown-eared Bulbuls, a native Japanese bird that regularly snacks on stick insects. After the lab subjects pooped out the remnants of their dinners, the scientists retrieved 14 eggs that survived intact—the others were crushed or otherwise damaged by digestion. A couple months later, two of the eggs hatched into baby walking sticks. (In the study, the authors note that stick insects have low hatch rates, even under ideal circumstances.)
Bird-poop survival is likely just a side benefit of having such hard eggs. Parasitic wasps pose a major threat to stick insect eggs and, in this study, Suetsugu assumes protection against wasps is a primary reason stick insect eggs developed such hard shells in the first place. Another reason might be the stick insect’s unusual approach to laying eggs, says Lynn Kimsey, director of the Bohart Museum of Entomology at the University of California, Davis. “The female produces the egg and then flips it into the air,” Kimsey says. Wherever the eggs land is where they will hatch. One time, says Kimsey, she visited a mesquite forest in Texas where “there was a huge outbreak of walking sticks and they were tossing eggs, and it sounded like it was raining.”
Suetsugu believes that if a female stick insect is eaten at just the right moment, the embryos in her belly could still be viable. But given the low survival rates in the experiment, he concedes it’s unlikely that they make their grand entrance through poop all that often. Even so, it’s a possibility worth exploring more, Suetsugu says. “Considering that stick insects are slow-moving and often flightless, the benefits of long-distance dispersal via bird predation should not be underestimated,” he writes. Hypothetically, the eggs travel as far as the birds do on a full stomach, which could be 120 miles or more at a time.
Kimsey, who wasn’t involved with the study, says she’d want to know if there’s evidence of eggs surviving in excrement in the wild. That’s what Suetsugu plans to test next: He’ll use stick insect genetics to map different populations along the migratory routes of predators. If birds really do help the bugs expand their range, he expects to see a geographical overlap. And even if they don't, just knowing that some of their eggs at least have the potential to survive being eaten makes the already fascinating stick insect all that more interesting.