It’s obvious why a stick insect’s wardrobe is the way it is. Look like a stick, avoid getting eaten.
But scientists in Japan noticed that despite their camouflage, stick insects became bird food quite frequently. And they also wondered: why do their eggs look like seeds?
In a paper published last week in the journal Ecology, the team of researchers suggest that these mostly flightless insects could sometimes benefit when they are eaten, using birds as carriers to disperse their eggs miles away, just like seeds. This passive dispersal mechanism could be why stick insects are found in places far from their original homes.
“It’s commonly assumed that when insects are eaten by birds, they and their unborn young have no chance of survival,” said Kenji Suetsugu, the leader of the study, a biologist who studies parasitic plants at Kobe University. But their results potentially overturn this dogma, he added.
Because plants can’t move around on their own, one way they disperse their seeds is by creating seed-containing fruits that animals eat. Thus, as animals travel, relieving themselves along the way, the plants travel too — through their seeds. But many birds also eat insects. So the researchers reasoned, that just like the fruits, the insects could be a means for dispersal, as long as their eggs can pass through the birds unharmed.
One thing that makes stick insect eggs different from most other insects is that they resemble seeds — same shape, size, color and texture. And they’re coated in a chemical layer of calcium oxalate, the stuff humans find in kidney stones. As you might imagine, it doesn’t dissolve easily. Some of these eggs, carried inside females in certain stick insect species, also don’t need to be fertilized to hatch into viable offspring.
An earlier hypothesis is that the thickness and shape of stick insect eggs protects them from parasitic wasps. But the researchers also wondered whether these hard-shell, every-ready eggs survive a bird’s digestive tract.
In 2015, Dr. Suetsugu and his team mixed dozens of these eggs into bird food and fed it to brown-eared bulbuls, birds that eat stick insects in Japan. Within three hours they were scavenging through bird excrement, looking for intact bug eggs under a microscope.
A small percentage of the eggs from three species of stick insects made it through intact. But two years later, none had hatched. It’s possible that the temperature and chemical environment inside the digestive tract “might have annihilated them, in spite of a lack of mechanical damages,” they wrote.
However, they repeated the experiment in 2017, with 70 eggs from a single species. Twenty percent made it through, and 14 of those hatched, proving that it was possible for birds that consumed stick insects to drop their eggs like seeds to new locations.
Previous reporting on stick insects
Perhaps not coincidentally, the insects’ egg production season overlaps with the birds’ migration season. But even with flocks adding up to hundreds of birds eating stick insects and excreting their eggs, dispersal would be a rare occurrence, given that stick insects don’t carry many fully-developed eggs inside them, and so few make it through the brown-eared bulbul’s digestive tract intact and viable.
They now want to determine whether stick insects share similar genes in different locations along the birds’ migration flight paths. They also want to know if stick insects might have some of the same genes as plants that rely on birds for seed distribution, which could be another sign that they’re even more plantlike than we think.