It's hard to be an attentive parent when your entire life strategy is predicated on looking and behaving like an inanimate object.
But members of the order Phasmatodea — better known as stick insects for their resemblance to leafy branches and dried twigs — try their best.
To protect their eggs from predators, some females lay them in hard-to-reach places, like the undersides of leaves. Others enlist the help of ants by attaching an irresistibly nutrient-rich knob called a capitulum to the ends of their eggs. The unsuspecting ants will carry an egg into their underground nest, eat the capitulum and toss the rest of the egg into their garbage zone, where it can incubate in safety, away from predators. In a few months, the hatchling stick insect will crawl back into the open, its inadvertent hosts none the wiser.
These animals may even have picked up a parenting pointer from the plants they emulate. A study published Monday in the journal Ecology found that some stick insect eggs can still hatch successfully after passing through the digestive tract of a bird, much as plant seeds do.
It's possible, the study authors argue, that stick insects use this strategy to spread their offspring far and wide, mimicking the way motionless plants rely on animals to distribute their seeds via eating and excreting them.
“Considering that stick insects are slow moving and often flightless, with a limited capacity for dispersal,” said lead author Kenji Suetsugu, a biologist at Kobe University in Japan, “the benefits of long-distance dispersal via bird predation should not be underestimated.”
Suetsugu has long been intrigued by the striking similarities between stick insect eggs and plant seeds. The eggs are encased in hard, tough shells. Their size, shape and color can resemble that of acorns, corn kernels and others seeds. Often, the eggs are also coated in a protective substance called calcium oxalate (the major component of kidney stones) that doesn't dissolve except under the most acidic conditions. Could these features protect an egg as it passed through a bird's gut?
Results from previous studies were not promising; a 2011 study found that only one in 1,000 eggs fed to chickens and quails could be recovered unbroken from the birds' feces.
In 2015, Suetsugu and his colleagues tried their own experiment, feeding 145 eggs from three stick insect species to brown-eared bulbuls, a common bird in Japan. Several hours later, they began examining the birds' poop.
Between 5 and 10 percent of the eggs survived digestion with no obvious damage. But none of them hatched. So two years later, the undaunted researchers tried again, offering up 70 eggs from the species Ramulus irregulariterdentatus for bulbuls to snack on.
This time, 14 of the eggs emerged unharmed, and two of those eventually hatched healthy infant insects.
A two-in-70 survival rate might not seem very promising. But given the limitations on stick insects' dispersal, even the occasional success could “profoundly affect the distribution, gene flow, and community composition,” Suetsugu said.
He also noted that female stick insects are capable of parthenogenesis — a type of cloning that produces offspring without the involvement of a male. A pregnant female could conceivably be eaten and still pass on her genes if her eggs were pooped out unharmed.
The next step for Suetsugu is to examine the phylogenies — or genetic family trees — of stick insects and compare them with bird migration routes.
“If avian dispersal is important to stick insects, the phylogeographical patterns should reflect occasional long-distance dispersal events,” he said.