AsianScientist (Jul. 6, 2018) – Last year, I wrote about the heartwarming story of the Lord Howe stick insect, thought extinct since the 1920s. In 2001, intrepid scientists discovered a surviving colony on Ball’s Pyramid, a desolate spire of rock hundreds of kilometers off the coast of mainland Australia. Happily, the insects, which can get so big that people used to refer to them as ‘tree lobsters,’ are now part of a survival breeding program run by the Melbourne Zoo.
Yet, one mystery remains. No one knows how the stick insects ended up on Ball’s Pyramid, which is separated by 20 kilometers of open ocean from the nearest land mass of Lord Howe Island, the bugs’ original home.
Now, scientists in Japan working on another species of stick insect may have uncovered a possible explanation. In a study published in the journal Ecology, they report that stick insect eggs can pass through the digestive tracts of birds unharmed, and also retain their ability to hatch. In other words, when stick insects are eaten by birds, their unborn offspring may gain a mode of long-distance transport, and with it a chance to poop-ulate faraway places they could never reach on their own.
The seed of an idea
The Japanese researchers drew inspiration from the way plants disperse their seeds, says study leader Kenji Suetsugu, an associate professor at Kobe University. While plants can’t move around, their fruit gets eaten by birds and other animals, which then excrete the seeds intact in other locations.
It turns out that seeds and stick insect eggs share many similarities in size, color, shape and texture. Like seeds, stick insect eggs are also remarkably durable—they are coated with a layer of calcium oxalate, which can withstand acidic environments such as digestive tracts.
The researchers mixed eggs from three different stick insect species with bird food and fed the concoctions to brown-eared bulbuls, which are predators of stick insects in Japan. (Suetsugu acknowledges that this is a departure from how birds would eat intact stick insects in nature, but says that this method allowed them to feed the birds many eggs). They then collected the resulting fecal pellets and combed through them under a microscope for signs of the eggs.
In the first trial, completed in 2015, between five and nine percent of eggs from each stick insect species survived their tour of the digestive tract intact, but none of them hatched (the researchers patiently waited two years). Although the eggs looked physically undamaged, the harsh chemical environment of the gut may still have rendered them unable to hatch, explains Suetsugu.
Undeterred, the researchers tried again in 2017, feeding the birds 70 eggs from a single stick insect species, Ramulus irregulariterdentatus. This time, 20 percent made it through, and two of these eggs eventually hatched about four months later.
Going places with predation
Although these experiments indicate that it is possible for stick insect eggs to survive the ordeal of being eaten by a bird, a success rate of two out of 70 doesn’t seem like a big deal. But given that stick insects are not the most mobile of creatures, Suetsugu says that even rare occurrences of dispersal via predation could make an impact on their ecology.
“Considering that stick insects are slow moving and often flightless, with a limited capacity for dispersal, the effect of long-distance dispersal via bird predation should not be underestimated,” he explains, adding that the phenomenon could be a factor in historical events such as stick insect invasions of oceanic islands.
Further, the stick insect egg production season in Japan nicely overlaps with the brown-eared bulbul migration season, a coincidence that potentially increases the role predation plays in the insects’ dispersal, the researchers suggest.
Suetsugu now wants to study the genetic makeup of stick insects found at various points along the birds’ flight paths, and hence piece together a map of their ancestry and movements. Stick insects look like sticks to avoid being eaten by birds, but if Suetsugu is right, then it’s lucky for the bugs—or rather for their offspring—that this disguise doesn’t always work.
This article is from a monthly column called The Bug Report. Click here to see the other articles in this series.
Copyright: Asian Scientist Magazine; Photo: Pixabay.
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