Stick insects are fascinating creatures that can mimic the appearance of twigs and leaves to avoid being eaten by predators.
However, some of them are also flightless, which means they have limited ability to move across long distances and overcome geographic barriers. How then do they manage to colonize new habitats and maintain genetic diversity?
A recent study by researchers from Kobe University in Japan suggests that birds may play a key role in dispersing the eggs of stick insects after eating the pregnant females.
This is similar to how many plants rely on birds to eat their fruits and seeds and carry them to new locations.
The evidence for bird-mediated egg dispersal
KARIM SAHIB/AFP via Getty Images
The researchers focused on a common stick insect species in Japan, Ramulus mikado, which is known to be parthenogenic, meaning that the females can produce viable eggs without fertilization.
They collected specimens from 17 locations across Japan and analyzed their genetic relationship patterns using a technique called microsatellite genotyping.
They found that most of the genes showed a positive correlation between genetic differentiation and geographic distance, indicating that the stick insects have limited active dispersal ability.
However, they also found a few genes that were identical or very similar among populations that were separated by hundreds of kilometers and across geographic features such as mountains and seas.
These genes were likely derived from a single or a few individuals that had been transported by birds.
The researchers also conducted experiments in the lab to test whether the eggs of Ramulus mikado could survive the passage through the digestive tract of birds.
They fed gravid females to brown-eared bulbuls, a common bird species in Japan, and collected their feces, and found that some of the eggs remained intact and viable after being excreted by the birds.
The implications for stick insect evolution and ecology
The study provides strong evidence that bird-mediated egg dispersal is a possible mechanism for the distribution of flightless stick insects.
This may explain how some stick insect species have colonized remote islands and maintained genetic connectivity among isolated populations.
The researchers suggested that this mechanism may also have evolutionary consequences for stick insects, such as promoting genetic diversity, facilitating adaptation to new environments, and increasing the risk of hybridization with other species.
The study also highlights the importance of understanding the interactions between predators and prey, and how they can affect the dispersal and distribution of organisms.
As the researchers conclude, "This finding invites researchers to delve deeper into the mechanisms of dispersal in various species and challenge long-held assumptions about the fate of organisms devoured by predators."
Stick insects belong to the insect order Phasmatodea - the name comes from the Greek word "phasm," meaning phantom.
The order includes about 3,000 species worldwide, with the most diversity around the tropics. New Zealand's temperate climate supports more than 20 described species of stick insect.
Stick insect species, often called walking sticks, range in size from the tiny, half-inch-long Timema cristinae of North America, to the formidable 13-inch-long Phobaeticus kirbyi of Borneo.
This giant measures over 21 inches with its legs outstretched, making it one of the world's longest insects. Females are normally larger than males.
Phasmids generally mimic their surroundings in color, normally green or brown, although some species are brilliantly colored and others conspicuously striped.
Many stick insects have wings, some spectacularly beautiful, while others resemble little more than a stump. Several species have spines and tubercles on their bodies.
Found predominantly in the tropics and subtropics-although several species live in temperate regions-stick insects thrive in forests and grasslands, where they feed on leaves.
Mainly nocturnal creatures, they spend much of their day motionless, hidden under plants.
The origin and evolution of stick insects
The fossil record of stick insects is scarce, but molecular studies suggest that they originated in the Jurassic period, about 200 million years ago.
They may have evolved from an ancestor that resembled a modern-day cockroach or mantis.
Some researchers have proposed that the diversification of stick insects was triggered by the emergence of birds and mammals as major predators in the Cretaceous period, about 145 to 66 million years ago.
These animals have keen vision and can detect small movements, which may have favored the evolution of camouflage and mimicry in stick insects.
However, other factors such as climate change, plant diversity, geographic isolation, and sexual selection may also have influenced the evolution of stick insects.
More studies are needed to unravel the complex history of these forest phantoms.
