Stick insects expand territory via bird poo

For most insects and their eggs, it’s “game over” when eaten by a predator such as a bird. Now, new research has revealed that stick insect eggs can pass through digestive tracts unharmed, meaning avian appetites may be facilitating a most novel form of dispersal for the slow-moving creatures.

Despite their excellent camouflage, stick insects are a favourite food of many bird species. Their eggs are intricate little vessels, with the size, colour, and shape resembling the seeds of local plant species. The eggs are coated in a layer of calcium oxalate, which forms a smooth outer shell tougher than most insect eggs.

Many stick insect species are capable of fertilising their own eggs – a phenomenon known as parthenogenesis. The birds prey upon females chock full of fertilised eggs, almost as if they were apples or other fruits full of seeds.

These factors – the predation by birds, and tough exterior of eggs – led a team of Japanese scientists to investigate the possibility that stick insect eggs are still viable after being eaten and excreted by an avian predator.

The researchers were led by Kenji Suetsugu from Kobe University. To test their theory, they selected one of the main predators of the insects in Japan, the brown-eared bulbul (Hypsipetes amaurotis), and fed it a blend of seeds and the eggs from three locally occurring species of stick insects (Ramulus irregulariterdentatus, Neohirasea japonica, and Micadina phluctaenoides).

For all three, between 5 and 20% of the eggs were exited the digestive tract unharmed. The scientists also confirmed that for one species (R. irregulariterdentatus), eggs retrieved from the bird’s excrement successfully hatched. These results suggest that dispersal via brown-eared bulbuls and other birds is a distinct possibility.

“Our next step is analysing the genetic structure of stick insects,” comments team leader Suetsugu.

“Based on this we’d like to investigate whether similar genetic structures of stick insects can be found along bird’’ migration flight paths, and whether there are genetic similarities between stick insects and plants that rely on birds for seed distribution”.

The findings were published in the journal Ecology.