How Bugs From Australia Settled African Islands

When the Mascarene islands—Mauritius, Réunion, and Rodrigues—were born, they were blank slates. Unlike some other islands, they didn’t break off from a larger landmass, taking plants and animals with them. Instead, they rose right out of the water, formed by volcanic activity in the Indian Ocean off the coast of East Africa. Life there had to start from scratch. 

The islands collected colonizers and wayward travelers—a seed here, an egg there—over about 10 million years. Some of these species, like the islands’ most famous resident, the dodo, are no longer around, but, German zoologist Sven Bradler says, the Mascarenes are still home to a “remarkably diverse, endemic, and threatened concentration of flora and fauna.”

The islands’ location led scientists to assume that the plants and animals there arrived from nearby Madagascar and the African mainland. That’s the case for most of them, but Bradler’s research, recently published in the journal BMC Evolutionary Biology, has revealed that at least some traveled from much farther away: Australia.  

Among the Mascarenes’ animals are several types of Phasmatodeans, commonly called stick bugs or walking sticks. These large insects mimic plants for camouflage. The Mascarene stick bugs are diverse in appearance and behavior, but share some similarities with other African stick bugs. Traditionally, they’ve been placed in four different groups and descend, the thinking goes, from different, unrelated lineages of African stick bugs that all found their way to the islands at different times.

When Bradler and his team mapped out the evolutionary histories and relationships of 120 stick bug species from around the world (including, for the first time, the ones on the Mascarenes and elsewhere in the Indian Ocean), though, they found that this traditional view was way off the mark.

Their results suggest that the Mascarene stick bugs aren’t the descendants of several types of African bugs that made separate journeys to the islands; in fact, they don’t appear to be closely related to African stick bugs at all. Instead, they descend from a lone species of stick bug that originated in Australia, some 3000 miles away. This common ancestor was a member of bug group Lanceocercata, from which they diverged around 22 million years ago. Over time, the Mascarene insects evolved and diversified, and converged on some of the same traits and features as their African neighbors, leading to confusing similarities. 

The trip from Australia to the Mascarenes would be a long, perilous one for most animals, and stick bugs seem to be at a distinct disadvantage when it comes to such long-distance travel. While many species have wings, they’re poor flyers and can usually only use their wings to slow their descent from trees to the ground. They’re not cut out for trans-oceanic journeys—but with some luck, their eggs could pull it off. Stick insect eggs are hard-shelled and seed-like, and can survive for months at a time floating on water. The researchers think that the eggs of the Masacarene bugs’ Australian ancestor floated on a strong ocean current to get to the islands, maybe attached to a stick (how apt) that acted like a raft for them.

If you’ve been paying attention to the numbers, you might have noticed that something is amiss with this explanation: These bugs are far older than the islands they call home. While the Mascarenes’ founding stick bugs appear to have arrived around 22 million years ago, the oldest of the islands only formed 8–10 million years ago.

There's a possible explanation for this mismatched timeframe. There’s evidence that the Mascarenes had sibling islands, formed from the same volcanic hot spot, that rose above sea level around 30 million years ago and have since sunk back under the ocean. Bradler thinks that the Australian stick bug’s eggs likely landed on one of these lost islands and the bugs used them as stepping stones to get where they are now. When the colonists hatched, they evolved and speciated before moving on to the newly formed Mascarenes.