Hitchhikers (Part 1)
When Animals Go Piggyback
“For my part, I travel not to go anywhere, but to go.
I travel for travel’s sake. The great affair is to move.”
– Robert Louis Stevenson
By Richard Conniff
It’s one of the most entertaining sights in the rain forest: Leafcutter ants marching back to their nest in a long line, bumper-to-bumper, each ant carrying a leaf fragment held upright, like a placard, on its back. People lucky enough to witness this spectacle tend to be impressed that those fragments can weigh three times as much as the ant itself, and that the ants are cooperating in the oldest farming society on Earth: They use the leaf fragments to farm the fungus that they eat.
What we tend not to notice, though, is that many of the ants also carry another ant riding on top of the leaf.
It’s an example of what scientists call “phoretic behavior,” from the Greek for “being carried.” Or “zoochory” (more Greek) meaning “being spread around by animals.” But in plain English, it’s hitchhiking—one animal catching a ride on another, and it’s a surprisingly common behavior in the animal world. The animals doing the carrying range from spiders and three-toed sloths to elephants.
For the individuals being carried, the motives include raising their young, catching a free ride to the next meal or mate, stealing a meal on the fly, getting a viewpoint on the world, or just plain having a lark—for instance, when a genet, a type of small cat-like African carnivore, turned up a few years ago riding two Cape buffalo and one rhino in the same night, back and forth in font of the camera trap, apparently just because it could.
Some of these associations border on magical. For instance, researchers in Vietnam recently discovered a beautiful cave-dwelling cockroach, with a helmet resembling a halo. (O.k., bear with me.) Most cave cockroaches feast on the frass dropped to the cave floor by bats. But the helmet cockroach (Helmablatta louisrothi) appears to feed on bacteria and fungi around the bat roosts. Its wings are too small for it fly, so scientist theorize that it travels to the cave ceiling instead as a passenger on a bat.
For the leafcutter ants, on the other hand, hitchhiking behavior has evolved for a deadly serious purpose. Any time the ants leave the nest, they risk being attacked by phorid flies. It’s a gruesome business, with the fly trying to insert an egg, and the ant thrashing its legs and antennae to keep that from happening. But it’s hard for an ant to defend itself while also toting its leaf fragment, and failure means the ant will become the unwilling incubator for the fly’s offspring, to be slowly eaten alive as it develops.
Hence the hitchhiker: She’s always a member of the minim caste of leafcutter ants, because minims are too small to be of interest to phorid flies. (And yes, she is always a female, as are all other workers in ant societies.) The hitchhiker climbs up on top of leaves carried by the largest and most vulnerable ants. And there she rides shotgun, or rather, acts as a traveling fly swatter to fend off the bad guys. (I mean bad flies).
NOTES:
The Stevenson quote is from Travels With A Donkey, p. 152.
For the hitchhiking genet, see this blog from the conservation group Wildlife Act.
For the cave cockroach, here’s the original description, and here’s a news item with a saintly head shot.