A surprising range of species hitch rides on—or inside—other creatures, an age-old strategy that science shows is still evolving.
Finding the perfect perch, a red-billed oxpecker sits on the horn of a Cape buffalo in Kenya’s Lake Nakuru National Park. These birds feast on blood-engorged ticks plucked from buffalo and other beasts. If ticks are absent, the birds will peck at wounds to keep the blood flowing.
IT’S ONE OF THE MOST ENTERTAINING SIGHTS IN THE RAIN FOREST: leafcutter ants marching back to their nest in a bumper-to-bumper line, each carrying a leaf fragment held upright like a placard. These ants are cooperating in the oldest farming society on Earth: They use the leaves to produce the fungus that they eat. What may go unnoticed, however, is that many of the ants also carry another ant riding on top of the leaf.
This piggybacking is an example of what scientists call from the Greek for “being carried.” In plain English, it’s hitchhiking—one animal catching a ride on another, more mobile creature—and it’s a surprisingly common behavior in the animal world, though for many different reasons.
Carriers range from spiders and bats to three-toed sloths and elephants. For the passengers, motives include catching a ride to the next meal or mate, feeding off their free ride, hiding from predators, staying safe with mom, dispersing young or just gaining a new vantage point to hunt or scout the landscape.
Motives can be mysterious—even comical. In 2014, for example, researchers with Wildlife ACT were using remote cameras to monitor animals in South Africa’s Hluhluwe-Imfolozi Park. For several nights, the cameras showed a genet, a type of small African cat, appearing to joyride on top of Cape buffalo and even a rhino. Zoologist Simon Morgan speculated the genet might be “picking ticks off the animals’ ears” or scanning for small prey being “flushed” by their lumbering taxis.
In other cases, hitchhiking pairs border on magical. In 2017, for instance, while working in a cave in Vietnam, a team led by researchers from the Slovak Academy of Sciences discovered a tiny cave-dwelling cockroach with a helmet resembling a halo. Most cave cockroaches feed on bat guano that drops to the floor. But the newly dubbed helmet cockroach (Helmablatta louisrothi) appears to feed on bacteria and fungi around bat roosts high on the cave walls and ceiling. The roach’s wings are too small for it to fly, so the scientists theorize that it travels to the cave ceiling as a passenger on a bat, making it one of the most “bizarre” cockroaches ever discovered.
For leafcutter ants, on the other hand, hitchhiking has evolved for self-defense. Leaf fragments can be attacked by fungal contaminants and the ants themselves by phorid flies. That’s a gruesome business, with the fly trying to insert an egg into the ant’s head 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 an incubator for the fly’s offspring—slowly eaten alive as the larva develops.
Hence the hitchhiker ant: She’s always a member of the smallest caste of leafcutter ants—called minims—and 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 on top of leaves carried by the largest ants, which are most vulnerable to attack. And there she rides shotgun, to harass and discourage the attackers.
Animals have been hitching rides on other animals almost as long as there have been other animals to hitch rides on. Researchers working on the border of Austria and Italy recently discovered fossils of marine invertebrates that appear to have been riding piggyback on other species 425 million years ago. The riders were juvenile crinoids, better known as “sea lilies” for their branchy, undulating arms. Modern crinoids use star-shaped attachments called “holdfasts” to cling to the sea floor. But the concave shape of the holdfasts left behind by these ancient juvenile crinoids indicates that they latched onto drifting algae or onto the shell of some nautiluslike creature, the better to get as far away from home as possible before settling down. It’s a clue, according to Ohio State University researcher William Ausich, to how certain crinoids achieved a global distribution “that defies all logic.”
Likewise, European researchers in 2017 announced the discovery of a mite riding on the back of a spider, both perfectly preserved in amber (fossilized tree resin) from the Baltic region. The mite was almost too small to see with the naked eye. But with the help of high-tech scanning devices, the researchers were able to examine it in detail and date the find to at least 50 million years ago. Also in 2017, a chunk of amber turned up containing the feather of a dinosaur from 99 million years ago and—shades of Jurassic Park—a passenger tick engorged with its blood.
This last example suggests one small complication in talking about animal hitchhikers: Roughly 40 to 50 percent of all species on the planet are parasites, according to a 2008 estimate by Princeton University ecologist Andrew Dobson and colleagues. Parasites spend their lives hitching rides on (or in) other species. But their motive is generally to steal nutrients at their host’s expense. This is a clear violation of hitchhiking etiquette. It also violates the technical definition of phoresy, meaning “a nonparasitic association for obtaining transportation.”
In the natural world, as on the highway, however, it can be hard to separate the friendly hitchhikers from the troublemakers. For instance, we tend to celebrate the colorful birds called oxpeckers for picking off ticks as they ride along on zebras, rhinos, giraffes and almost any other large mammal in the African bush. But researchers now say these birds also pick open wounds to drink the blood of their host animals. Likewise, copepods and other small crustaceans often pass their lives innocently lounging about on jellyfish or other gelatinous marine creatures. “But if seaborne food runs short, they might eat up the host as well,” says Larry Madin of the Woods Hole Oceanographic Institution.
Even honest hitchhikers can inadvertently do damage. For instance, off the coast of British Columbia, the larval stage of the coldwater shrimp Pandalus danae likes to ride atop a small jellyfish. It is, in theory, surfing for its dinner: All it has to do is hang on, open its mouth and inhale the passing zooplankton. But spending extra energy hauling the shrimp around sometimes kills the jellyfish.
Among the most intriguing hitchhikers are several species that use other species as the vehicles for their reproduction. For instance, a small crustacean called Phronima hijacks the gelatinous bodies of salps, siphonophores and other marine species to make a barrellike, translucent house in which to raise its young. Marine biologist Carol Diebel’s account of this interaction begins: “The female entered the salp, cut out the brain and gill bar and consumed them….” Better to leave the rest to your imagination.
Freshwater mussels also hitchhike on other species to rear their young. The North American genus Lampsilis, for instance, is an unspectacular creature—until it opens its shell and sticks out a fleshy protrusion that brilliantly mimics a small, wriggling fish. This is an ingenious fishing lure to attract a local bass, walleye or other predatory fish. When the fish bites, the fleshy protrusion breaks open like a piñata, releasing a dense cloud of mussel larvae. They clamp onto the would-be predator’s scales and gills, turning it into their nursery and delivery vehicle for weeks or months of development. The larvae actually become embedded in their host’s flesh until they mature enough to break out and settle in a new home.
“Here you have this animal that has no head,” much less eyes to see either the fish they are mimicking or the one they are trying to attract, says Michael Gangloff, a conservation biologist at Appalachian State University. And yet, “using a limited evolutionary toolkit, they have come up with some pretty amazing tools for attracting fish.”
Some mussels, says Gangloff, are less subtle than Lampsilia. Species in the genus Epioblasma, for example, use an insectlike lure to attract darters, then grab the fish by the snout and pump it full of larvae before letting it go. Still other mussel species target seagoing fish returning to their breeding grounds, so their larvae can catch rides to distant parts no mussel could ever reach under its own power.
Charles Darwin thought hard about animal hitchhiking as he was coming to terms with the radical theory that one species could evolve from another. Conventional wisdom at that time held that God had created each species and made it exquisitely adapted to its particular habitat. But if that were so, why did species on offshore islands almost always resemble those on the nearest mainland?
The obvious answer, Darwin thought, was that mainland species somehow made it out to islands such as the Galápagos and slowly evolved into new forms there. But how could relatively stationary animals such as freshwater snails cross 600 miles of open seawater?
Darwin tried an experiment. He had noticed how ducks and other waterbirds sometimes carried duckweed as they flew from one pond to another, and he knew that duckweed sometimes contains small snails. So he immersed severed duck feet in a fish tank where freshwater snails were hatching. The feet emerged covered with tiny snail hatchlings that “clung to them so firmly … they could not be jarred off, though at a somewhat more advanced age they would voluntarily drop off.”
The snails survived and remained attached “in damp air, from twelve to twenty hours,” Darwin noted, “and in this length of time a duck or heron might fly at least six or seven hundred miles, and if blown across the sea to an oceanic island, or to any other distant point, would be sure to alight on a pool or rivulet.”
This kind of hitchhiking helped Darwin explain how species could find their way to surprising new habitats—and never come back—in the course of their evolution. He later received confirmation of this kind of hitchhiking when an American correspondent sent him an illustrated account of a particularly ambitious mussel clamped onto the toe of a blue-winged teal shot by a duck hunter in West Newbury, Massachusetts. Modern scientists also credit waterbirds with carrying saltwater mollusks to saline lakes in the Sahara, 300 miles from the ocean, and with responsibility for the quirky home range of species in the genus Tryonia, an aquatic snail found only in Guatemala, Florida and the western United States.
Does this suggest that notorious invasive species like zebra and quagga mussels found their way to new habitats, such as the Great Lakes, by some form of natural hitchhiking? It’s far more likely that these invaders arrived from Eurasia in the ballast water or on the hulls of ships. We know that invasive species also can spread by hitchhiking on recreational boaters, hunters, anglers or their gear—good reason to scour boats and boots before moving to new waters.
It sometimes seems as if every living thing—even a mussel seemingly glued for life to a fixed spot—now and then gets the urge to move, and by whatever means at hand. Creatures travel on the winds, on the currents of the oceans, on drifting bits of debris and on the bodies of other living things, destination always unknown. Thus we populate the Earth, and the Earth populates us. The poet John Donne had it right: “No man is an island entire of itself.”
He’s a minivan.
Writer Richard Conniff wrote about the vertical migration of ocean life in the December–January 2018 issue.
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