Hitchhikers (Part 3)

Darwin, Duck Feet, & Evolution

Apr 11, 2024

Beyond its curiosity value, the idea of animal hitchhiking helped change our understanding of life on Earth. Charles Darwin thought hard about animal travel 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? Or as Darwin put it, “Why should the species which are supposed to have been created in the Galapagos Archipelago, and nowhere else, bear so plain a stamp of affinity to those created in America?”

The obvious answer, he thought, was that colonizers from the mainland had somehow made it out to the Galapagos and slowly evolved into new forms there. But how could freshwater snails, for instance, cross 600 miles of open seawater?

Darwin tried an experiment. He had noticed how ducks and other waterbirds sometimes carry hitchhikers as they fly from one pond to another. So he immersed duck feet in a fish tank where freshwater snails were just hatching. (Yes, the feet were detached from their former owner, which had presumably made an appearance at the Darwin family’s dinner table.) 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 that this kind of hitchhiking really happens 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 now 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 one saltwater mollusk found only in Guatemala, Florida, and the western United States.

For years, researchers also puzzled over the seemingly miraculous appearance of the large freshwater fairy shrimp in waterholes at vast distances from any similar habitat on the Antarctic Peninsula. They now know that the dormant eggs of these shrimp, called cysts, can withstand a long list of unpleasant conditions—including inside the digestive tract of waterbirds. These birds inadvertently consume the cysts at one waterhole and then deposit them at later waterholes on their travels.

It sometimes seems as if every living thing--even mussels, glued for life to their place on the bottom—now and then gets the urge for going, and by whatever means turn up: We travel on the winds, on the currents of the oceans, on drifting bits of debris, and on the backs 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—no Earthling--is an island.

He’s a minivan.

Richard Conniff’s Substack

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