Back in 350 B.C., Aristotle devised a theory on why birds in Greece vanished every fall. Some, like swallows, hibernated in holes, he reasoned, whereas Common Redstarts transformed themselves into European Robins, which look somewhat alike.
We now know, of course, that the great philosopher was wrong; redstarts and swallows fly south to Africa every winter. But while we’ve made incredible strides since Aristotle’s time in tracking the migrations of species that span every branch of the animal kingdom, a new study in Scientific Reports on tiger shark movements is a reminder that many migrants still remain a mystery.
Here are seven unexpected drifters—a saltwater fish, a bird, an insect, a mammal, a crustacean, a freshwater fish, and a reptile—whose impressive nomadic feats are only now being uncovered by scientists.
For the first time ever, researchers documented tiger shark migrations by attaching satellite transmitters to 24 of the large predators that they baited with chopped-up bonito in Bermuda. The GPS data showed that mature sharks travel back and forth each year between the coral reefs of the Caribbean and the deep waters of the mid-North Atlantic—a round-trip journey of more than 4,660 miles
Only two other sharks—salmon and great white—are known to migrate from coastal regions to the middle of the ocean. But senior author Mahmood Shivji, director of Nova Southeastern University's Guy Harvey Research Institute in Florida, thinks makos and some other species follow the same itinerary—it’s just that nobody has fully recorded it yet.
Uncovering these paths could help protect the fearsome fish, about 100 million of which are slaughtered every year. “Sharks are badly overfished in most parts of the world,” Shivji says. Once scientists know where the predators spend most of their time, the findings can help focus conservation on those hotspots.
Of all the wild creatures out there, birds are best known for their truly extraordinary migrations. But it’s not only the stout and rugged seabirds that make these epic journeys. On a pound-for-pound basis, the Northern Wheatear is a true champion, being the only songbird to breed in the North American Arctic and winter in sub-Saharan Africa.
Through the use of geolocators, in 2012 researchers determined that this thrush, which weighs less than an ounce, travels up to 9,000 miles each way—a migration so time consuming that some individuals spend nearly half of their lives wayfaring. Birds that breed in Alaska were found to take the terrestrial route across Asia, whereas those that summer in eastern Canada traverse the Atlantic Ocean, with a possible stopover in Greenland, before dipping down through Western Europe. It’s a trip that even Marco Polo would envy.
Birds aren’t the only winged creatures capable of migrating great distances. In 2009, biologist Charles Anderson deduced that millions of dragonflies—mostly wandering gliders—undertake a 8,700- to 11,000-mile round-trip journey from India to East Africa, with key refueling stops in the Maldives and Seychelles (That’s about twice as long as the famed monarch migration in North America.) Anderson monitored when swarms of dragonflies were arriving and departing from multiple locations along the route to build his hypothesis.
Three years later, Anderson revealed that the wandering glider’s migration might even include a high-altitude traverse over the Himalayas. “The very broad hind wings probably contribute to its ability to glide on air currents and stay airborne for days at a time,” Kenn Kaufman, Audubon’s field editor, says of this globally widespread species, which he sees regularly in his Ohio backyard. The U.S. population is believed to winter in the New World tropics.
Just like with monarchs, no single wandering glider (also aptly called the “globe skimmer”) completes the long journey on its own. Instead, it flies as part of a relay race; each generation completes a designated leg, lays its eggs, and then dies. No need for retirement, apparently.
Sometimes even large mammals can migrate without being detected. Such was the case with several herds of mule deer in Wyoming. In 2014, wildlife biologist Hall Sawyer revealed that the deer undertake the longest known terrestrial migration in the Lower 48. Individuals with radio collars traveled about 150 miles each way from the Red Desert of southwestern Wyoming to the mountains of northwest Wyoming.
Herds can swell to 5,000 mule deer, but the study showed that only about 500 ungulates make the full trip. Each leg of the journey takes a few months and involves numerous river, highway, and fence crossings. Conservationists are now trying to protect the migration route from further development by buying up land at key crossover points. Will green bridges be coming soon to Wyoming?
Christmas Island Red Crab
Around November, when the rains hit Christmas Island and the Cocos Islands, tens of millions of spectacularly colored crustaceans begin marching from the islands’ lush interior rainforests to the beach. The crabs are endemic to these two Australian territories, which are located far out in the Indian Ocean.
This is no leisurely trip to the coast, however. The crabs face many obstacles during the two-week trudge, including car-filled roads, steep cliffs, dehydration, exhaustion, and yellow crazy ants, an invasive species that sprays formic acid. There are also brawls with other crabs over prime real estate (burrows in the sand). Yet the final reward is immense: a mating orgy that produces a giant spawning event in the ocean during high tide of the new moon.
Unfortunately, climate change may be upsetting the precise nature of this migration by making rainfall more erratic, a 2013 study found. And if the rains never come, the crabs don’t reproduce, which is what happened in 1997 during a strong El Niño. Given all their hard work, these eager little crustaceans don’t want to have to skip an annual mating frenzy. But then again, who would?
Food, sex, and weather are three of the main reasons animals migrate. But the roach, a common Eurasian fish in the carp family, is an anomaly, researchers discovered when they tagged more than 2,200 of the swimmers in two Danish lakes. The roach, it turns out, goes to great distances to avoid being eaten—a pattern best known in zooplankton, the researchers reported in a 2013 study.
Roach usually relocate from lakes to nearby streams for the winter. Although food is scarcer in these narrow straits, the move reduces the chance that they’ll end up in the stomach of a cormorant, their main adversary. To show this, the researchers placed antennas at the entrance to every connecting stream, which picked up the signals of tagged fish. That way, the team knew which fish were in the streams and which remained in the lakes. They then dug through cormorant feces to see which roach had been consumed, and lo and behold, most were of the lake variety. Moral of the story: avoid becoming cormorant poop.
At top speed, giant tortoises can lumber along at less than 0.2 miles per hour. Yet even these 500-pound plodders heed the need to migrate. Researchers discovered the unusual pattern in 2013 by tracking 17 individuals on Santa Cruz Island in the Galapagos.
Expanding on the work of Charles Darwin and others who noted that giant tortoises like to travel, the researchers determined that adult males and some adult females head to the island’s highlands each dry season to consume vegetation there that sucks up moisture from the thick and constant fog. Once the rainy season arrives the tortoises trek back to the lowlands to enjoy a different and more nutritious buffet of plants.
The reptiles cover 650 to 1,000 feet each day, and up to six miles in total. With an average life span of 100 years, these slowpokes can afford to devote some time to wanderlust.