Biologists are discovering new details about how wild species find their way around
01-27-2014 // Doug Stewart
ONE OF THE WONDERS OF THE NATURAL WORLD is the ability of salmon, after swimming thousands of miles in the ocean, to find their way back to the freshwater streams where they were hatched years before. Exactly how they do it has long puzzled researchers. Now a new study suggests that the fish know their location by sensing variations in the Earth’s magnetic field as they travel.
The study focused on sockeye salmon that leave British Columbia’s Fraser River and wander the Pacific Ocean for two years before returning to spawn. The mouth of the Fraser, like any spot on the planet, has a distinctive magnetic signature, and the salmon appear to recognize it. “We looked at data on fish migration going back to the 1950s,” says the study’s lead author, Nathan Putman, a post-doctoral researcher at Oregon State University. On the home stretch, returning salmon have to swim around Vancouver Island’s northern or southern end. Historical records show which route was more popular in any given year; other records show how the intensity of the local magnetic field has fluctuated yearly.
“We found that when the magnetic field at the northern end of the island was similar to what it had been at the mouth of the Fraser two years earlier, more fish used the northern route,” Putman says. “The converse was true for the southern route.” This suggests the sockeye weren’t simply using their magnetic sense to get a compass direction, he says. Rather, they seem to be using it to position themselves on a kind of magnetic map that they have learned in the course of their travels.
Putman is one of a host of experimental biologists teasing out details of how wild species find their way around, whether returning to a burrow after a foraging trip or migrating between hemispheres. With each new study, the science of animal navigation is becoming less mysterious but more amazing.
At the simplest level, animals on the move use visual landmarks, just as people do. Eagles fly along Adirondack ridges. Gray whales follow California’s coast, bobbing upright from time to time to keep tabs on nearby headlands. Nearsighted honey bees rely so heavily on visual landmarks that they’ll make midflight detours to pass close by a bush they’re using as a guidepost.
Slightly trickier is a strategy called “path integration,” which an animal can use to determine its position by assessing how far and in what direction it has been moving. In the absence of a landmark or other positional fix, path integration is the best method available. Sailors call it dead reckoning and use it reluctantly—the farther they travel and the more turns they make, the more errors accumulate.
Even so, fiddler crabs (left) scurrying on mudflats to and from their burrows depend for their lives on path integration, making dead reckoning into an exact science. Notable for a single oversized claw (which reminds some people of a violin, hence the name), these tiny crustaceans live in marshes and estuaries around the world, feeding on organic matter they sift from sediment. Typically less than an inch long, fiddler crabs venture from their burrows at low tide to forage.
“Once they’re three or four body lengths from the burrow, they can’t see it,” says biologist John Layne of the University of Cincinnati. “Yet even 25 body lengths away, they know exactly where this unseen spot is out there on the mud.” When frightened, crabs make a beeline straight for their holes.
From fieldwork in Beaufort, North Carolina, Layne and colleague Michael Walls determined that the crabs aren’t using visual landmarks or odor cues to stay oriented. Instead, they’re measuring how far their eight tiny legs have propelled them—or how far their legs feel as if they’ve propelled them. The researchers rubbed baby oil on acetate sheets and lay them near burrows, then filmed the fiddler crabs slipping and skating across the slippery plastic. When Layne startled the crabs by waving his hands, they darted to the precise spot their hole should have been, based on the number of slip-sliding steps they’d taken. The fiddler crabs evidently keep a running measurement at all times of where they are in relation to their holes.
“Path integration is just about the worst navigational system you can design, but somehow fiddler crabs get away with it,” Layne says. “It’s one reason I find these little guys fascinating.” He suspects they’re good at dead reckoning in part because they avoid error-inducing turns as they forage. They hold their bodies in a more or less fixed orientation and just crab-walk forward, backward or sideways.
Eyes on the Stars
Scientists have long known that many birds and other animals navigate by looking at the sun, moon and brighter stars, freeing themselves from sticking close to familiar landmarks near home. Navigation by sun or stars works anywhere, if the sky is clear.
One of the more improbable animals to use celestial navigation is the dung beetle, or scarab. Common worldwide, the beetle can derive all the nutrition it needs, even water, from the waste deposits of larger animals. Females also lay their eggs in dung; the larvae eat their way out from inside. After swarming over a cattle-dung pile, dung beetles will quickly roll away and bury food balls larger than their bodies.
To avoid having their precious balls stolen by other insects, the beetles roll a straight course away from the pile, traveling 100 feet or more in less than 10 minutes. To stay on course, they need a navigational guide. “Without one, any animal will move around in circles,” says biologist Marie Dacke of Lund University in Sweden. Blindfolded humans wearing earplugs will do the same, she says.
For years, Dacke has studied how nocturnal South African dung beetles keep their bearings. “We found that even if the moon was far below the horizon, the beetles did not get lost,” she says. Yet something in the sky was important to Dacke’s dung beetles—when she fitted them with little hats that blocked out the sky, they became disoriented. On overcast nights, too, they were lost. “They could still see trees and other landmarks on the ground, but they seemed to totally ignore them. So what were they using?”
Dacke’s group recently answered the question by borrowing the Johannesburg Planetarium for two weeks and running beetle-rolling contests under different projections of night skies, such as bright stars, dim stars or no stars. “We concluded the beetles were using the Milky Way,” she says. Without it, the insects wandered aimlessly. Perhaps they couldn’t see individual stars, but they evidently had no trouble seeing the wide streak of light running across in the sky and using it as a directional bearing. Dacke imagines that other nocturnal insects do the same thing.
Perhaps the richest and most widely available navigational aid is the Earth’s magnetic field, a force generated by the planet’s molten core and serving as a 4,000-mile-long bar magnet. Creatures from bats and mole rats to butterflies and bacteria carry within them bits of a crystalline iron oxide, called magnetite, that helps them orient in relation to the Earth’s magnetic force lines. Recent studies demonstrate that many animals use magnetic sensing for more than just direction-finding. Like the Fraser River’s sockeye salmon, they’re identifying where they are and where they need to go as if they were carrying a GPS, though biologists are not exactly sure how the mechanism works.
One species that has demonstrated this ability spectacularly is the loggerhead sea turtle (left). On Florida’s Atlantic coast, loggerheads hatch on beaches months after their mothers have lumbered back into the surf. If they can evade pelicans, gulls and other predators, the 2-inch hatchlings crawl into the ocean and begin a multiyear, transatlantic migration—alone. Weak swimmers, they’re aided by the North Atlantic’s clockwise currents. Keeping them on course is their ability to tune into the Earth’s magnetic field.
“There are two features of the magnetic field that the turtles are known to detect: its inclination angle and its intensity,” says Kenneth Lohmann, a biology professor at the University of North Carolina–Chapel Hill and an authority on oceanic navigation. Magnetic inclination, the tilt of the magnetic field at any given spot on the Earth’s surface, varies with latitude—it’s horizontal at the magnetic equator, vertical at the poles and slanted in between—so the turtles can use it to gauge latitude, he says. The second feature, magnetic intensity, varies across the globe in a slightly different direction from inclination. As a result, different places around the world have distinctive, possibly unique, magnetic signatures.
“The turtles seem to have a mental map based on these two features, nothing more,” Lohmann says. Certain areas along their migratory route are apparently so distinctly marked they might as well be midocean signposts. “They’ve never been to these areas before and probably have no conception of where they are, yet they know what to do. Young loggerheads probably inherit a set of instructions encoded in a very simple way, like: ‘If you encounter a magnetic field like this, swim south.’”
Lohmann recently led a team that tested the idea of inherited maps by placing magnets around captive Florida loggerheads in his lab. The magnets mimicked the Earth’s magnetic field off the coast of Portugal, roughly the midpoint of the turtles’ migration. Sure enough, with the magnets turned on, the turtles tried swimming southward in their tanks, just as they would off Portugal when changing course for northwest Africa.
About 10 years ago, Lohmann tested spiny lobsters in a similar experiment. Spiny lobsters off the Florida coast migrate 20 or 30 miles seasonally between relatively deep and shallow water, walking single file, one behind the other. “When we gave captive lobsters a magnetic field that exists north of their home area, they responded by walking south. When we gave them a field that exists south of their home area, they walked north. The lobsters seem to have a magnetic map quite similar to the ones sea turtles have.” Lohmann is willing to bet that if two such dissimilar animals share this ability, plenty of other species do, too.
Not all animals, even those that travel long distances, depend entirely on innate navigation skills. Thomas Mueller, a biologist at the University of Maryland, and his colleagues studied captive-bred, endangered whooping cranes that were released in Wisconsin’s Necedah National Wildlife Refuge. Their research indicates that older birds—cranes can live in excess of 25 years—play a significant role in helping younger cranes learn migratory routes between Necedah and their winter habitat at Florida’s Chassahowitzka National Wildlife Refuge.
After analyzing eight years of data on the birds and their travels, Mueller and his team concluded that migratory groups in which the eldest individuals were only a year old deviated from their courses by about 47.3 miles, while flocks that included birds up to 8 years old deviated by only 29.1 miles. “Social learning from older birds reduced deviations from a straight-line path, with seven years of experience yielding a 38 percent improvement in migratory accuracy,” the team reported recently in the journal Science.
The findings suggest that the birds learn migration routes over many years, with older birds more likely to become leaders. “Such learning probably applies to other birds that are long-lived and social, including storks and geese, but probably is not important to smaller birds that migrate alone,” Mueller says.
Probably no animal possesses the uncanny navigational skill of the homing pigeon. Known in the wild as the rock dove (below), it can be blindfolded, anesthetized, surrounded by magnets, placed in a sealed box with its own air supply and shipped to another time zone, and it still encounters no trouble flying back to its loft. Unlike sea turtles (and a bit like salmon), young pigeons need to make an initial trip, during which they imprint on their home turf. The birds apparently memorize not just local magnetic cues but also visual landmarks and patterns of sunlight. Other long-distance avian travelers likely do the same.
At the Baylor College of Medicine in Houston, researchers led by neuroscientists David Dickman and Le-Qing Wu recently placed homing pigeons in a magnetic “clean room,” where the Earth’s natural magnetic field had been blocked. They then rotated artificial magnetic fields around the birds’ heads and studied the response of 53 neurons in a part of the brain associated with navigation.
“We found that all of the cells we recorded from are tuned to a particular magnetic direction,” Dickman says. “This tells us that these cells together can encode for all directions of the Earth’s magnetic field,” allowing the pigeons to construct a mental geomagnetic map based on where they’ve been. It’s a travel guide they can use for the rest of their lives.
Dickman says the aerospace industry is already looking into navigational systems that would do something similar—rely for precision guidance on the Earth’s natural magnetism rather than on expensive GPS satellites. If such a system is ever perfected, don’t expect to see supersonic jets that look like pigeons. But deep down, they’ll share a kinship. And their pilots can only hope their planes’ navigational systems work as unerringly as a homing pigeon’s.