Resume Reading — By the Light of the Moon, the Poles of the Earth


By the Light of the Moon, the Poles of the Earth

The remarkable ways animals get around.

Back in January, science news was abuzz with reports that the lowly dung-beetle—shellacked trundler of balled-up excreta, stuck with one of nature’s least glamorous jobs—used a majestic method to find its way around: the Milky Way. The critters had already been shown to exploit the polarized light of the moon to orient and move their smelly cargo in straight lines. But researchers knew there was a back-up mechanism, since on nights when the moon rose late, the insects’ sense of direction was still pretty good. A team from Lund University in Sweden tested their subjects under the night sky of the Kalahari Desert and in the controlled environment of a planetarium. In both settings, beetles with a view of the burning band of the galaxy moved competently, while those which were offered only the brightest stars or none at all were lost. “It’s corny, but it’s a highway in the sky, a great big pathway: the Milky Way,” one of the researchers told The New Yorker.

The critters had already been shown to exploit the polarized light of the moon to orient and move their smelly cargo in straight lines.

We may pride ourselves on our ingenious means of transportation, but animals themselves are capable of remarkable feats of swimming, scuttling, and flying, often with pinpoint accuracy. Monarch butterflies orient by the sun to travel the thousands of miles between the northern United States and Mexico; seals appear to guide themselves through the seas by a lone star. While animals’ motives for moving around are simple—to breed and feed—the mechanisms by which they do it are not. Animals possess an exceptional array of compasses, clocks, distance-sensing techniques, and internal maps. The examples below are guided by the light of the 2012 book Nature’s Compass: The Mysteries of Animal Navigation, by James and Carol Gould.

The Sugar Rush of Bacteria

To move toward a food source, bacteria compare the concentration of chemicals—usually sugars—in one place over another, and move toward the area of higher concentration. They do so through a time-lapse technique. The microbe stops every few seconds and charges off in a random direction, usually with the aid of a flagellum, a microscopic tentacle, to propel it. It will “taste” the spot it finds itself in; special proteins in the cell walls then release a flood of charged ions into the cell according to the concentration of chemicals, and control whether the bacterium moves or stays still. If the new location has richer pickings than the last, the microorganism will continue in that direction for longer before trying out something new. And the quicker the concentration is increasing, the longer the little critter will wait before trying its luck again.

The Sex Wiggle of the Bermuda Fireworm

Getting from place to place is as much about when as where. Perhaps the best display of chronological bravura is the luminescent Bermuda fireworm, which wriggles up from the mud to the surface of the ocean to mate in a burst of light, once every lunar cycle—to be more precise, on the third evening after the full moon, 57 minutes after sunset. The little annelids appear to have at least three timepieces (or maybe one that performs all these functions): a lunar clock to pick the right day; a daily clock to judge when the sun sets, in case the day is cloudy; and a stopwatch to figure out the number of minutes after sunset and to pinpoint “the consensual moment,” as the Goulds call it.

The March of the Cataglyphis Ant

While ants in friendly climates can rely on scent trails or landmarks to find their way back to a burrow after scavenging for food, the hot sun, sharp winds, and parched sands of the Sahara make that strategy unworkable for the desert-dwelling Cataglyphis ant. Yet, no matter how tortuous the route to find food, these creatures almost unfailingly make it back home. A team of German and Swiss researchers used a novel and ghoulish method to figure out how. They kidnapped ants after they had found a food source, sticking pig bristles to the legs of some of the insects, and amputating part of the legs of others. When the unfortunate abductees were released, those with “stilts” overshot their nests, and those with “stumps” fell short. It seems the ants had been using an “odometer” to keep tally of their steps on the outward journey—probably using proprioreceptors, nerve cells wedged in parts of the ants’ anatomy that register their movements. This odometer allows the insects to perform what is called “path integration:” keeping track of the direction and distance travelled, so they know where they are relative to the starting point at all times. Once the length of the ants’ stride changed, their calculation of the route home was no longer accurate. But when the surgically enhanced ants were returned to their nest and then set out to forage the next day, they counted the steps accurately both ways and made it home just fine.

The Dance of the Honeybees

Buzzing back and forth to the hive with bundles of pollen, honeybees travel up to five miles per trip and make up to 500 trips a day. When they find a rich source of food, they perform a “waggle dance” back at the hive: a figure eight turned on its side that encodes directions to the feast for the other bees. 

The further away the food, the longer the bee will dance; each waggle communicates a unit of distance, with the length depending on the subspecies. The tilt of the line running through the two halves of the figure eight corresponds to the angle of the food relative to the direction of the sun. Baby bees can learn that the sun rotates clockwise (in the northern hemisphere) or counter-clockwise (in the southern hemisphere), and compensate for how far it will have moved across the sky in the time it takes to get back home to spread the news. When the sun is obscured, bees appear able to see the unique pattern of how light is polarized in the sky1 to determine where the sun is.

When they leave the hive, bees keep track of distance by measuring “visual flow,” the rate of movement of textured surfaces beneath them. This seems to let them determine their position by way of dead reckoning, in which the direction and speed of movement away from a known location lets them plot a position. Bees may also be able to form cognitive maps—internal representations of a local area, rather than a set of orienteering instructions.

The further away the food, the longer the bee will dance; each waggle communicates a unit of distance.

In an experiment, James Gould trained bees to travel to a feeder station in a boat. When they got back to the hive, the scouts danced out the location. Bees hate to fly over water. When the dance pointed to a spot in the middle of a lake, none of the other bees could be convinced to follow it—but when the boat moved to the far shore, the recruits were happier to home in on it. One interpretation of the results is that the reluctant insects compared where the dance pointed them to their own maps of the area, and figured out in advance that it was surrounded by water. But when the dance pointed them somewhere close to shore, they were prepared to check it out.

The Brainy Navigation of Rats

Rats appear capable of creating maps of their environment. Let rats loose in a maze and they will learn its layout, even without the incentives of hidden food and punishment. Remove barriers to a reward and rats will know to take a shortcut. Take a rat out of a labyrinth with an electrified section, and days later it will remember to scuttle to the safe side. Both male and female rats methodically explore the eight arms of a spoke-shaped maze, although female rats prefer to use visual marks to identify which parts have been explored, while male rats tend to rely on recalling the angles of the turns they have taken.

At least three kinds of neurons appear to play a role in rat navigation. “Grid cells” fire as the rat moves through certain locations in space, corresponding to points on a remarkably regular hexagonal grid, while “head-direction cells” fire according to the way the creature is facing. These data trigger a third kind of neuron, “place cells,” which set themselves off when the rat moves through a familiar location. The Goulds compare this system, found in the hippocampus, to a “mental drafting table” that is hardwired at birth and filled in through experience and exploration. It is likely that this system exists in other mammals, including humans. One of the common early symptoms of Alzheimer’s patients is disorientation and inability to navigate, and one of the first areas of the brain to degenerate is the hippocampus.

The Smelly Flight of Seabirds

The big-winged family of seabirds known as Procellariiformes—which includes albatrosses, shearwaters, and petrels—are thought to sniff their way across the oceans. Their neuroanatomy suggests that they have a highly developed sense of smell, based on the size of the slug-shaped regions of their forebrains known as “olfactory bulbs.” During foraging expeditions for fish, carrion, and krill, albatrosses cover thousands of miles of sea before returning to the tiny slices of rock in the middle of the ocean where they like to make their homes.

Research by the neurobiologist Gabrielle Nevitt at the University of California Davis indicates that at small distances, up to a few hundred square miles, albatrosses use visual markers and odor plumes (such as from a dead fish or squid) to move toward their dinner. Across larger spaces, she speculates that they may be able to memorize olfactory maps based on the smell of microorganisms that accumulate, in part, according to the depth and topography of the ocean floor. Certain oceanic features, such as subaquatic mountains, tend to attract lots of plankton whose digestion produces a smelly sulfur-based compound that the birds can detect. At night the birds get lazy: they float on the water and wait for the current to drag them into the path of food.

The Internal Maps of Migrating Birds

Migratory birds accomplish remarkable feats of navigation, but the science behind their skills is contentious and evolving. Night migrants like the meadow pipit, a songbird, are thought to set their star by the pole point, the spot in the sky due north around which the stars and sun rotate. Many species fly at dusk, when the dark band of highly polarized light slides across the highest point in the celestial vault. At this moment, the band is perpendicular to the line running between where the sun rose and where it set, known as the azimuth, and so makes it easy for these birds to determine true north, which lies where the polarized band disappears below the horizon, halfway between where the sun rises and where it sets.

But like all good navigators, migrating birds have back-up systems. If a European robin flies into a dark space from which it can’t escape, the pattern of its claw-prints will show it hopping northwest, regardless of the season. The birds are detecting the lines of magnetic attraction that emanate from the north and south poles to orient flight direction. However, magnetism is not the most reliable guide. The earth’s magnetic field lines do not run precisely north-south between the poles, which produces a divergence known as “declination” between the direction of a compass needle and true north that depends on where you are on the planet. The earth’s magnetic poles also wander, and switch their polarity every few thousand years. Consequently, it seems likely that any compasses or maps the birds derive from the magnetic poles would need to be calibrated against other measurements, such as their celestial compass.

Some scientists, such as the neurobiologist Gerta Fleissner at Goethe University in Frankfurt, think that tiny magnetite crystals in the upper beaks of certain birds might alert the creatures to the direction of the magnetic field. This appears to be the mechanism at work in the confined robin example, and is also thought to be behind the orientation of newts, spiny lobsters, and sea turtles. However, research by Roswitha and Wolfgang Wiltschko back in the 1970s revealed that when a magnetic field is angled downwards—a metric known as “dip” or “inclination,” and which shifts slowly from straight up at the south pole to straight down at the north pole—robins will interpret that dip as north, regardless of the direction of the magnetic pull or “polarity.” Magnetite receptors can’t account for this phenomenon, and so researchers have been forced to look for more exotic explanations.

More recent research suggests that the magnetic crystals may be part of the birds’ immune systems, rather than detectors of magnetic information. Other studies point to the presence of a quantum mechanism in the eyes of avian migrants that enables them, perhaps literally, to see magnetic fields. An optical protein known as cryptochrome is thought to change into a quantum state when light strikes it, with two unpaired electrons zipping around two molecules in a configuration known as a radical pair. If these electrons spin in parallel, the system exists in what’s known as a triplet state; if they spin against each other, it’s known as a singlet. Running a magnetic field through this delicate biochemical web of subatomic particles will push the system towards either a singlet or a triplet state, depending on the direction of the field lines. The theory is that sensitivity to the balance of these states in the eye would allow the bird to perceive magnetic field lines, like a thread running through three-dimensional space, and use it to set their compasses for flight.

Other studies point to the presence of a quantum mechanism in the eyes of avian migrants that enables them, perhaps literally, to see magnetic fields.

Birds may even be able to internalize maps that encompass whole continents or even the globe itself. The long-beaked waterbird known as the bar-tailed godwit flies around 7,000 miles from Alaska to New Zealand every winter, sometimes stopping off at the tiny Melanesian Islands in the South Pacific. It is hard to account for such accuracy in the absence of a cognitive map. Similarly, each fall, flocks of white-crowned sparrows in northern Canada and the northwest of the United States fly several thousand miles south to breeding grounds in the southwest and Mexico. Researchers from the University of Lund in Sweden captured some of these birds in Washington state during their annual migration, and carried them in darkness to New Jersey, on the other side of the country. The adolescent birds flew south, apparently failing to register the displacement. But adults corrected for the change, flying west-south-west towards the correct destination, indicating that they have some kind of large-scale map acquired through experience.

The Magnetic Swim of the Loggerhead Turtle

Loggerhead turtles swim vast distances across featureless oceans, then tirelessly find their way back to the beach where they were born. Whether it hatched on the coast of Mozambique or Florida, a baby turtle will first detect the spread of light around it and scuttle toward the brightest area it can see. Because of the reflective qualities of the water, this is almost always toward the ocean. Once they are surrounded by water in the sea, this method becomes ineffective, so the hatchlings switch to heading in the direction that the waves are coming from in the open sea. 

Turtles also appear to have internal magnetic maps of the Atlantic. In 2011, a team of researchers from the University of North Carolina at Chapel Hill plopped a clutch of baby sea turtles into a swimming pool and re-created the geomagnetic conditions of two points on the earth. Experiments had already shown that turtles could use the intensity of the earth’s magnetic field, which is strongest near the poles and varies on a roughly north-south axis, to judge latitude. But longitude sensitivity is more mysterious, since there are no obvious cues that run reliably east-west. Yet when the researchers plugged the magnetic conditions of the western Atlantic off Puerto Rico into the system, the hatchlings swam towards the northeast, as they would if they were navigating toward the swirl of currents known as the Atlantic gyre. When the team set the system to the waters around the Cape Verde Islands, 2,300 miles away in the eastern Atlantic at the same latitude but a different longitude, the turtles correctly oriented toward the northwest. In the way that humans use latitude and longitude to pinpoint a location on a map, the turtles parse magnetic vectors to determine where they are, relative to their destination.

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