Get Your Bearings – Background

Animals are always on the move, traveling from place to place to look for mates, find food, tend their young, or migrate to and from a faraway winter home. To do so, animals must have a sense of where they are and a way to navigate between destinations. Many aspects of the physical world provide important cues for animals on the go, some that we can sense and others that are outside our range of perception. What environmental cues do animals use to get their bearings, and how do people find their way?

From an early age, we begin to develop a sense of direction, an understanding of where we are inside our homes, which soon expands to school and beyond. One of the ways we orient is by forming a mental map of familiar places – a picture in our mind’s eye of house, backyard, neighborhood. Do animals form mental maps as well? Digger wasps nest colonially, digging tunnels in sandbanks in which to lay their eggs. A wasp finds her own particular nest hole among many similar ones by memorizing the location of landmarks like twigs, cones, and pebbles near the entrance. She becomes disoriented when the landmarks are moved around experimentally. From honeybees to homing pigeons, many animals have been shown to form a mental map that lets them navigate easily within their home ranges.

A different system is used by desert ants and fiddler crabs. They rely on a kind of dead reckoning to find their way. By keeping a running tally of how far and in what direction they go on each leg of an outward journey, they always know where they are with respect to “home.” For example, if you remember that you walked ten steps north, then five steps east and ten steps south, then you know right where you are, and that five steps west will bring you home. Sailors of old used dead reckoning too, when skies were cloudy, but they were always glad to see sun or stars again for a more accurate reading.

Many animals make long-distance migrations of thousands of miles, some traveling in the dark, over oceans, without any prior experience of the route. Sea turtles, salmon, sharks, and whales travel migration routes through trackless oceans. Many shorebirds, seabirds, songbirds, and raptors cover huge distances over land or sea, migrating between summer and winter ranges in different hemispheres. Tiny honeybees forage a mile from their hives, and monarch butterflies make a trip of some two thousand miles, by a route they’ve never traveled, to some fir trees they’ve never seen, in the mountains of Mexico. These feats of navigation astound us, for we would need sophisticated technology to accomplish such a task, or at least a map and compass.

A map is a flat representation of the landscape set out on a grid based on the cardinal directions, with north, by modern convention, at the top. From a map we can find our position and the direction and distance to our destination. But to get there we need to know how to orient the map with respect to the real world. For this we need a compass. A compass contains a needle that aligns with the Earth’s magnetic field, the red end pointing towards the magnetic north pole, which is close to the geographic North Pole. A compass tells us direction, but it can’t tell us our position. With map and compass together, though, we should be able to navigate to any place on Earth. In most places we can now use the Global Positioning System (GPS) which uses satellites to provide us with our longitude and latitude, and plot our position on a map, even as we move along. Like us, for traveling long distances, animals must have both a map sense, a way to know their position with respect to their destination, and a compass sense, a way to maintain a particular heading to get there.

Animals have internal compasses based on different environmental cues – the sun, stars, the Earth’s magnetic field. A sun compass was first discovered in the honeybee, later in birds like starlings, pigeons, and thrushes, and more recently in monarch butterflies. Since the sun rises in the east, a bird or insect flying with the sun on its left wing would be flying south. But, the sun changes position in the sky over the course of the day. Because animals have an internal clock, they are able to compensate for the sun’s movement and stay on course. Cloudy days might present a problem, but many birds and insects can see polarized light, which tells them the direction of the sun, even if it is hidden behind clouds.

Other cues can be important compasses for finding and maintaining a course. Hatchling sea turtles sense the direction of waves along the beach, and by swimming perpendicular to wave movement, they navigate out to sea. Birds that migrate over land in the daytime use prominent features of the landscape, like ridgelines, coastlines, and rivers, as guides. Hawks and eagles follow mountain ranges, making use of thermals, rising warm air columns, to carry them along. Some birds like ibises, Canada geese, and whooping cranes learn the route to their wintering grounds by following their parents, while others find their way by some innate instructions, for they migrate without their parents.

Many birds, especially small songbirds, migrate at night when the danger of predators is less. As they depart they set their course by the setting sun, and on cloudy days by using polarized light. But once it is dark they must rely on other cues. Night migrators call to each other, which helps them stay in contact with a group. Some birds are able to hear infrasound, the extremely low vibrations produced by waves crashing on the shore, a way to follow a coastline in the dark. Indigo buntings, Savannah sparrows, thrushes, and many others use the stars to navigate, learning through experience the patterns of the constellations and how they move in the night sky.

The sense of smell is important for some animals to find their way. Smell is a key help for homing in pigeons. Salmon fry imprint on the particular odor of the freshwater stream in which they hatch. Later they migrate out to sea where they live for several years before returning to breed and lay eggs. Salmon can recognize the scent of their natal stream and follow it upstream because the intensity of the odor increases as they near their goal – the spawning grounds where they were hatched as fry years before.

A scent works for short-range navigation, but to find the mouth of their river, salmon rely on a different cue – the Earth’s magnetic field. In fact salmon, sharks, seals and sea turtles, bees and butterflies, bats and mole rats, and birds of many kinds have been found to have a magnetic compass, an ability to sense the Earth’s magnetic field and orient by it. Since the Earth is like a giant bar magnet with north and south poles, an animal that is able to sense the magnetic field can use this ability to maintain a bearing.

Remarkably, recent studies have shown that for some animals, the Earth’s magnetic field is both map and compass, like a GPS. The Earth’s magnetic field varies in both its angle and intensity. It is strongest at the poles and weakest near the equator. The tilt (angle of inclination) of the magnetic field is vertical at the poles, parallel to the Earth at the equator, and irregularly slanted in between. So, any location has a particular set of magnetic coordinates. Sea turtles roam the oceans for many years but return to lay their eggs on exactly the same beach where they hatched, locating it by its unique magnetic signature. Similarly, salmon learn the magnetic profile of the Pacific coastline on their migration out to sea and use it to navigate back to their river’s mouth years later. Pigeons also memorize the magnetic cues close to home, as well as those on all their travels. Do humans have a magnetic sense? Possibly, though the results so far are not conclusive. We do have memory, landmarks, trails, compasses, maps, road signs, and GPS to help us find our way.

People and animals have many different ways to orient and navigate, and we rely on all of them at one time or another. Being able to move from place to place, sometimes over great distances, and still find the way home, is of great importance in our lives. The more we learn about how animals navigate, the more we marvel at their ability to get their bearings and find their way around the globe.

 

Suggested Reading:

Dingle, Hugh. Migration: The Biology of Life on the Move. New York, NY: Oxford University Press, 1996.

Heinrich, Bernd. The Homing Instinct: Meaning and Mystery in Animal Migration. Boston, MA: Houghton-Mifflin-Harcourt, 2014.

Kjellström, Björn. Be Expert with Map and Compass. New York, NY: Macmillan, 1994.

http://www.nwf.org/news-and-magazines/national-wildlife/animals/archives/2014/animal-navigation.aspx

http://www.unc.edu/depts/oceanweb/turtles/

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