Navigating by the Stars

by Belinda Recio

Seals, birds, whales and dung beetles, too, use celestial navigation to find their way at night. If you have ever looked out at the sea and noticed a pointy ear-less head suddenly pop out of the water, it may have been a harbor seal “spyhopping.” Seals and other marine mammals spyhop by moving their tails back and forth—similar to how we tread water with our feet—in order to rise to the surface and check out what’s going on above.

For some time now, scientists have known that marine mammals spyhop to search for prey, watch for predators, investigate something unusual (like a kayaker) or to get their bearings by looking for reference points on the shore. But recently, scientists Björn Mauck, Nele Gläser, Wolfhard Schlosser and Guido Dehnhardt discovered a new reason that harbor seals might spyhop at night: They may be navigating by the stars, as ancient sailors once did.

For most of human seafaring history, sailors crossing the ocean plotted their direction by measuring the angle between a designated lodestar (a celestial object such as the sun, the moon, a planet or a single star) and an azimuth (a point on the earth’s horizon directly below the lodestar). This measurement allowed sailors to estimate how far north or south they were from the equator. For example, the Vikings used Polaris (the North Star) as their lodestar, measuring the distance between it and the azimuth to calculate their latitude.

Mauck and his colleagues had a hunch that harbor seals might use a similar process to navigate at night. To find out, the researchers turned a circular pool into a planetarium, raising a dome above it onto which they projected roughly 6,000 points of light reproducing the stars and constellations of the Northern Hemisphere’s night sky. They then separately trained two male harbor seals, Nick and Malte, to locate a single lodestar—in this case Sirius, the brightest star in the Earth’s night sky. The researchers used a laser pointer to highlight Sirius. The seals then had to swim from the center of the pool toward the azimuth—the point on the horizon underneath Sirius—and touch it with their snouts. If they touched the correct location, they were rewarded with food. The seals were easy to train and were soon ready to take their astro-navigational test. No laser pointer was used during the test, and during each round of testing, the researchers randomly spun the planetarium dome so the lodestar Sirius would be in a new position.

Right from the start, the seals did pretty well. Most of the time, they were still able to locate Sirius and touch its azimuth point on the horizon despite the change in position. But by the end of the test, both seals were finding the correct azimuth 100 percent of the time!

Mauck and his colleagues believe that the seals’ amazing precision at identifying lodestars suggests that they can navigate by the stars. Of course, just because seals are capable of astronavigation doesn’t mean they actually use this method to find their way. More research is needed before we can know that for certain. But this test does provide a plausible explanation for how seals navigate at night in the open sea.

Seals aren’t the only animals who navigate by the stars. In the late 1960s, one of the first celestial navigation planetarium experiments demonstrated that birds—indigo buntings—navigate by the stars and can even adjust their orientation as stars move throughout the night. Other birds, such as mallard ducks, owls, cuckoos and orioles, also use stars to determine their travel route. Scientists speculate that whales, too, might use stars as reference points to keep them on track during their long migrations. Humpback whales, for example, cover up to five thousand miles during their seasonal migration. They somehow manage to travel in nearly straight lines even when encountering currents, storms and ever-changing sea depths. Scientists know that humpbacks use the Earth’s magnetic field to help keep them on course, but they doubt that magnetism alone is responsible for the accuracy of their beeline journeys. So it is possible that the whales might also be using the sun and stars to help them navigate.

Even the lowly dung beetle uses celestial navigation to find his way in the desert at night. Through a series of planetarium experiments similar to the one conducted with seals, scientists from Sweden’s Lund University figured out that dung beetles navigate a straight path between two locations by getting their bearings from the Milky Way. There is something poetic in the realization that other species look up at the stars and use them to find their way, just as we do. It is a poignant reminder that we share the world with them, and that the sparks of light that shine in our night, shine in theirs, too.

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