Humans have been navigating by the stars since ancient times, but a small yet diverse group of species also use the night sky to get around. Some recognise the movement of star patterns, while others get their bearings via particularly bright individual stars. A few even plot their course via our galaxy, the Milky Way.
From dung beetles to seals, steering by the stars is a critical skill, as it aids them in migrating, finding food, or searching out mates. Even a creature as small as a dung beetle, with a brain the size of a grain of rice, can gaze up at the starry night and decide where to go. (See five more amazing animal navigators.)
“I find this extremely fascinating,” says Marie Dacke, an expert in animal navigation at Sweden’s Lund University. “Every time I look at [dung beetles], I am really impressed by what they are able to do.”
A stellar compass is often one of many navigational cues, such as visual landmarks, the position of the moon, wind direction, or even Earth’s magnetic field.
Overall, scientists hope to understand how the eyes and brains of such celestial navigators have evolved to respond to visual cues from far beyond our own solar system. Such research could offer new insights into the effects of light pollution on wildlife, and may even improve the design of robots that need to find their way around.
Here are some examples of stellar navigators.
Like many migratory songbirds, indigo buntings of North America fly south for winter—and prefer to do so at night.
In 1967, researchers in Michigan temporarily captured several indigo buntings during their autumn migration. They brought the birds to the Robert T. Longway Planetarium in Flint, Michigan, and placed them in special cages that afforded a view of the starlit dome.
To determine their direction, Indigo buntings observe the rotation of close star patterns around a centre point, such as the North Star.
PHOTOGRAPH BY GEORGE GRALL, NAT GEO IMAGE COLLECTION
During migration season, songbirds orient themselves before takeoff by hopping in the direction they want to travel. As the planetarium sky rotated about the North Star, mimicking the apparent movement of the natural night sky, the birds attempted to hop south, as expected.
However, when the researchers removed constellations within 35 degrees of the North Star, something interesting happened: The birds became disoriented.
The research revealed individual stars don’t matter to the birds as much as seeing the rotation of close star patterns around a centre point. In the wild, this enables them to determine where north is, and then use this information to fly south.
Polynesian sailors have relied on guiding stars, called lodestars, to find their way at sea for thousands of years, but harbour seals may have beat them to it.
The marine mammals, which live along the Atlantic and Pacific coastlines of the Northern Hemisphere, spend a great deal of their time foraging for food at night. In those situations, they lack terrestrial landmarks.
To investigate their potential for steering by the stars, in 2006, German and Danish scientists placed two captive harbour seals from the Marine Science Centre in Germany in a specially constructed floating planetarium.
The team trained the two pinnipeds, named Nick and Malte, to swim in the direction of specific lodestars, and then discovered the animals could identify a single star out of a realistic projection of the Northern Hemisphere night sky, according to a 2008 study.
This suggests seals may use specific lodestars as navigational cues to venture far from shore—the first scientific evidence of a marine mammal orienting by the stars.
If this is true in the wild, it's likely such star-based navigation enables the seals to efficiently search an area for food, notes Dacke.
Seals can see individual stars because, like all vertebrates, their eye is “much like a camera—it has a lens and one opening where light comes in and hits the retina, which is like a film,” says Dacke. This anatomical setup allows in a lot of light, enabling detection of small, relatively dim objects like stars.
Insects, by contrast, cannot see individual stars: Their compound eyes can’t detect fine detail like single points of light. However, a collection of stars as big and dense as the Milky Way would appear to them as a single, luminous streak. For the nocturnal African dung beetle Scarabaeus satyrus, this becomes a useful reference point.
At the first whiff of fresh excrement, these beetles fly in. But competition is fierce, so some will make a small ball of dung and roll it somewhere more secluded as fast as possible. “It’s like take-away from a restaurant, basically,” says Dacke.
A dung beetle of the S. sacer species pushes a dung ball in Uganda. Several dung beetles are known to navigate using celestial bodies.
PHOTOGRAPH BY RONAN DONOVAN, NAT GEO IMAGE COLLECTION
Once again, efficiency is key—beetles travelling in a straight line can traverse the greatest distance away in the least amount of time. (Read more about dung beetles’ incredible navigation skills.)
To accomplish this, S. satyrus usually navigates by the polarised light of the moon. But on moonless nights, the Milky Way provides a backup, according to Dacke’s research. This beetle is the only animal that has been confirmed to navigate by the Milky Way, Dacke says.
“As long as they keep it in the same orientation to their initial position, they will move straight,” she says.
There are likely many more animals that navigate by the stars, Dacke adds. For instance, initial evidence suggests that European robins, yellow underwing moths, and perhaps even cricket frogs can do so.
Lead Image: Harbour seals (pictured, two curious individuals on Alaska's South Sawyer Glacier) are the first marine mammals known to orient by the stars.
PHOTOGRAPH BY MICHAEL MELFORD, NAT GEO IMAGE COLLECTION