Not just a tuxedo: African penguins identify mates by their polka dots

African penguins, as well as members of two closely related species, sport individually unique patterns of black dots on their white chest feathers. In a study published last week in Animal Behaviour , researchers have discovered the birds use these dots like name tags to help identify their mates , perhaps to recognize them amid throngs of similar-looking penguins.

“It is an original study with a remarkable finding,” says animal physiologist Andreas Nieder, who directs the Institute of Neurobiology at the University of Tübingen and was not involved in the new research.

Many experts in animal behavior consider the ability to identify individuals within a local group a mark of advanced social awareness; some believe this talent may have contributed to the evolution of cooperation. This type of recognition has long been established in primates, but it can also be seen in a variety of social animals, including insects. Paper wasps recognize the faces of certain other wasps. Some parrots not only distinguish their offspring, but appear to give them the squawking equivalent of individual names. With birds, the talent has appeared to be almost exclusively based on audio rather than visual cues. That makes sense for creatures that hide in trees and bushes, hearing one another more often than catching glimpses. Although some birds, such as Beswick swans, can recognize individuals by sight, it’s not clear—to the human eye—how they tell one another apart.

The new clue to how birds might recognize one another comes from a colony of African penguins ( Spheniscus demersus ) at Zoomarine Italia, a zoo and marine park near Rome. Luigi Baciadonna, a psychologist who did the work while at the University of Turin, was struck by the dots on the animals’ chests. The patterns are unique to each individual, and zookeepers use them to keep track of the animals in their care. Do the birds also do that, Baciadonna wondered?

The team set up an experiment at the marine park, which keeps more than a dozen of the African penguins. As the birds were being fed, Baciadonna and others would gently herd one penguin into a waist-high enclosure. “You have to be quite calm and relaxed,” Baciadonna says. At the far end of the pen were two life-size photographs of other penguins: one depicting enclosed animal’s mate, and another showing a different member of the colony.

The researchers measured how long the penguin in the enclosure looked at either photograph and how much time it spent near one or the other. A good long stare, they hypothesized, indicates the penguin is familiar with the photograph’s subject.

The researchers found that the penguins spent much more time, 23 seconds on average, looking at a photo of their mate compared with another bird, and twice as long standing near their mate’s mugshot. Next, the team showed the penguins two images of their mate, but one had the chest markings digitally removed. The birds spent more time looking at the photo with the familiar dots.

In a third test, the birds were again shown two photographs—one of their mate and another of a different bird from the colony—and this time the dots were removed from both pictures. The birds no longer seemed to recognize their mate: There was no statistically significant difference in the amount of time the birds spent looking at or near the photos.

Recognizing individuals based on their looks is “a trait that we used to think was very special and cognitively difficult,” says Mark Hauber, a comparative psychologist at the City University of New York Graduate Center who studies animal behavior. The new study shows penguins can do it, too. “I love it.”