As any dog owner knows, dogs pay close attention to each other's growls—and with good reason. A new study reveals that dogs can tell another canine's size simply by listening to its growl. The size information is so accurate that a dog hearing a growl can match the sound to a photograph of the growler—a complex cognitive talent previously seen only in primates.
In a previous study, Péter Pongrácz, an ethologist at Eötvös Loránd University in Hungary, and colleagues showed that dogs use a specific growl ("this bone is mine") when guarding a tasty bone. The growl always causes a listening dog to stop in its tracks. In their new study, the scientists tested the responses of dogs seated next to their owners, so that the animals felt comfortable in the lab. Twenty-four of the 96 dogs were shown images of two dogs projected onto a screen in front of them (see picture). One image showed a small dog less than 52 centimeters tall; the other image was of the same dog but projected as being taller than 60 cm (a 30% increase in size). The other dogs were shown control images, either of large and small triangles or of the silhouettes of large and small cats.
The researchers then played recorded food-guarding growls—from either a large or a small dog—on a speaker placed between the two projected images. The scientists filmed the dogs, recording where the canines looked as they listened to the growls (see video).
Dogs that listened to the growls when either images of cats or dogs were shown spent more time looking at the pictures than did dogs looking at the triangles, the team reports online today in PLoS ONE. And 20 of the 24 dogs that were shown two dog images while listening to the growl matched the sound and photograph by looking at the correct image first and for a longer period of time. Dogs that were shown a cat's image while listening to the growl generally looked to the left—a reaction that supports other studies suggesting that dogs look to the left when they encounter something new and unexpected.
The findings mean that "when growling, dogs don't lie about their size," says Pongrácz. "So a listening dog can find out exactly the other dog's size"—and then decide whether to fight or step away. "It's the first time," he adds, "that scientists have shown that a listening animal can determine the size of another animal via its call."
"It's a great study," says Marc Bekoff, a cognitive ethologist at the University of Colorado, Boulder, "and a clear demonstration that dogs have mental representations" of their world. "It just makes so much sense and demonstrates what animals in the wild have to do daily. They have to have some noninteractive way of assessing the size of a rival, especially if they're likely to be beaten up, or lose their territory or their mates."
The study is also likely to spark other research projects using dogs' ability to match sounds to photographs, says Brian Hare, an animal cognition researcher at Duke University in Durham, North Carolina. "If dogs have expectations regarding the size of a dog based on its growl, does that mean that they do something similar with humans?"