New sense discovered in dog noses: the ability to detect heat

Dogs’ noses just got a bit more amazing. Not only are they up to 100 million times more sensitive than ours, they can sense weak thermal radiation—the body heat of mammalian prey, a new study reveals. The find helps explain how canines with impaired sight, hearing, or smell can still hunt successfully.

“It’s a fascinating discovery,” says Marc Bekoff, an ethologist, expert on canine sniffing, and professor emeritus at the University of Colorado, Boulder, who was not involved in the study. “[It] provides yet another window into the sensory worlds of dogs’ highly evolved cold noses.”

The ability to sense weak, radiating heat is known in only a handful of animals: black fire beetles, certain snakes, and one species of mammal, the common vampire bat, all of which use it to hunt prey.

Most mammals have naked, smooth skin on the tips of their noses around the nostrils, an area called the rhinarium. But dogs’ rhinaria are moist, colder than the ambient temperature, and richly endowed with nerves—all of which suggests an ability to detect not just smell, but heat.

To test the idea, researchers at Lund University and Eötvös Loránd University trained three pet dogs to choose between a warm (31°C) and an ambient-temperature object, each placed 1.6 meters away. The dogs weren’t able to see or smell the difference between these objects. (Scientists could only detect the difference by touching the surfaces.) After training, the dogs were tested on their skill in double-blind experiments; all three successfully detected the objects emitting weak thermal radiation, the scientists reveal today in Scientific Reports .

Next, the researchers scanned the brains of 13 pet dogs of various breeds in a functional magnetic resonance imaging scanner while presenting the pooches with objects emitting neutral or weak thermal radiation. The left somatosensory cortex in dogs’ brains, which delivers inputs from the nose, was more responsive to the warm thermal stimulus than to the neutral one. The scientists identified a cluster of 14 voxels (3D pixels) in this region of the dogs’ left hemispheres, but didn’t find any such clusters in the right, and none in any part of the dogs’ brains in response to the neutral stimulus.

Together, the two experiments show that dogs, like vampire bats, can sense weak hot spots and that a specific region of their brains is activated by this infrared radiation, the scientists say. They suspect dogs inherited the ability from their ancestor, the gray wolf, who may use it to sniff out warm bodies during a hunt.

“The study is consistent with other research that describes the combined dog nose and brain as a sophisticated platform for processing a broad range of signals,” says Gary Settles, an emeritus professor of mechanical engineering at Pennsylvania State University, University Park, who has studied dogs’ sniffing abilities. He doubts, however, “that the dog rhinarium can distinguish patterns of hot and cold objects at a distance,” suggesting dogs’ thermal detection skills may not be useful for long distance hunting. “[T]hat needs further study.”