The amazing canine nose is more impressive than we thought, demonstrating the ability to detect weak infrared radiation from a distance, according to new research.
The rhinarium—the cold, wet, bare tip of a dog’s snout—is capable of sensing weak thermal radiation, according to new research published today in Scientific Reports. The canine nose, therefore, in addition to picking up scents, effectively serves as a thermal radiation sensor. Instead of detecting heat through conduction (direct contact between surfaces) or convection (heat transferred via a medium such as through the air), the nose might be able to directly detect weak infrared radiation released by a warm body or object through photons. This ability would conceivably help the carnivore to detect warm-blooded prey. This capacity was uncovered by a collaborative team from Lund University in Sweden and Eötvös Loránd University in Hungary, who are the first to report this ability in dogs, or any carnivorous mammal for that matter.
Other mammals, such as moles and raccoons, have rhinaria, which they use for tactile sensitivity. This type of sensitivity, however, is reduced in mammals with cold rhinaria, such as dogs, leading scientists to suspect that the canine rhinarium is used for something other than tactile sense.
That said, there is an exception in the animal kingdom: pit vipers, which are sometimes referred to as crotaline snakes. These reptiles have sensitive pit-organs located between each eye and nostril that operate more optimally at low temperatures, allowing the snakes to strike their prey with tremendous accuracy.
Inspired by pit vipers, the researchers, led by Anna Bálint from Lund University, wondered if wolves and other carnivores who hunt large mammals might also have the ability to detect weak infrared signals emanating from their warm prey.
Now, the ability to sense heat remotely may not seem like a big deal. A human holding a hand above a hot stove can most certainly sense heat. The difference here, according to Bálint, has to do with the different mechanisms of heat transfer, of which there are at least three basic types: thermal conduction, thermal convection, and thermal radiation.
In conduction, heat is transferred through the direct contact between two objects, while in convection heat is transferred through a medium, like a fluid or gas. For radiation, heat is transferred via photons as electromagnetic radiation and can even happen in a vacuum, Bálint told Gizmodo in an email.
“So, we humans—and also dogs—have thermoreceptors in our skin, and we can feel heat by all means of heat transfer,” for example, we “can feel the warmth of the Sun on our skin through thermal radiation,” wrote Bálint. “The difference here is that this is very low-intensity thermal radiation—or weak thermal radiation—because the temperature of the mammalian bodies that emit it is not very high, unlike the Sun for instance.” This means an animal has have to have “very sensitive sensors to detect it,” she told Gizmodo.
Bálint and her colleagues don’t yet know how this sense could work in dogs, but they do know that, among very thermal-sensitive animals such as crotaline snakes, the “mechanism of the reception of thermal radiation is not qualitatively different from other types of thermoreception, it’s just simply way more sensitive,” she said.
To test this assumption that carnivores might use their rhinaria to detect weak thermal radiation, the researchers conducted two sets of tests in dogs, who share a recent common evolutionary ancestor with modern wolves.
The first test, held at Lund University, involved three dogs who were trained to detect warmth emanating from an object. This object measured 102 millimetres (4 inches) wide and was warmed to around 11 to 13 degrees Celsius higher than the ambient temperature, which was meant to mimic the body temperature of a furry mammal. A second object, the control, had a neutral temperature equal to the ambient environment.
During the test, both of these objects were placed around 1.6 meters (5.2 feet) away from the dogs, which the researchers described as a reasonable hunting distance. The dogs then had to detect the warmer object, but from this distance. Importantly, the objects were not visually distinguishable—both were covered by the same black electric tape, and the handlers themselves did not not know which of the two objects was the correct one (so they were unable to influence the dogs, even unconsciously).
“All three dogs could detect stimuli of weak thermal radiation in double-blind experiments,” according to the study.
The second phase of the test, held at Eötvös Loránd University, saw 13 dogs placed inside an fMRI scanner. The dogs were exposed to similar objects—one warm and one at ambient temperature—while their brains were scanned. When exposed to the warm object, the left somatosensory cortex in their brains lit up, pointing to an increased neural response to the warmer thermal stimulus.
“Demonstrating that there is a region in the cortex that is more responsive to a warmer object than to an ambient-temperature one complements and lends further support to the behavioural results,” said Bálint.
Taken together, this evidence suggests dogs, and possibly wolves and other carnivores with cold rhinaria, are capable of detecting weak thermal radiation from a distance and that this information might assist with hunting. Bats are the only other mammal with a similar capacity, and they use this skill to find skin areas rich in blood.
The rhinarium was pegged as the organ most likely responsible for the capacity, as no other piece of canine anatomy, such as the eyes, were deemed capable of the feat, according to the researchers.
“There are only two naked pieces of skin in the dog face that may receive heat radiation: the eyes and the rhinarium,” wrote Ronald Kröger, a co-author of the study and a researcher at Lund University, in an email to Gizmodo. “The eyes are not suitable for receiving infrared radiation, because the sensitive structures are hidden behind a thick layer of tissue.”
Importantly, the researchers didn’t perform a test to see if this newly discovered ability actually helps dogs find prey. The dogs were able to detect the warm objects at a distance of 1.6 meters, which seems a relatively close distance. How this ability might help in a real-world hunting scenario remains an unanswered question.
At the same time, no cellular or molecular mechanism was identified in the new research that could be attributed to this capacity, nor did the researchers measure the exact wavelengths that dogs are sensitive to. That said, Kröger suspects special cells in the rhinarium are sensitive to infrared photons, most likely at wavelengths less than 7 micrometres.
“The exact mechanism of the thermoreception is as yet unclear,” Bálint told Gizmodo. “The structure of the dog rhinarium is different from known infrared-sensitive organs, such as the infrared-sensitive pit organs of crotaline snakes, so it may be that the underlying cellular-molecular mechanisms are different.”
Future research should now focus on these gaps, and also consider other mammals. Looking ahead, Bálint said her team would like to determine the limits and threshold of the sense, investigate the molecular details of the process, and test the abilities of other species.
Indeed, that dogs might sense weak thermal radiation is a fascinating revelation, one certainly deserving of further study. The next time your dog pokes you with her uncomfortably frigid, wet nose, perhaps you can better appreciate the physics at work.