Dogs and humans have lived alongside one another for approximately 15,000 years. Across that time, the dog has been one of the most carefully observed animals in the world — studied by hunters, herders, biologists, behaviourists, and most recently by neuroscientists with access to MRI machines and genetic sequencing equipment. The basic capabilities of the domestic dog are, by now, supposedly well-understood. Their hearing is roughly four times the range of human hearing. Their sense of smell is approximately 10,000 to 100,000 times more sensitive than ours. Their vision is poorer in colour discrimination but better in low light and motion detection. And, as a team of Swedish and Hungarian researchers reported in 2020, dogs can detect heat radiation with the tip of their nose, in a capability that had gone unnoticed for the entire 15,000 years of dog-human cohabitation, and that is now understood to make them one of only two mammals known to possess this sense.
According to the original paper, published in Scientific Reports by Anna Bálint and colleagues at Lund University in Sweden and Eötvös Loránd University in Hungary on 28 February 2020, the canine rhinarium — the bare, wet, conspicuously cold patch of skin at the tip of a dog’s nose — functions as an infrared sensor capable of detecting weak thermal radiation from warm objects at distances of more than a metre. The discovery emerged from a hypothesis the research team had been developing for several years: that the unusual coldness of the dog rhinarium, which sits roughly five degrees colder than the rest of the dog’s body, was a clue that the organ might have a sensory function beyond simple touch. Cold tissue, paradoxically, is more sensitive to infrared radiation than warm tissue, because the contrast between the tissue and the radiating source is greater when the tissue is cold.
What the experiments showed
The Bálint study used two complementary experimental approaches. According to the Science news coverage of the paper, the Lund University team trained three pet dogs to discriminate between two objects placed 1.6 metres away. One object was at ambient room temperature. The other was warmed to approximately 31 degrees Celsius — the surface temperature of a small furry mammal. The dogs could not see the temperature difference, smell the temperature difference, or feel the temperature difference at 1.6 metres of distance. (The researchers themselves could only detect the difference by touching the surfaces directly.) After training the dogs to choose the warm object for a food reward, the researchers ran double-blind tests in which the dogs were presented with the two objects in random positions, and the experimenters running the trials did not themselves know which object was warm.
All three dogs detected the warm object at well above chance levels. The result, taken alone, would have been suggestive but not definitive — three dogs is a small sample, and the behavioural choice might in principle have been driven by some subtle cue the experimenters had failed to control for. The second arm of the study, conducted at Eötvös Loránd University in Budapest, addressed this through functional magnetic resonance imaging. Thirteen awake pet dogs were placed in an fMRI scanner and exposed to either warm or neutral-temperature objects at varying distances. The brain scans showed that exposure to the warm objects produced a specific pattern of activation in the left somatosensory association cortex — the part of the canine brain that processes sensory information arriving from the skin, including specifically from the rhinarium. Neutral objects produced no such activation. The neural signature of thermal detection, in other words, was present, localised, and consistent with input from the nose-tip rather than from any other body surface.
The vampire bat parallel
The 2020 finding placed dogs in unexpectedly select company. Among mammals, only one other species was previously known to have evolved infrared sensing: the vampire bat, Desmodus rotundus, native to South America. Vampire bats use IR sensing to locate the warmest spots on their prey — typically large mammals or birds — where blood vessels run closest to the skin surface and a small bite will produce maximum blood flow. According to a 2011 Nature paper by Elena Gracheva and colleagues at the University of California, San Francisco, the vampire bat’s IR sensing is mediated by specialised pit organs surrounding the nose, in which a variant of the heat-sensitive ion channel TRPV1 — produced by alternative RNA splicing — has been tuned to detect heat at temperatures as low as approximately 30 degrees Celsius. The mechanism is somewhat different from the dog’s, which appears to use the cold rhinarium itself as the sensor rather than specialised pit organs.
Outside the mammals, infrared sensing has been documented in three lineages of snakes (pit vipers, pythons, and boas), in certain beetles, and in some fish. The snake mechanism, also studied by the Gracheva team and by others, uses a different ion channel — TRPA1, normally not heat-sensitive in most species — that has been evolutionarily co-opted to detect IR radiation in specialised pit organs on the snake’s face. The dog mechanism appears to be a third, independent evolution of the same basic capability, mediated by different molecular machinery and a different anatomical structure, but producing functionally similar results.
What dogs might be using it for
The natural question that follows from the Bálint discovery is what dogs actually use this sense for. The research team has speculated that the most likely answer involves predation. Dogs, like other canids — wolves, foxes, jackals, coyotes — are predominantly predators that evolved to track and hunt warm-blooded prey across varied terrain and in varied conditions of visibility. According to the Lund University press release accompanying the paper, principal investigator Ronald Kröger noted that “the canine heat sense had been overlooked for thousands of years” and proposed that the ability may be widespread among carnivorous mammals — possibly extending to cats, bears, foxes, wolves, and other Carnivora — but had simply never been tested.
For prey animals, having a predator that can detect their body heat across a room, even when vision is obscured or the prey is concealed, would be a substantial selective pressure. Some prey species may have evolved counter-adaptations — fur patterns that mask heat signatures, behaviours that hide warm body parts, denning habits that reduce thermal exposure — that have until now been interpreted purely in terms of visual or olfactory concealment. The discovery suggests that some of these counter-adaptations may have evolved in part to defeat IR detection by canine and feline predators. Predator-prey biology, in other words, may need to be reconsidered with body-heat sensing in mind.
What we still don’t know
The 2020 study is the foundational paper in a new line of research, not a final answer. The sample sizes were small — three dogs in the behavioural arm, 13 in the fMRI arm — and the breeds tested were limited. Whether the sensitivity varies by breed, by age, by individual, or by training is unknown. The maximum distance at which dogs can detect warm objects is also unknown; the experiments tested only at 1.6 metres, and the actual limit of canine IR sensing may be substantially greater. The mechanism by which the rhinarium converts thermal radiation into a neural signal has not yet been worked out at the molecular level, and the relationship between the dog rhinarium mechanism and the vampire bat pit organ mechanism is still being characterised. Whether other carnivorans share the ability — and whether the dog rhinarium evolved this function once, or whether multiple independent canine lineages developed it separately — are open questions that subsequent research is now beginning to address.
What is clear from the existing evidence is that an entirely new sense, with no previous indication in 15,000 years of human-dog cohabitation, was identified in the world’s most-studied domestic animal in 2020. The discovery suggests that the dog beside you on the sofa, dozing through the afternoon, is in fact detecting the heat radiating from your body across the room — registering your presence not just through hearing and smell and (when its eyes are open) sight, but through a sixth sense that places it in the same biological category as a sleeping vampire bat, scanning a dark forest floor for the warmest patch of skin on its next meal. The dog, presumably, has been doing this all along, and we have only now figured out that it can.
