The dog rhinarium (naked and often moist skin on the nose-tip) is prominent and richly innervated, suggesting a sensory function. Compared to nose-tips of herbivorous artio- and perissodactyla, carnivoran rhinaria are considerably colder. We hypothesized that this coldness makes the dog rhinarium particularly sensitive to radiating heat. We trained three dogs to distinguish between two distant objects based on radiating heat; the neutralobject was about ambient temperature, the warmobject was about the same surface temperature as a furry mammal. In addition, we employed functional magnetic resonance imaging on 13 awake dogs, comparing the responses to heat stimuli of about the same temperatures as in the behavioural experiment. The warmstimulus elicited increased neural response in the left somatosensory association cortex. Our results demonstrate a hitherto undiscovered sensory modality in a carnivoran species.
A conspicuous feature of most mammals is the glabrous skin on the nose-tip around the nostrils, called a rhinarium. In moles (Talpidae) in general and in the star-nosed mole (Condylura cristata) in particular, the rhinarium has exquisite tactile sensitivity, mediated by a special sensory structure in the skin, Eimer’s organ. In the raccoon (Procyon lotor) and the coati (Nasua nasua), two carnivoran species with well-developed rhinaria without Eimer’s organs, activity was elicited in the trigeminal ganglion by stimulation of the rhinarium skin with various non-chemical stimulus modalities. The authors concluded that the rhinaria of the studied species seem to have a primary function other than gathering tactile information. Curiously, the temperature of the carnivoran rhinarium in awake animals is considerably lower than in other mammalian groups. In alert dogs (Canis familiaris), the temperature of the rhinarium follows a characteristic pattern. At 30 °C, it is about 5 °C colder than ambient temperature, about equal at 15 °C, and about 8 °C at 0 °C ambient temperature. Rich innervation by the trigeminus nerve asuggests a sensory function.
A role of the wet rhinarium in thermoregulation is unlikely, because its surface area is too small in relation to body size. Furthermore, if a dog is exposed to moderate heat stress and starts to pant, it extends the tongue from the open mouth (Fig. 1). The tongue is wet and warm, despite the airflow generated by panting, and is thus effectively dissipating surplus body heat by radiation and evaporation. The rhinarium, however, remains cold (Fig. 1) and is therefore ineffective.
Thermograph of a dog in the shade at 27 °C ambient temperature. The colour scale on the right is in °C and can be used to read out approximate temperatures. Note the warm tongue and the cold rhinarium (hairless nose tip). Scale bar: 50 mm.
Low tissue temperature affects metabolic functions in general and sensory sensitivity in particular, with one known exception: crotaline snakes cool their infrared-sensitive pit-organs by respiratory evaporation of water and strike more accurately with colder pit-organs. Furthermore, colder snakes are more sensitive to thermal radiation.
Detection of thermal radiation is challenging because of the low energy contents of long-wavelength photons. Ferrets (Mustela putorius furo) are able to see electromagnetic radiation of up to almost 1 µm in wavelength (near-infrared, NIR). A photon of thermal radiation in the middle-infrared (MIR, 3–5 µm) and far-infrared (FIR, >7 µm) bands cannot excite a photoreceptor because it has too little energy to isomerize a photopigment. For biological detectors of thermal radiation, the only option is to detect the warming of the tissue by the absorption of many long-wavelength photons.
The sensory membranes of snake pit-organs react to temperature changes as small as 0.001 degrees, possibly even smaller. It is still unclear how the snakes transduce such tiny temperature differences to useful nervous signals because the molecular mechanism suggested cannot account for the performance of the snakes at temperatures below 25 °C.
Crotaline snakes can strike at prey guided exclusively by the thermal radiation emanating from a warm body. The closest wild relative of domestic dogs, the grey wolf (Canis lupus), preys predominantly on large endothermic prey and the ability to detect the radiation from warm bodies would be advantageous for such predators.
With this in mind, we designed two complementary series of experiments to test whether dogs can sense thermal radiation. We trained dogs on weak signals of thermal radiation at Lund University, Sweden, in a two-alternative forced-choice paradigm. At the Eötvös Loránd University in Budapest, Hungary, we performed functional magnetic resonance imaging (fMRI) experiments on awake dogs to elucidate where in the brain activity occurs if the animals detect a source of weak thermal radiation.
Results - Behavioural experiment
All three dogs could detect stimuli of weak thermal radiation in double-blind experiments
Data: 28 febbraio 2020
Authors: A. Bálint, A. Andics, M. Gácsi, A. Gábor, K. Czeibert, C.M. Luce, A. Miklósi & R.H.H. Kröger.