nature.com - Anna Bálint,   Attila Andics,   Márta Gácsi,   Anna Gábor,   Kálmán Czeibert,   Chelsey M. Luce,   Ádám Miklósi &  Ronald H. H. Kröger
Abstract

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 neutral object was about ambient temperature, the warm object 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 warm stimulus elicited increased neural response in the left somatosensory association cortex. Our results demonstrate a hitherto undiscovered sensory modality in a carnivoran species.
Introduction
A conspicuous feature of most mammals is the glabrous skin on the nose-tip around the nostrils, called a rhinarium1. 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 organ2. 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 modalities3. 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 groups4. 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 temperature5. Rich innervation by the trigeminus nerve6a suggests 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.
www.nature.com/articles/s41598-020-60439-y