Rhinarium

[4] It also acts as a wind-direction detector: cold receptors in the skin of the rhinarium detect the orientation where evaporative cooling is highest, as determined by the wind direction.

Arguments supporting the former position consider the rhinarium "an outward extension of the olfactory ... skin that covers the nasal passages, [which] contains nerve receptors for smell and touch.

"[2] If that interpretation is correct, and the rhinarium is an extension of the olfactory epithelium lining the nasal passages, then it derives from the main system.

[7] But one opposing view traces a path from the philtrum over a notch in the upper lip, through a gap between the first incisors and premaxillae, along a "midline palatal groove" to "a canal that connects with the duct of the vomeronasal organ," suggesting that the rhinarium belongs to the accessory system.

[5] Mammals with rhinaria tend to have a more acute sense of smell, and the loss of the rhinarium in the haplorrhine primates is related to their decreased reliance on olfaction, being associated with other derived characteristics such as a reduced number of turbinates.

In mammals that dig or root with their noses, the rhinarium often develops into a resilient pad, with the nostrils off to the side (or below) and capable of closing to keep out dust.

For example, the lack of an obvious rhinarium in Tarsiiformes has been interpreted by some scholars as the consequence of the enormous development of the eyeballs, rather than a loss of relevance of olfaction,[10] but the significance is currently debatable, because there currently is an influential body of opinion favouring inclusion of the tarsiers in the Haplorhini rather than in the Strepsirrhini as had been traditional.