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The rhinarium (New Latin, "belonging to the nose"; plural: rhinaria) is the moist, naked surface around the nostrils of the nose in most mammals. In actual scientific usage it is typically called a "wet snout" or "wet nose" from its moist and shiny appearance. The groove in the center of it, which reaches the mouth, is called the philtrum.
Anatomically the rhinarium is certainly part of the olfactory system. Whether it evolved from and is part of the main olfactory system, which captured media-borne odors, or the "second nose", the Accessory Olfactory System, which sampled chemicals in fluid solution more directly, has been debated. Ankel-Simons views the rhinarium as "an outward extension of the olfactory ... skin that covers the nasal passages, [which] contains nerve receptors for smell and touch." If that interpretation is true, and the rhinarium extends the olfactory epithelium, the tissue that lines the nasal passages, then the rhinarium is part of the main system. In an opposing point of view, the philtrum typically traces a path that continues 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," part of the accessory system. Where on the one hand the moisture (mucus) may have trapped odiferous molecules in the medium, on the other hand it may be the remnant of a fluid transmission system for molecules of pheromones. The rhinarium is typically crenellated (wrinkled) to increase its sensory area, but, "contra Ankel-Simons," it has no "olfactory receptors" and there is no clear path to the main system.
Mammals with rhinaria tend to have more acute olfaction, 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. The rhinarium is very useful to animals with good sense of smell because it acts as a wind direction detector. The cold receptors in the skin respond to the place where evaporation is the highest. Thus the detection of a particular smell is associated with the direction it came from.
The rhinarium is adapted for different purposes in different mammals according to ecological niche. In aquatic mammals, development of lobes next to the nostrils allow them to be closed for diving. In mammals that root, the rhinarium often develops into a hard pad, with the nostrils off to the side. In the elephants it has become a tactile organ. In the walrus, it is covered with stiff bristles to protect it during foraging for shellfish. In many animals the form and purpose of the rhinarium remains to be elucidated.
Relation to classification
The rhinarium is a general mammalian feature and therefore is likely to have been present in the proto-mammal stage.
Primates are phylogenetically divided into Strepsirrhini ("curly-nosed" primates with rhinaria, which is the ancestral condition) and Haplorhini ("simple-nosed" primates which have replaced the rhinarium with a more mobile, continuous, dry upper lip).
Note that the traditional paraphyletic "prosimian" division of primates cannot be characterised by the presence of a rhinarium, due to its absence in the tarsiers, and loss of the rhinarium is not a synapomorphy of the simians or anthropoids, but a homoplasy shared with the tarsier outgroup.
- "rhinarium, -arium". Webster's Third New International Dictionary (Unabridged ed.). Encyclopædia Britannica, Inc. 1986.
- Ankel-Simons, Friderun (2000). Primate anatomy: an introduction. San Diego: Academic Press. pp. 349–350.
In most mammals we find a moist and shiny glandular area around the nostrils....
- Aspinall, Victoria; O'Reilly, Melanie (2004). Introduction to veterinary anatomy and physiology. Edinburgh; New York: Butterworth-Heinemann. p. 98.
The chambers and the turbinates are covered by a ciliated mucous epithelium ... These nerve fibers reach the olfactory bulbs of the forebrain ....
- Smith, Timothy; Rossie, James (2006), "Primate olfaction: anatomy and evolution", in Brewer, Warrick; Castle, David; Pantelis, Christos, Olfaction and the Brain, Cambridge; New York: Cambridge University Press, p. 139
- S. Dijkgraaf, D. I. Zandee, Alberti Daniel François Addink, ed. (1978). Vergelijkende dierfysiologie (Comparative animal physiology) (in Dutch) (2nd ed.). Utrecht: Bohn, Scheltema & Holkema. ISBN 9789031303229.
- Fleagle, J. G. (1988). Primate adaptation and evolution. San Diego: Academic Press.
- Mammalian Rhinarium Group - Lund University