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The rhinarium (New Latin, "belonging to the nose"; plural: rhinaria) is the naked skin surface surrounding the external openings of the nostrils in most mammals. In common usage, for example among breeders of cats or dogs, the standard term is nose leather. More informally people may speak of a "wet snout" or "wet nose" because its surface commonly is moist in a healthy cat or dog. Down the middle of the rhinarium in many species there is a philtrum, a groove down the middle.
The study of the structure and of its associated functions has proved to be of considerable importance in mammalian evolution and taxonomy.
In an analogous connection of no relevance to vertebrate morphology, the term rhinarium sometimes is applied to chemosensory structures in invertebrates. For example, microscopic sensilla in the form of flattened sense organs on the antennae of aphids are referred to as rhinaria.
Morphologically the rhinarium certainly is part of the olfactory system, but it still is open to debate which part of the system it derived from. On the one hand it might be part of the main olfactory system, which captures media-borne odors; on the other, it might be part of the "second nose", the Accessory Olfactory System, which samples chemicals dissolved in fluids. An example of the former view is that the rhinarium is "an outward extension of the olfactory ... skin that covers the nasal passages, [which] contains nerve receptors for smell and touch." If that interpretation is correct, and the rhinarium extends the olfactory epithelium that lines the nasal passages, then the rhinarium is part of the main system.
In an opposing view, the philtrum ideally 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.
Typically the rhinarium is crenellated (wrinkled or embossed), which, as a generalisation has been speculated to increase its sensory area, but there are many exceptions and variations among different mammalian taxa, and also variations in its innervation and sensilla, so it is appropriate to treat generalisations with caution in this matter.
Mammals with rhinaria tend to have 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. 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 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 for exclusion of dust. Examples include the common wombat, marsupial mole, and members of the Chrysochloridae. 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.
The evolutionary pressures also are not always clearly distinguishable and there have been upheavals in late 20th and early 21st century taxonomy. For example, the lack of an obvious rhinarium in Tarsiiformes has been suggested to be the consequence of the enormous development of the eyeballs, rather than a loss of relevance of olfaction., 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.
Mammalian taxonomy in general
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 ("twisted-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 tarsiers used to be included in the Strepsirrhini and have no rhinarium, so its presence or absence was not seen as strictly diagnostic of the traditional paraphyletic prosimian/simian division of primates. Accordingly, loss of the rhinarium would not have been a synapomorphy of the simians or anthropoids, but a homoplasy shared with the Tarsiiformes outgroup. However, current taxonomy favours inclusion of the Tarsiiformes in the Haplorhini, in which view the attribute would indeed be diagnostic.
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- 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....
- Lund University Faculty of Science Department of Biology Mammalian Rhinarium Group 
- Basbaum, Allan I. Kaneko, Akimichi, Shepherd, Gordon G. Westheimer, Gerald (editors). The Senses: A Comprehensive Reference. Academic press 2007. ISBN 978-0126394825
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- 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
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- Smith, T. D., & Bhatnagar, K. P. (2004). Microsmatic primates: reconsidering how and when size matters. The Anatomical Record Part B: The New Anatomist, 279(1), 24-31.
- Frederick S. Szalay; Eric Delson (22 October 2013). Evolutionary History of the Primates. Elsevier Science. pp. 189–. ISBN 978-1-4832-8925-0.
- Fleagle, J. G. (1988). Primate adaptation and evolution. San Diego: Academic Press.
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- Mammalian Rhinarium Group - Lund University