1. Morphology

2. Function

3. Taxonomy
    Mammalian   Primate  

4. Use of term in invertebrates

5. See also

6. References

7. Bibliography

8. External links

The rhinarium (New Latin, "belonging to the nose"; plural: rhinaria)^[1] is the furless skin surface surrounding the external openings of the nostrils in many mammals. Commonly it is referred to as the tip of the snout, and breeders of cats and dogs sometimes use the term nose leather. Informally, it may be called a "wet snout" or "wet nose", because its surface is moist in many species, e.g. healthy dogs and cats.^[2]

In many species the rhinarium has a mid-line groove (cleft) - the philtrum - and the surface is wrinkled (crenellated).^[3] Although the rhinarium is claimed to enhance the sense of smell{{Sfn|Rowe|1996|p=13}}, it is actually a touch-based sense organ that connects with a well-developed vomeronasal organ (VNO). Since pheromones are usually large, non-volatile molecules, the rhinarium is used to touch a scent-marked object and transfer the pheromone molecules down the philtrum to the VNO via the nasopalatine ducts that travel through the incisive foramen of the hard palate.{{Sfn|Ankel-Simons|2007|pp=392–514}}. It also acts as a wind direction detector: the cold receptors in the skin of the rhinarium respond to the location where evaporation is the highest, which is determined by the wind direction.

The study of the structure of the rhinarium and of its associated functions has proved to be of considerable importance in mammalian evolution and taxonomy.^[4]

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.^[5]

Morphology

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.^[7] 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.^[4]

Function

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.^[8]

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.,^[9] 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.^[10]

Taxonomy

Mammalian

The rhinarium is a general mammalian feature and therefore is likely to have been present in the stem mammals.

Primate

Primates are phylogenetically divided into Strepsirrhini, and the Haplorhini. The Strepsirrhini, which means "twisted-nosed", are primates with rhinaria, which is the ancestral condition. The Strepsirrhini consists of the prosimians: the lorises, and the lemurs. The Haplorhini, which means "simple-nosed", are primates which have replaced the rhinarium with a more mobile, continuous, dry upper lip. The Haplorhini consists of the Simians: the monkeys, the apes, and humans.

Prior to the 1970s, taxonomy ignored the fact that tarsiers have no rhinarium when placing them in the Strepsirrhini. However, since then the tarsiers have been placed in with the Haplorhini, and thus the absence of a rhinarium became diagnostic of the Haplorhini.^[10]

Use of term in invertebrates

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.^[11]

See also

• Nose
• Snout

References

Bibliography

• {{cite book|author=Fleagle, J. G.|year=1988|title=Primate adaptation and evolution|publisher=Academic Press|location=San Diego|ref=CITEREFFleagle1988}}

External links

• Mammalian Rhinarium Group - Lund University

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