The rhinarium ( New Latin, "belonging to the nose"; plural: rhinaria) is the furless skin surface surrounding the external openings of the nostrils in many

mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...

s. Commonly it is referred to as the tip of the ''

snout A snout is the protruding portion of an animal's face, consisting of its nose, mouth, and jaw. In many animals, the structure is called a muzzle, rostrum, or proboscis. The wet furless surface around the nostrils of the nose of many mammals is c ...

'', and breeders of cats and dogs sometimes use the term ''nose leather''. Informally, it may be called a "truffle", "wet snout," or "wet nose” because its surface is moist in some

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...

: for example, healthy

dog The dog (''Canis familiaris'' or ''Canis lupus familiaris'') is a domesticated descendant of the wolf. Also called the domestic dog, it is derived from the extinct Pleistocene wolf, and the modern wolf is the dog's nearest living relative. Do ...

s and cats. In many species, the rhinarium has a mid-line groove (cleft)the

philtrum The philtrum ( la, philtrum from Ancient Greek ''phíltron,'' lit. "love charm"), or medial cleft, is a vertical indentation in the middle area of the upper lip, common to therian mammals, extending in humans from the nasal septum to the tubercl ...

and a wrinkled (

crenellate A battlement in defensive architecture, such as that of city walls or castles, comprises a parapet (i.e., a defensive low wall between chest-height and head-height), in which gaps or indentations, which are often rectangular, occur at interva ...

d) surface. The rhinarium is a separate sense organ: it is a touch-based chemosensory organ that connects with a well-developed

vomeronasal organ The vomeronasal organ (VNO), or Jacobson's organ, is the paired auxiliary olfactory (smell) sense organ located in the soft tissue of the nasal septum, in the nasal cavity just above the roof of the mouth (the hard palate) in various tetrapods. ...

(VNO). The rhinarium is used to touch a scent-marked object containing pheromones (usually large, non-volatile molecules), and transfer these pheromone molecules down the philtrum to the VNO via the nasopalatine ducts that travel through the incisive foramen of the

hard palate The hard palate is a thin horizontal bony plate made up of two bones of the facial skeleton, located in the roof of the mouth. The bones are the palatine process of the maxilla and the horizontal plate of palatine bone. The hard palate spans ...

. 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. The study of the rhinarium's structure and associated functions has proven of considerable importance in the fields of mammalian evolution and taxonomy.Basbaum, Allan I. Kaneko, Akimichi, Shepherd, Gordon G. Westheimer, Gerald (editors). The Senses: A Comprehensive Reference. Academic press 2007. For example,

primate Primates are a diverse order of mammals. They are divided into the strepsirrhines, which include the lemurs, galagos, and lorisids, and the haplorhines, which include the tarsiers and the simians (monkeys and apes, the latter including huma ...

s are

phylogenetically In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...

divided into those, such as lemurs, with the primitive rhinarium (

Strepsirrhini Strepsirrhini or Strepsirhini (; ) is a Order (biology), suborder of primates that includes the Lemuriformes, lemuriform primates, which consist of the lemurs of Fauna of Madagascar, Madagascar, galagos ("bushbabies") and pottos from Fauna of A ...

) and the dry-nosed primates (

Haplorhini Haplorhini (), the haplorhines (Greek for "simple-nosed") or the "dry-nosed" primates, is a suborder of primates containing the tarsiers and the simians (Simiiformes or anthropoids), as sister of the Strepsirrhini ("moist-nosed"). The name is some ...

, including apes and humans). In an analogous wayentirely unrelated to vertebrate morphologythe term rhinarium is sometimes 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.

Morphology

Derivation

Morphologically, the rhinarium belongs to the olfactory system, but which part of the system it derives from is open to debate. One possibility is the

main olfactory system The olfactory system, or sense of smell, is the sensory nervous system, sensory system used for smelling (olfaction). Olfaction is one of the special senses, that have directly associated specific organs. Most mammals and reptiles have a main ol ...

, which captures media-borne odors; another is the "second nose," the

accessory olfactory system The sense of smell, or olfaction, is the special sense through which smells (or odors) are perceived. The sense of smell has many functions, including detecting desirable foods, hazards, and pheromones, and plays a role in taste. In humans, it ...

, which samples chemicals dissolved in fluids. Arguments supporting the former position consider the rhinarium "an outward extension of the olfactory ... skin that covers the nasal passages, hichcontains nerve receptors for smell and touch." 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. 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

premaxilla The premaxilla (or praemaxilla) is one of a pair of small cranial bones at the very tip of the upper jaw of many animals, usually, but not always, bearing teeth. In humans, they are fused with the maxilla. The "premaxilla" of therian mammal has b ...

e, along a "midline palatal groove" to "a canal that connects with the duct of the

," suggesting that the rhinarium belongs to the accessory system. It is unclear if moisture (

mucus Mucus ( ) is a slippery aqueous secretion produced by, and covering, mucous membranes. It is typically produced from cells found in mucous glands, although it may also originate from mixed glands, which contain both serous and mucous cells. It is ...

) there functions to trap odiferous molecules — or is the remnant of a fluid transmission system for pheromonal molecules.

Crenellations

Typically, the rhinarium is crenellated (wrinkled, crackled, or embossed), which may, in theory, increase its sensory area, but there are many exceptions and variations among different mammalian taxa, and also variations in the innervation and

sensilla A sensillum (plural ''sensilla'') is an arthropod sensory organ protruding from the cuticle of exoskeleton, or sometimes lying within or beneath it. Sensilla appear as small hairs or pegs over an individual's body. Inside each sensillum there are ...

of the rhinarium, so such generalized speculation should be treated with caution regarding this matter.

Function

Mammals with rhinaria tend to have a more acute sense of smell, and the loss of the rhinarium in the

haplorrhine Haplorhini (), the haplorhines (Greek for "simple-nosed") or the "dry-nosed" primates, is a suborder of primates containing the tarsiers and the simians (Simiiformes or anthropoids), as sister of the Strepsirrhini ("moist-nosed"). The name is so ...

primates is related to their decreased reliance on olfaction, being associated with other derived characteristics such as a reduced number of

turbinate In anatomy, a nasal concha (), plural conchae (), also called a nasal turbinate or turbinal, is a long, narrow, curled shelf of bone that protrudes into the breathing passage of the nose in humans and various animals. The conchae are shaped like ...

s. The rhinarium is also very useful to animals with good sense of smell because of its role 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 comes from. The rhinarium is adapted for different purposes in different mammals, according to

ecological niche In ecology, a niche is the match of a species to a specific environmental condition. Three variants of ecological niche are described by It describes how an organism or population responds to the distribution of resources and competitors (for ...

. In aquatic mammals, the development of lobes beside the nostrils allows them to close 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 to keep out dust. Examples include the

common wombat The common wombat (''Vombatus ursinus''), also known as the coarse-haired wombat or bare-nosed wombat, is a marsupial, one of three extant species of wombats and the only one in the genus ''Vombatus''. The common wombat grows to an average of lo ...

marsupial mole Marsupial moles, the Notoryctidae , are highly specialized marsupial mammals, known from two species found at the Australian interior. * ''Notoryctes typhlops'' (southern marsupial mole, known as the ''itjaritjari'' by the Pitjantjatjara and Yan ...

, and members of the Chrysochloridae. In elephants, the rhinarium has become a tactile organ. And in the walrus, a covering of stiff bristles protects it while the animal forages for shellfish. In many animals, the form and purpose of the rhinarium remains unclear. Evolutionary pressures also are not always unequivocally distinguishable, and there have been upheavals in late 20th and early 21st century taxonomy. For example, the lack of an obvious rhinarium in ''

Tarsiiformes Tarsiiformes are a group of primates that once ranged across Europe, northern Africa, Asia, and North America, but whose extant species are all found in the islands of Southeast Asia. Tarsiers (family Tarsiidae) are the only living members of ...

'' has been interpreted by some scholars as the consequence of the enormous development of the eyeballs, rather than a loss of relevance of

olfaction The sense of smell, or olfaction, is the special sense through which smells (or odors) are perceived. The sense of smell has many functions, including detecting desirable foods, hazards, and pheromones, and plays a role in taste. In humans, it ...

,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. but the significance is currently debatable, because there currently is an influential body of opinion favouring inclusion of the tarsiers in the

rather than in the

as had been traditional.

Phylogeny

Mammalian

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

Primate

Primate Primates are a diverse order of mammals. They are divided into the strepsirrhines, which include the lemurs, galagos, and lorisids, and the haplorhines, which include the tarsiers and the simians (monkeys and apes, the latter including huma ...

s are

divided into those with a rhinarium, the

(the

prosimians Prosimians are a group of primates that includes all living and extinct strepsirrhines (lemurs, lorisoids, and adapiforms), as well as the haplorhine tarsiers and their extinct relatives, the omomyiforms, i.e. all primates excluding the simia ...

: the lorises, and the

lemurs Lemurs ( ) (from Latin ''lemures'' – ghosts or spirits) are wet-nosed primates of the superfamily Lemuroidea (), divided into 8 families and consisting of 15 genera and around 100 existing species. They are endemic to the island of Madagasc ...

); and those without a rhinarium, the

, (the Simians:

monkey Monkey is a common name that may refer to most mammals of the infraorder Simiiformes, also known as the simians. Traditionally, all animals in the group now known as simians are counted as monkeys except the apes, which constitutes an incomple ...

ape Apes (collectively Hominoidea ) are a clade of Old World simians native to sub-Saharan Africa and Southeast Asia (though they were more widespread in Africa, most of Asia, and as well as Europe in prehistory), which together with its siste ...

s, and

human Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, ...

s). In place of the rhinarium, Haplorhini have a more mobile, continuous, dry upper lip.

Use of term in invertebrates

In an analogous usage unrelated to vertebrate morphology, the term rhinarium is sometimes applied to chemosensory structures in invertebrates. For example, microscopic sensilla in the flattened sense organs on the antennae of aphids are referred to as rhinaria.Du Yongjun Yan Fushun Tang Jue. Structure and Function of Olfactory Sensilla on the Antennae of Soybean Aphids, Aphis glycines. ACTA ENTOMOLOGICA SINICA 1995, Vol. 38 Issue (1): 1-

/ref>

References

Bibliography

External links

{{wiktionary, rhinarium
Mammalian Rhinarium Group
- Lund University Mammal anatomy Dog anatomy Felidae anatomy