Interdigital webbing refers to the presence of skin membranes. Normally, in

mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...

s, webbing is present but resorbed later in development, but in various mammal species, it occasionally persists in adulthood. In humans, it can be found in those suffering from

LEOPARD syndrome Noonan syndrome with multiple lentigines (NSML) which is part of a group called Ras/MAPK pathway syndromes, is a rare autosomal dominant, multisystem disease caused by a mutation in the protein tyrosine phosphatase, non-receptor type 11 gene (''P ...

and from

Aarskog–Scott syndrome Aarskog–Scott syndrome (AAS) is a rare disease inherited as X-linked and characterized by short stature, facial abnormalities, skeletal and genital anomalies. This condition mainly affects males, although females may have mild features of the sy ...

. Webbing between the digits of the hindfoot is also present in several mammals that spend part of their time in the water.Voss, 1988, p. 455 Webbing accommodates movement in the water.Voss, 1988, p. 458 Interdigital webbing is not to be confused with

syndactyly Syndactyly is a condition wherein two or more digits are fused together. It occurs normally in some mammals, but is an unusual condition in humans. The term is . Classification Syndactyly can be simple or complex. * In simple syndactyly, adja ...

, which is a fusing of digits and occurs rarely in humans. Syndactyly specifically affecting feet occurs in birds (such as

duck Duck is the common name for numerous species of waterfowl in the family (biology), family Anatidae. Ducks are generally smaller and shorter-necked than swans and goose, geese, which are members of the same family. Divided among several subfam ...

s), amphibians (such as

frog A frog is any member of a diverse and largely semiaquatic group of short-bodied, tailless amphibian vertebrates composing the order (biology), order Anura (coming from the Ancient Greek , literally 'without tail'). Frog species with rough ski ...

s), and mammals (such as the

kangaroo Kangaroos are marsupials from the family Macropodidae (macropods, meaning "large foot"). In common use, the term is used to describe the largest species from this family, the red kangaroo, as well as the antilopine kangaroo, eastern gre ...

Mammals with interdigital webbing

Rodents

oryzomyines Oryzomyini is a tribe (taxonomy), tribe of rodents in the subfamily Sigmodontinae of the family Cricetidae. It includes about 120 species in about thirty genera,Weksler et al., 2006, table 1 distributed from the eastern List of mammals of the Uni ...

, a mainly South American

rodent Rodents (from Latin , 'to gnaw') are mammals of the Order (biology), order Rodentia ( ), which are characterized by a single pair of continuously growing incisors in each of the upper and Mandible, lower jaws. About 40% of all mammal specie ...

group, the

marsh rice rat The marsh rice rat (''Oryzomys palustris'') is a semiaquatic North American rodent in the family Cricetidae. It usually occurs in wetland habitats, such as swamps and salt marshes. It is found mostly in the eastern and southern United States, f ...

, '' Pseudoryzomys simplex'', and ''

Sigmodontomys alfari ''Sigmodontomys alfari'', also known as the short-tailed sigmodontomys, Alfaro's rice water rat, Cana rice rat, or Allen's rice rat,Jones and Engstrom, 1986 is a species of rodent in the subfamily Sigmodontinae of family Cricetidae. It is found f ...

'' all have small webs, which do not extend to the end of the

proximal phalanges The phalanges (: phalanx ) are digital bones in the hands and feet of most vertebrates. In primates, the thumbs and big toes have two phalanges while the other digits have three phalanges. The phalanges are classed as long bones. Struct ...

, whereas ''

Amphinectomys savamis ''Amphinectomys savamis'', also known as the Ucayali water ratMusser and Carleton, 2005 or amphibious rat,Duff and Lawson, 2004 is a rodent from the Peruvian Amazon. It is placed as the only member of genus ''Amphinectomys'' in the tribe Oryzomyi ...

'', ''

Lundomys molitor ''Lundomys molitor'', also known as Lund's amphibious ratMusser and Carleton, 2005, p. 1124 or the greater marsh rat, is a semiaquatic rat species from southeastern South America. Its distribution is now restricted to Uruguay and nearby Ri ...

'' and the members of the genera ''

Holochilus ''Holochilus'' is a genus of semiaquatic rodents in the tribe Oryzomyini of family Cricetidae,* sometimes called marsh rats. It contains five living species, '' H. brasiliensis'', '' H. chacarius'', '' H. nanus'', '' H. oxe'', and '' H. sciureus ...

'' and ''

Nectomys ''Nectomys'' is a genus of rodent in the tribe Oryzomyini of family Cricetidae. Musser and Carleton, 2005. It is closely related to '' Amphinectomys'' and was formerly considered congeneric with '' Sigmodontomys''. It consists of five species, wh ...

'' have more expansive webbing, which extends beyond the proximal phalanges.Weksler, 2006, p. 25 Webbing apparently developed several times in oryzomyines and may also have been lost in some groups.Weksler, 2006, p. 79 Most

ichthyomyines Ichthyomyini is a tribe of New World rats and mice in the subfamily Sigmodontinae. The species within this tribe share the characteristics of all being carnivorous semiaquatic rodents. The following genera are recognized in this tribe: * Subtribe ...

, an exclusively semiaquatic South and Central American rodent group, have small webbing, but members of the genus ''

Rheomys ''Rheomys'' is a genus of Mexican and Central American semiaquatic rodents in the family Cricetidae The Cricetidae are a family of rodents in the large and complex superfamily Muroidea. It includes true hamsters, voles, lemmings, muskrat ...

'' have more expansive webs. Webbing is also present in the Australasian semiaquatic hydromyines (subfamily

Murinae The Old World rats and mice, part of the subfamily Murinae in the family Muridae, comprise at least 519 species. Members of this subfamily are called murines. In terms of species richness, this subfamily is larger than all mammal families excep ...

) of the genera ''

Baiyankamys '' Baiyankamys'' is a genus of amphibious murid rodents. It was originally described, along with the species '' Baiyankamys shawmayeri'' by Hinton in 1943 after he found the remains of a single individual in south east of the Bismarck Mountai ...

'', ''

Hydromys ''Hydromys'' is a genus (biology), genus of semiaquatic rodents in the subfamily Murinae. Three species are endemism, endemic to New Guinea and nearby islands. The fourth species, the rakali, is also found on Australia. The most recently discovere ...

'', and '' Crossomys''; in the latter, it is most well-developed. The

Africa Africa is the world's second-largest and second-most populous continent after Asia. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 20% of Earth's land area and 6% of its total surfac ...

n semiaquatic rodents '' Colomys goslingi'' and ''

Nilopegamys plumbeus The Ethiopian amphibious rat, also known as the Ethiopian water mouse (''Nilopegamys plumbeus''), is an insectivorous and semiaquatic species of rodent in the monotypic genus ''Nilopegamys'' of the family Muridae. There has only been one known sp ...

'', also members of the

, lack interdigital webbing. Webbing is present in the hind feet of the

coypu The nutria () or coypu () (''Myocastor coypus'') is a herbivore, herbivorous, semiaquatic rodent from South America. Classified for a long time as the only member of the family Myocastoridae, ''Myocastor'' has since been included within Echimy ...

(''Myocastor coypus'') of South America, which is currently classified in its own family.

Soricomorphs

Among

shrew Shrews ( family Soricidae) are small mole-like mammals classified in the order Eulipotyphla. True shrews are not to be confused with treeshrews, otter shrews, elephant shrews, West Indies shrews, or marsupial shrews, which belong to dif ...

s, the members of the genera ''

Chimarrogale The Asiatic water shrews are the members of the genus ''Chimarrogale''. They are mammals in the subfamily Soricinae of the family Soricidae. They are aquatic, with some species inhabiting streams. The genus contains the following species: * Mala ...

'' of southeastern Asia and ''

Neomys The genus ''Neomys'' is a group of four Eurasian water shrews from the subfamily Soricinae of the family Soricidae. These shrews are found in most of Europe and parts of northern Asia, as well as Turkey and Iran Iran, officially the ...

'' of western Eurasia have interdigital webbing, as does the American water shrew (''Sorex palustris'') of North America, but it is more well-developed in '' Nectogale elegans'' of montane Asia. Webbing is also present in the Pyrenean desman (''Galemys pyrenaicus'').

Tenrecs

The

tenrec A tenrec () is a mammal belonging to any species within the afrotherian family Tenrecidae, which is endemic to Madagascar. Tenrecs are a very diverse group, as a result of adaptive radiation, and exhibit convergent evolution, some resemble hed ...

family, which occurs in Africa and mainly on

Madagascar Madagascar, officially the Republic of Madagascar, is an island country that includes the island of Madagascar and numerous smaller peripheral islands. Lying off the southeastern coast of Africa, it is the world's List of islands by area, f ...

, includes several semiaquatic forms, and the small otter-shrews ('' Micropotamogale'') and the aptly named

web-footed tenrec The web-footed tenrec, Malagasy otter shrew, or aquatic tenrec (''Microgale mergulus'') is the only known semiaquatic tenrec (the related African otter shrews have similar habits), and is found in eastern Madagascar, especially in and around Rano ...

(''Limnogale mergulus'') have developed interdigital webbing.

Opossums

The

water opossum The water opossum (''Chironectes minimus''), also locally known as the yapok (), is a marsupial of the family Didelphidae.* It is the only monotypic species of its genus, ''Chironectes''. This semiaquatic creature is found in and near freshwat ...

(''Chironectes minimus'') of South America is the only

opossum Opossums () are members of the marsupial order Didelphimorphia () endemic to the Americas. The largest order of marsupials in the Western Hemisphere, it comprises 126 species in 18 genera. Opossums originated in South America and entered North A ...

with interdigital webbing.

Carnivorans

Several

semiaquatic In biology, being semi-aquatic refers to various macroorganisms that live regularly in both aquatic and terrestrial environments. When referring to animals, the term describes those that actively spend part of their daily time in water (in ...

carnivora Carnivora ( ) is an order of placental mammals specialized primarily in eating flesh, whose members are formally referred to as carnivorans. The order Carnivora is the sixth largest order of mammals, comprising at least 279 species. Carnivor ...

ns have interdigital webbing, including the

greater grison The greater grison (''Galictis vittata'') is a species of mustelid native to Southern Mexico, Central America, and South America. Description The greater grison is a slender animal with short legs, a long neck, and a short, bushy tail. They are ...

(''Galictis vittata''), the

Colombian weasel The Colombian weasel (''Neogale felipei''), also known as Don Felipe's weasel, is a very rare species of New World weasel only known with certainty from the departments of Huila and Cauca in Colombia and nearby northern Ecuador (where it is on ...

(''Neogale felipei''), the

Amazon weasel The Amazon weasel (''Neogale africana''), also known as the tropical weasel, is a species of New World weasel native to South America. It was first identified from a museum specimen mislabelled as coming from Africa, hence the scientific name. ...

(''Neogale africana''), and the

American mink The American mink (''Neogale vison'') is a semiaquatic species of Mustelidae, mustelid native to North America, though human introduction has expanded its range to many parts of Europe, Asia, and South America. Because of range expansion, the Am ...

(''Neogale vison''). All

otter Otters are carnivorous mammals in the subfamily Lutrinae. The 13 extant otter species are all semiaquatic, aquatic, or marine. Lutrinae is a branch of the Mustelidae family, which includes weasels, badgers, mink, and wolverines, among ...

s have interdigital webbing, in the fore or hind limbs or both, to aid in aquatic propulsion. In sea otters, the webbing is covered with hair, at a density of 3300 hairs per square centimeter.

Whales

Pits present on the sides of fossil proximal phalanges of

pakicetid Pakicetidae ("Pakistani whales") is an extinct family of early whales that lived during the Early Eocene in Pakistan. Unlike modern cetaceans, they had well-developed limbs and were capable of walking. The species included were fox to wolf-sized. ...

s, ancestral

whale Whales are a widely distributed and diverse group of fully Aquatic animal, aquatic placental mammal, placental marine mammals. As an informal and Colloquialism, colloquial grouping, they correspond to large members of the infraorder Cetacea ...

s, suggest that these animals had interdigital webbing, a development hypothesized to lead to the fluke, spurred by

FGF8 Fibroblast growth factor 8 (FGF-8) is a protein that in humans is encoded by the ''FGF8'' gene. Function The protein encoded by this gene belongs to the fibroblast growth factor (FGF) family. FGF proteins are multifunctional signaling molecul ...

, a

fibroblast growth factor Fibroblast growth factors (FGF) are a family of cell signalling proteins produced by the macrophages. They are involved in a wide variety of processes, most notably as crucial elements for normal development in animal cells. Any irregularities in ...

.Cooper and Thewissen, 2009

Citations

Literature cited

* Braun, J.K. and Díaz, M.M. 1999. Key to the native mammals of Catamarca Province, Argentina. Occasional papers of the Oklahoma Museum of Natural History 4:1–16. * Cooper, L.N., and J.G.M. Thewissen. 2009 The role of FGF-8 in the origin of interdigital webbing in cetaceans. Presentation, Society of Integrative and Comparative Biology, Boston, Massachusetts. * Fish, Frank E
Biomechanical Perspective on the Origin of Cetacean Flukes
J. G. M. Thewissen, ed. The emergence of whales: evolutionary patterns in the origin of Cetacea. Springer, 1998. . 303-24. * * * * * * Rumbaugh, D.M. and Chiarelli, A.B. 1972. Evolution, ecology, behavior, and captive maintenance. S. Karger, 263 pp. * Tate, G.H.H. 1951
The rodents of Australia and New Guinea
Bulletin of the American Museum of Natural History 97:187–430. * Voss, R.S. 1988
Systematics and ecology of ichthyomyine rodents (Muroidea) : patterns of morphological evolution in a small adaptive radiation
''Bulletin of the American Museum of Natural History'' 188:260–493. * Voss, R.S. and Jansa, S.A. 2009
Phylogenetic relationships and classification of didelphid marsupials, an extant radiation of New World metatherian mammals
''Bulletin of the American Museum of Natural History'' 322:1–177. * Weksler, M. 2006
Phylogenetic relationships of oryzomyine rodents (Muroidea: Sigmodontinae): separate and combined analyses of morphological and molecular data
''Bulletin of the American Museum of Natural History'' 296:1–149. * {{cite journal, author1=Yensen, E. , author2=Tarifa, T. , year=2003, title=Galictis vittata, journal=Mammalian Species , volume=727, pages=1–8, url=http://www.science.smith.edu/departments/Biology/VHAYSSEN/msi/pdf/727_Galictis_vittata.pdf, archive-url=https://web.archive.org/web/20060830153937/http://www.science.smith.edu/departments/Biology/VHAYSSEN/msi/pdf/727_Galictis_vittata.pdf, url-status=dead, archive-date=2006-08-30, doi=10.1644/727, s2cid=198121748 Mammal anatomy