Webbed foot
The webbed foot is a specialized limb with interdigital membranes (webbings) that aids in aquatic locomotion, present in a variety of tetrapod vertebrates. This adaptation is primarily found in semiaquatic species, and has convergently evolved many times across vertebrate taxa.
It likely arose from mutations in developmental genes that normally cause tissue between the digits to apoptose. These mutations were beneficial to many semiaquatic animals because the increased surface area from the webbing allowed for more swimming propulsion and swimming efficiency, especially in surface swimmers.[2] The webbed foot also has enabled other novel behaviors like escape responses and mating behaviors. A webbed foot may also be called a paddle to contrast it from a more hydrofoil-like flipper.
Morphology
A webbed foot has connecting tissue between the toes of the foot. Several distinct conditions can give rise to webbed feet, including
Webbed feet are a compromise between aquatic and terrestrial locomotion. Aquatic control surfaces of non-piscine vertebrates may be paddles or hydrofoils. Paddles generate less lift than hydrofoils, and paddling is associated with drag-based control surfaces. The roughly triangular design of webbed feet, with a broad distal end, is specialized to increase propulsive efficiency by affecting a larger mass of water over generating increased lift. This is in contrast to a more hydrofoil-like flipper of many permanently aquatic animals.[7]
Evolution
Development
Webbed feet are the result of mutations in genes that normally cause interdigital tissue between the toes to apoptose.[8] Apoptosis, or programmed cell death, in development is mediated by a variety of pathways, and normally causes the creation of digits by death of tissue separating the digits. Different vertebrate species with webbed feet have different mutations that disrupt this process, indicating that the structure arose independently in these lineages.
In humans, syndactyly can arise from as many as nine unique subtypes with their own clinical, morphological, and genetic fingerprints. In addition, the same genetic mutations can underlie different phenotypic expressions of syndactyly.[10] While these conditions are disorders in humans, the variability in genetic cause of webbed digits informs our[who?] understanding of how this morphological change arose in species where webbed feet were selectively advantageous. These conditions also demonstrate a variety of genetic targets for mutation resulting in webbed feet, which may explain how this homologous structure could have arisen many times over the course of evolutionary history.
One pathway implicated in interdigital necrosis is the
Webbed feet could also arise due to being linked to other morphological changes, without a selective advantage. In salamanders, webbed feet have arisen in multiple lineages, but in most do not contribute to increased function. However, in the cave salamander species Chiropterotriton magnipes (bigfoot splayfoot salamander), their webbed feet are morphologically unique from other salamanders and may serve a functional purpose.[13] This demonstrates that webbed feet arise from developmental changes, but do not necessarily correlate with a selective advantage functionally.
Phylogeny
Webbed feet have arisen in all major vertebrate lineages with limbed animals. Most webbed-footed species spend part of their time in aquatic environments, indicating that this homologous structure provides some advantage to swimmers. Some examples from each class are highlighted here, but this is not a complete listing.
Amphibians
Of the three orders of amphibians, Anura (frogs and toads) and Urodela (salamanders) have representative species with webbed feet. Frogs that live in aquatic environments, like the common frog (Rana temporaria), have webbed feet. Salamanders in arboreal and cave environments also have webbed feet, but in most species, this morphological change does not likely have a functional advantage.[13]
Reptiles
Reptiles have webbed-footed representatives that include freshwater turtles and geckos. While turtles with webbed feet are aquatic, most geckos live in terrestrial and arboreal environments.
Birds
- Palmate: only the anterior digits (2–4) are joined by webbing. Found in skimmers).[15][16] Diving ducks also have a lobed hind toe (1), and gulls, terns and allies have a reduced hind toe.[17]
- Totipalmate: all four digits (1–4) are joined by webbing. Found in gannets and boobies, pelicans, cormorants, anhingas, frigatebirds, and tropicbirds. Some gannets have brightly colored feet used in display.[14][16]
- Semipalmate: a small web between the anterior digits (2–4). Found in some plovers (Eurasian dotterels) and sandpipers (semipalmated sandpipers, stilt sandpipers, upland sandpipers, greater yellowlegs and willet), avocet, herons (only two toes), all grouse, and some domesticated breeds of chicken. Plovers and lapwings have a vestigial hind toe (1), and sandpipers and their allies have a reduced and raised hind toe barely touching the ground. The sanderling is the only sandpiper having 3 toes (tridactyl foot).[14]
- Lobate: the anterior digits (2–4) are edged with lobes of skin. Lobes expand or contract when a bird swims. In grebes, coots, phalaropes, finfoots and some palmate-footed ducks on the hallux (1). Grebes have more webbing between the toes than coots and phalaropes.[15][18][16]
The palmate foot is most common.
Mammals
Some semiaquatic mammals have webbed feet. Most of these have interdigital webbing, as opposed to the syndactyly found in birds. Some notable examples include the platypus, the beaver, the otter, and the water opossum.[19][20][21]
Function
Swimming propulsion
In many species, webbed feet likely evolved to aid in generation of propulsion during swimming. Most webbed-footed animals utilize paddling modes of locomotion where their feet stroke backwards relative to their whole body motion, generating a propulsive force. The interdigital membrane increases the surface area, which increases the propulsive drag the animal can generate with each stroke of its foot.[22][23] This is a drag-based mode of propulsion. However, some waterfowl also utilize lift-based modes of propulsion, where their feet generate hydrodynamic lift due to the angle of attack of the foot and the relative water velocity. For example, great-crested grebes use solely lift-based propulsion due to their lateral foot stroke and asymmetric, lobated toes.[6] Most waterfowl use a combination of these two modes of propulsion, where the first third of their foot stroke generates propulsive drag and the last two-thirds of the stroke generates propulsive lift.[1]
The stroke of the foot through the water also generates vortices that aid propulsion. During the transition from drag-based to lift-based propulsion in ducks, leading edge vortices formed on the front of the foot are shed, which creates a flow of water over the foot that likely aids lift production.[1] Other species also create these vortices during their webbed foot stroke. Frogs also create vortices that shed off their feet when swimming in water. The vortices from the two feet do not interfere with each other; therefore, each foot is generating forward propulsion independently.[24]
Most fully aquatic vertebrates do not use paddling modes of locomotion, instead using undulatory modes of locomotion or flipper locomotion. Fully aquatic mammals and animals typically have flippers instead of webbed feet, which are a more heavily specialized and modified limb.[2] It is hypothesized that an evolutionary transition between semiaquatic and fully aquatic higher vertebrates (especially mammals) involved both the specialization of swimming limbs and the transition to underwater, undulatory modes of motion.[25] However, for semiaquatic animals that mainly swim at the surface, webbed feet are highly functional; they trade-off effectively between efficient terrestrial and aquatic locomotion.[2] In addition, some waterfowl can also use paddling modes for underwater swimming, with added propulsion from flapping their wings. Diving ducks can swim underwater to forage. These ducks expend more than 90% of their energy to overcome their own buoyancy when they dive.[26] They can also achieve higher speeds underwater due to surface speeds being limited to their hull speed; at this speed, the wave drag increases to the point where the duck cannot swim faster.[27]
Other behaviors
In ducks, webbed feet have also enabled extreme forms of propulsion that are used for escape behaviors and courtship display. Surface swimmers are speed-limited due to increasing drag as they approach a physically defined hull speed, which is determined by their body length. In order to achieve speeds higher than hull speed, some ducks, like eider ducks, use distinctive modes of locomotion that involve lifting the body out of the water. They can hydroplane, where they lift part of their body out of the water and paddle with their webbed feet to generate forces that allow them to overcome gravity; they also use paddle-assisted flying, where the whole body is lifted out of the water, and the wings and feet work in concert to generate lift forces.[28] In extreme cases, this type of behavior is used for sexual selection. Western and Clark's grebes utilize their lobated feet to generate nearly 50% of the force required to allow them to walk on water in elaborate sexual displays; they are likely the largest animal to "walk" on water, and are an order of magnitude heavier than the well-known lizards that exhibit a similar behavior.[29]
Terrestrial locomotion
While webbed feet have mainly arisen in swimming species, they can also aid in terrestrial locomotors by increasing contact area on slick or soft surfaces. For P. rangei, the Namib sand gecko, their webbed feet may serve as sand shoes that enable them to move atop sand dunes.[30] However, some ecologists believe that their webbed feet do not aid aboveground locomotion, but are mainly utilized as shovels for burrowing and digging in the sand.[31] In salamanders, most species do not benefit from the increased surface area of their feet. However, some, like the bigfoot splayfoot salamander (Chiropterotriton magnipes) increase their body size to foot surface area ratio enough to provide increased suction. This species lives in cave environments where they often encounter wet, slick surfaces. Therefore, their webbed feet may enable them to move on these surfaces with ease.[13]
See also
- Webbed toes
- Interdigital webbing
- Syndactyly
- Bird feet and legs
- Tradeoffs for locomotion in air and water
References
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- ^ "Webbed Wonders". www.ducks.org. Retrieved 2017-04-17.
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- ^ "Why Don't Ducks' Feet Freeze?". Ask a Naturalist.com. 2010-04-22. Retrieved 2017-04-18.
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- ^ a b c Kochan 1994; Proctor & Lynch 1993; Elphick, Dunning & Sibley 2001
- ^ a b Gill 2001; Kochan 1994; Proctor & Lynch 1993; Elphick, Dunning & Sibley 2001
- ^ a b c Kalbe, Lothar (1983). "Besondere Formen für spezielle Aufgaben der Wassertiere [Special adaptations of aquatic animals to specific lifestyles]". Tierwelt am Wasser [Wildlife by the Water] (in German) (1st ed.). Leipzig-Jena-Berlin: Urania-Verlag. pp. 72–77.
- ^ Kochan 1994; Elphick, Dunning & Sibley 2001
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- ^ "Web-Footed Geckos". National Geographic. Archived from the original on February 7, 2010. Retrieved 2017-04-28.
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Sources
- Elphick, John B.; Dunning, Jack B. Jr.; Sibley, David Allen (2001). National Audubon Society: The Sibley Guide to Bird Life & Behavior. New York: Alfred A. Knopf. ISBN 978-0-679-45123-5.
- Gill, Frank B. (2001). Ornithology (2nd ed.). New York: W.H. Freeman and Company. ISBN 978-0-7167-2415-5.
- Kochan, Jack B. (1994). Feet & Legs. Birds. Mechanicsburg: Stackpole Books. ISBN 978-0-8117-2515-6.
- Proctor, Noble S.; Lynch, Patrick J. (1993). "Chapters: 6. Topography of the foot, 11. The pelvic girdle, and 12. The bones of the leg and foot Family". Manual of Ornithology. Avian Structure & Function. New Haven and London: ISBN 978-0-300-07619-6.