Flipper (anatomy)

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Humboldt penguin swimming. Penguin wings evolved into short, strong flippers causing flightlessness. Schwimmender-Pinguin.jpg
Humboldt penguin swimming. Penguin wings evolved into short, strong flippers causing flightlessness.
This green turtle is about to break the surface for air at Kona, Hawaii. Chelonia mydas is going for the air.jpg
This green turtle is about to break the surface for air at Kona, Hawaii.

A flipper is a broad, flattened limb adapted for aquatic locomotion. It refers to the fully webbed, swimming appendages of aquatic vertebrates that are not fish.

Contents

In animals with two flippers, such as whales, the flipper refers solely to the forelimbs. In animals with four flippers, such as pinnipeds and sea turtles, one may distinguish fore- and hind-flippers, or pectoral flippers and pelvic flippers. [2] [3]

Animals with flippers include penguins (whose flippers are also called wings), cetaceans (e.g., dolphins and whales), pinnipeds (e.g., walruses, earless and eared seals), sirenians (e.g., manatees and dugongs), and marine reptiles such as the sea turtles and the now-extinct plesiosaurs, mosasaurs, ichthyosaurs, and metriorhynchids.

Usage of the terms "fin" and "flipper" is sometimes inconsistent, even in the scientific literature. However, the hydrodynamic control surfaces of fish are always referred to as "fins" and never "flippers". Tetrapod limbs which have evolved into fin-like structures are usually (but not always) called "flippers" rather than fins. The dorsal structure on cetaceans is called the "dorsal fin" and the large cetacean tails are referred to primarily as flukes but occasionally as "caudal fins"; neither of these structures are flippers.

Some flippers are very efficient hydrofoils, analogous to wings (airfoils), used to propel and maneuver through the water with great speed and maneuverability (see Foil). Swimming appendages with the digits still apparent, as in the webbed forefeet of amphibious turtles and platypus, are considered paddles rather than flippers. [3]

Locomotion

For all species of aquatic vertebrates, swimming performance depends upon the animal's control surfaces, which include flippers, flukes and fins. Flippers are used for different types of propulsion, control, and rotation. In cetaceans, they are primarily used for control while the fluke is used for propulsion. [4]

The evolution of flippers in penguins was at the expense of their flying capabilities, in spite of evolving from an auk-like ancestor that could 'fly' underwater as well in the air. Form constrains function, and the wings of diving flying species, such as the murre or cormorant have not developed into flippers. The flippers of penguins became thicker, denser and smaller while being modified for hydrodynamic properties. [1]

Hydrodynamics

Cetacean flippers may be viewed as being analogous to modern engineered hydrofoils, which have hydrodynamic properties: lift coefficient, drag coefficient and efficiency. Flippers are one of the principal control surfaces of cetaceans (whales, dolphins and porpoises) due to their position in front of the center of mass, and their mobility which provides three degrees of freedom. [4]

The tubercles on the flippers of humpback whales improve the hydrodynamics of the flipper at their size. Breaking up channels of fast-moving water allows humpbacks to retain their "grip" on the water, and turn at sharper angles even at low velocities. Humpback stellwagen.JPG
The tubercles on the flippers of humpback whales improve the hydrodynamics of the flipper at their size. Breaking up channels of fast-moving water allows humpbacks to retain their "grip" on the water, and turn at sharper angles even at low velocities.

Flippers on humpback whales ( Megaptera novaeangliae ) have non-smooth leading edges, yet demonstrate superior fluid dynamics to the characteristically smooth leading edges of artificial wings, turbines and other kinds of blades. The whale's surprising dexterity is due primarily to its non-conventional flippers, which have large, irregular looking bumps called tubercles across their leading edges. The tubercles break up the passage of water, maintaining even channels of the fast-moving water, limiting turbulence and providing greater maneuverability. [5]

The foreflippers used by the pinnipeds act as oscillatory hydrofoils. Both fore and hind flippers are used for turning. [3] A 2007 study of Steller's sea lion found that a majority of thrust was produced during the drive phase of the fore flipper stroke cycle. Although previous findings on eared seals suggested that thrust was generated by the initial outward movement of the fore flippers or the terminal drag-based paddling phase, the 2007 study found that little or no thrust was generated during those phases. Swimming performance in sea lions is modulated by changes in the duration and intensity of movements without changing their sequence. Using criteria based on velocity and the minimum radius of turns, pinnipeds' maneuverability is superior to cetaceans but inferior to many fish. [6]

Evolution of flippers

Marine mammals have evolved several times, developing similar flippers. The forelimbs of cetaceans, pinnipeds, and sirenians presents a classic example of convergent evolution. There is widespread convergence at the gene level. [7] Distinct substitutions in common genes created various aquatic adaptations, most of which constitute parallel evolution because the substitutions in question are not unique to those animals. [8]

When comparing cetaceans to pinnipeds to sirenians, 133 parallel amino acid substitutions occur. Comparing and contrasting cetaceans-pinnipeds, cetaceans-sirenians, and pinnipeds-sirenians, 2,351, 7,684, and 2,579 substitutions occur, respectively. [8]

Digit processes

Close up skeletal of fin whale flipper Flipper eines Finnwales.jpg
Close up skeletal of fin whale flipper

Whales and their relatives have a soft tissue flipper that encases most of the forelimb, and elongated digits with an increased number of phalanges. [9] Hyperphalangy is an increase in the number of phalanges beyond the plesiomorphic mammal condition of three phalanges-per-digit. [10] This trait is characteristic of secondarily aquatic vertebrates with flippers. Hyperphalangy was present among extinct ichthyosaurs, plesiosaurs, and mosasaurs. [11]

Illustration of limbs of aquatic reptiles: the mosasaurs Platecarpus and Clidastes, and the ichthyosaurs Ophthalmosaurus and Platypterygius. From The Osteology of the Reptiles (1925) The Osteology of the Reptiles p193.png
Illustration of limbs of aquatic reptiles: the mosasaurs Platecarpus and Clidastes , and the ichthyosaurs Ophthalmosaurus and Platypterygius. From The Osteology of the Reptiles (1925)

Cetaceans are the sole mammals to have evolved hyperphalangy. Though the flippers of modern cetaceans are not correctly described as webbed feet, the intermediate webbed limbs of ancient semiaquatic cetaceans may be described as such. The presence of interdigital webbing within the fossils of semi-aquatic Eocene cetaceans was probably the result of BMP antagonists counteracting interdigital apoptosis during embryonic limb development. Modifications to signals in these tissues likely contributed to the origin of an early form of hyperphalangy in fully aquatic cetaceans about 35 million years ago. The process continued over time, and a very derived form of hyperphalangy, with six or more phalanges per digit, evolved convergently in rorqual whales and oceanic dolphins, and was likely associated with another wave of signaling within the interdigital tissues. [10]

Although toothed cetaceans have five digits, most baleen whales have four digits and even lack a metacarpal. In the latter (mysticetes), the first digit ray may have been lost as late as 14 million years ago. [9]

Flipper evolution in turtles

Illustration of plesiosaur flippers. Meyerasaurus and Trinacromerum. From The Osteology of the Reptiles (1925) The Osteology of the Reptiles p194.png
Illustration of plesiosaur flippers. Meyerasaurus and Trinacromerum. From The Osteology of the Reptiles (1925)

Sea turtles evolved in the Cretaceous. Their flippers developed gradually by a series of stepwise adaptations, with the most fundamental traits of flippers appearing in the deepest nodes (the earliest times) in their phylogeny. These initial traits evolved only once among chelonioids, and the bauplan was refined through a secondary process of specialization. [12]

Evers et al. identified characters related to the pectoral girdle and forelimb that are related to the modification of sea turtle arms and hands into flippers. [12]

Key biomechanical features of flippers

Fundamental traits for flipper movement

Foraging behavior

Because of the specialization of flippers and their hydrodynamic constraints, it was thought that they were not used to significantly interact with the environment, unlike the legs of terrestrial tetrapods. However, the use of limbs for foraging is documented in marine tetrapods. [13] Use of the flippers for foraging behavior is observed in marine mammals such as walruses, seals, and manatee, and even in reptiles such as sea turtles. Among turtles, observed behaviors include a green turtle holding a jellyfish, a loggerhead rolling a scallop on the sea floor, and a hawksbill turtle pushing against a reef for leverage to rip an anemone loose. [14] Based on presumed limb use in ancestral turtles, these behaviors may have occurred as long ago as 70 million years. [13]

See also

Related Research Articles

<span class="mw-page-title-main">Fin</span> Thin component or appendage attached to a larger body or structure

A fin is a thin component or appendage attached to a larger body or structure. Fins typically function as foils that produce lift or thrust, or provide the ability to steer or stabilize motion while traveling in water, air, or other fluids. Fins are also used to increase surface areas for heat transfer purposes, or simply as ornamentation.

<span class="mw-page-title-main">Ungulate</span> Group of animals that walk on the tips of their toes or hooves

Ungulates are members of the diverse clade Euungulata, which primarily consists of large mammals with hooves. Once part of the clade "Ungulata" along with the clade Paenungulata, "Ungulata" has since been determined to be a polyphyletic and thereby invalid clade based on molecular data. As a result, true ungulates had since been reclassified to the newer clade Euungulata in 2001 within the clade Laurasiatheria while Paenungulata has been reclassified to a distant clade Afrotheria. Living ungulates are divided into two orders: Perissodactyla including equines, rhinoceroses, and tapirs; and Artiodactyla including cattle, antelope, pigs, giraffes, camels, sheep, deer, and hippopotamuses, among others. Cetaceans such as whales, dolphins, and porpoises are also classified as artiodactyls, although they do not have hooves. Most terrestrial ungulates use the hoofed tips of their toes to support their body weight while standing or moving. Two other orders of ungulates, Notoungulata and Litopterna, both native to South America, became extinct at the end of the Pleistocene, around 12,000 years ago.

<span class="mw-page-title-main">Marine mammal</span> Mammals that rely on marine environments for feeding

Marine mammals are mammals that rely on marine (saltwater) ecosystems for their existence. They include animals such as cetaceans, pinnipeds, sirenians, sea otters and polar bears. They are an informal group, unified only by their reliance on marine environments for feeding and survival.

<i>Rodhocetus</i> Genus of mammals

Rodhocetus is an extinct genus of protocetid early whale known from the Lutetian of Pakistan. The best-known protocetid, Rodhocetus is known from two partial skeletons that taken together give a complete image of an Eocene whale that had short limbs with long hands and feet that were probably webbed and a sacrum that was immobile with four partially fused sacral vertebrae. It is one of several extinct whale genera that possess land mammal characteristics, thus demonstrating the evolutionary transition from land to sea.

<span class="mw-page-title-main">Dactyly</span> Arrangement of digits on hands and feet

In biology, dactyly is the arrangement of digits on the hands, feet, or sometimes wings of a tetrapod animal. The term is derived from the Greek word δακτυλος meaning "finger."

<span class="mw-page-title-main">Evolution of cetaceans</span>

The evolution of cetaceans is thought to have begun in the Indian subcontinent from even-toed ungulates (Artiodactyla) 50 million years ago (mya) and to have proceeded over a period of at least 15 million years. Cetaceans are fully aquatic marine mammals belonging to the order Artiodactyla and branched off from other artiodactyls around 50 mya. Cetaceans are thought to have evolved during the Eocene, the second epoch of the present-extending Cenozoic Era. Molecular and morphological analyses suggest Cetacea share a relatively recent closest common ancestor with hippopotami and that they are sister groups. Being mammals, they surface to breathe air; they have five finger bones (even-toed) in their fins; they nurse their young; and, despite their fully aquatic life style, they retain many skeletal features from their terrestrial ancestors. Research conducted in the late 1970s in Pakistan revealed several stages in the transition of cetaceans from land to sea.

<i>Ambulocetus</i> Genus of extinct mammals of the order Cetacea

Ambulocetus is a genus of early amphibious cetacean from the Kuldana Formation in Pakistan, roughly 48 or 47 million years ago during the Early Eocene (Lutetian). It contains one species, Ambulocetus natans, known solely from a near-complete skeleton. Ambulocetus is among the best-studied of Eocene cetaceans, and serves as an instrumental find in the study of cetacean evolution and their transition from land to sea, as it was the first cetacean discovered to preserve a suite of adaptations consistent with an amphibious lifestyle. Ambulocetus is classified in the group Archaeoceti—the ancient forerunners of modern cetaceans whose members span the transition from land to sea—and in the family Ambulocetidae, which includes Himalayacetus and Gandakasia.

<span class="mw-page-title-main">Phalanx bone</span> Digital bone in the hands and feet of most vertebrates

The phalanges 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.

<span class="mw-page-title-main">Forelimb</span> One of the paired articulated appendages attached on the cranial end of a vertebrates torso

A forelimb or front limb is one of the paired articulated appendages (limbs) attached on the cranial (anterior) end of a terrestrial tetrapod vertebrate's torso. With reference to quadrupeds, the term foreleg or front leg is often used instead. In bipedal animals with an upright posture, the term upper limb is often used.

Many vertebrates are limbless, limb-reduced, or apodous, with a body plan consisting of a head and vertebral column, but no adjoining limbs such as legs or fins. Jawless fish are limbless but may have preceded the evolution of vertebrate limbs, whereas numerous reptile and amphibian lineages – and some eels and eel-like fish – independently lost their limbs. Larval amphibians, tadpoles, are also often limbless. No mammals or birds are limbless, but some feature partial limb-loss or limb reduction.

The reniculate kidney is a multilobed kidney found in marine and aquatic mammals such as pinnipeds and cetaceans but absent in terrestrial mammals except bears. Kidneys of this morphology have increased surface area for removing toxins from the body more efficiently than a non-lobed kidney.

<span class="mw-page-title-main">Evolution of sirenians</span> Development from a Tethytherian ancestor and radiation of species

Sirenia is the order of placental mammals which comprises modern "sea cows" and their extinct relatives. They are the only extant herbivorous marine mammals and the only group of herbivorous mammals to have become completely aquatic. Sirenians are thought to have a 50-million-year-old fossil record. They attained modest diversity during the Oligocene and Miocene, but have since declined as a result of climatic cooling, oceanographic changes, and human interference. Two genera and four species are extant: Trichechus, which includes the three species of manatee that live along the Atlantic coasts and in rivers and coastlines of the Americas and western Africa, and Dugong, which is found in the Indian and Pacific oceans.

A limb is a jointed, muscled appendage of a tetrapod vertebrate animal used for weight-bearing, terrestrial locomotion and physical interaction with other objects. The distalmost portion of a limb is known as its extremity. The limbs' bony endoskeleton, known as the appendicular skeleton, is homologous among all tetrapods, who use their limbs for walking, running and jumping, swimming, climbing, grasping, touching and striking.

<span class="mw-page-title-main">Marine vertebrate</span> Marine animals with a vertebrate column

Marine vertebrates are vertebrates that live in marine environments. These are the marine fish and the marine tetrapods. Vertebrates are a subphylum of chordates that have a vertebral column (backbone). The vertebral column provides the central support structure for an internal skeleton. The internal skeleton gives shape, support, and protection to the body and can provide a means of anchoring fins or limbs to the body. The vertebral column also serves to house and protect the spinal cord that lies within the column.

Interdigital webbing refers to the presence of skin membranes. Normally, in mammals, 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 and from Aarskog–Scott syndrome.

<span class="mw-page-title-main">Fish fin</span> Bony skin-covered spines or rays protruding from the body of a fish

Fins are moving appendages protruding from the body of fish that interact with water to generate thrust and help the fish swim. Apart from the tail or caudal fin, fish fins have no direct connection with the back bone and are supported only by muscles.

<span class="mw-page-title-main">Webbed foot</span> Animal feet with non-pathogenic interdigital webbing

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.

<i>Jeholochelys</i> Extinct genus of turtles

Jeholochelys is an extinct genus of sinemydid turtle that lived during the Early Cretaceous of what is now China. The holotype specimen was discovered in the Jiufotang Formation of Sihedang in Lingyuan, western Liaoning. In 2018, the Chinese palaeontologist Shuai Shao and colleagues named the new genus and species Jeholochelys lingyuanensis based on the specimen. The generic name consists of "Jehol", which refers to the Jehol biota, and "chelys", which is Greek for turtle. The specific name refers to the type locality. Seven skeletons were described in the study, five nearly complete, and two consisting of shells, and hundreds of turtle fossils have been found in the area. The described specimens are kept in the Paleontological Museum of Liaoning.

The physiology of underwater diving is the physiological adaptations to diving of air-breathing vertebrates that have returned to the ocean from terrestrial lineages. They are a diverse group that include sea snakes, sea turtles, the marine iguana, saltwater crocodiles, penguins, pinnipeds, cetaceans, sea otters, manatees and dugongs. All known diving vertebrates dive to feed, and the extent of the diving in terms of depth and duration are influenced by feeding strategies, but also, in some cases, with predator avoidance. Diving behaviour is inextricably linked with the physiological adaptations for diving and often the behaviour leads to an investigation of the physiology that makes the behaviour possible, so they are considered together where possible. Most diving vertebrates make relatively short shallow dives. Sea snakes, crocodiles, and marine iguanas only dive in inshore waters and seldom dive deeper than 10 meters. Some of these groups can make much deeper and longer dives. Emperor penguins regularly dive to depths of 400 to 500 meters for 4 to 5 minutes, often dive for 8 to 12 minutes, and have a maximum endurance of about 22 minutes. Elephant seals stay at sea for between 2 and 8 months and dive continuously, spending 90% of their time underwater and averaging 20 minutes per dive with less than 3 minutes at the surface between dives. Their maximum dive duration is about 2 hours and they routinely feed at depths between 300 and 600 meters, though they can exceed depths of 1,600 meters. Beaked whales have been found to routinely dive to forage at depths between 835 and 1,070 meters, and remain submerged for about 50 minutes. Their maximum recorded depth is 1,888 meters, and the maximum duration is 85 minutes.

References

  1. 1 2 "Why Did Penguins Stop Flying? The Answer Is Evolutionary". National Geographic News. 2013-05-21. Archived from the original on October 13, 2019. Retrieved 2019-12-18.
  2. ↑ William F. Perrin; Bernd Würsig; J.G.M. Thewissen (26 February 2009). Encyclopedia of Marine Mammals. Academic Press. ISBN   978-0-08-091993-5.
  3. 1 2 3 Fish, F.E. (2004). "Structure and Mechanics of Nonpiscine Control Surfaces". IEEE Journal of Oceanic Engineering. 29 (3): 605–621. Bibcode:2004IJOE...29..605F. doi:10.1109/joe.2004.833213. ISSN   0364-9059. S2CID   28802495.
  4. 1 2 Weber, Paul W.; Howle, Laurens E.; Murray, Mark M.; Fish, Frank E. (2009-07-15). "Lift and drag performance of odontocete cetacean flippers". Journal of Experimental Biology. 212 (14): 2149–2158. doi: 10.1242/jeb.029868 . ISSN   0022-0949. PMID   19561204.
  5. 1 2 "Flippers provide lift, reduce drag : Humpback Whale". AskNature. Retrieved 2019-12-18.
  6. ↑ Cheneval, O.; Blake, R.; Trites, Andrew; Chan, K. (2007-01-01). "Turning maneuvers in Steller sea lions (Eumatopias jubatus)". Marine Mammal Science. 23 (1): 94–109. Bibcode:2007MMamS..23...94C. doi:10.1111/j.1748-7692.2006.00094.x.
  7. ↑ Chikina, Maria; Robinson, Joseph D.; Clark, Nathan L. (2016-09-01). "Hundreds of Genes Experienced Convergent Shifts in Selective Pressure in Marine Mammals". Molecular Biology and Evolution. 33 (9): 2182–2192. doi:10.1093/molbev/msw112. ISSN   0737-4038. PMC   5854031 . PMID   27329977.
  8. 1 2 Zhou, Xuming; Seim, Inge; Gladyshev, Vadim N. (2015-11-09). "Convergent evolution of marine mammals is associated with distinct substitutions in common genes". Scientific Reports. 5 (1): 16550. Bibcode:2015NatSR...516550Z. doi:10.1038/srep16550. ISSN   2045-2322. PMC   4637874 . PMID   26549748.
  9. 1 2 Cooper, Lisa Noelle; Berta, Annalisa; Dawson, Susan D.; Reidenberg, Joy S. (2007). "Evolution of hyperphalangy and digit reduction in the cetacean manus". Anatomical Record. 290 (6): 654–672. doi: 10.1002/ar.20532 . ISSN   1932-8486. PMID   17516431. S2CID   14586607.
  10. 1 2 Cooper, Lisa; Sears, Karen; Armfield, Brooke; Kala, Bhavneet; Hubler, Merla; Thewissen, J G M (2017-10-01). "Review and experimental evaluation of the embryonic development and evolutionary history of flipper development and hyperphalangy in dolphins (Cetacea: Mammalia)". Genesis. 56 (1): e23076. doi: 10.1002/dvg.23076 . PMID   29068152.
  11. ↑ Fedak, Tim J; Hall, Brian K (2004). "Perspectives on hyperphalangy: patterns and processes". Journal of Anatomy. 204 (3): 151–163. doi:10.1111/j.0021-8782.2004.00278.x. ISSN   0021-8782. PMC   1571266 . PMID   15032905.
  12. 1 2 3 Evers, Serjoscha W.; Barrett, Paul M.; Benson, Roger B. J. (2019-05-01). "Anatomy of Rhinochelys pulchriceps (Protostegidae) and marine adaptation during the early evolution of chelonioids". PeerJ. 7: e6811. doi: 10.7717/peerj.6811 . ISSN   2167-8359. PMC   6500378 . PMID   31106054.
  13. 1 2 Fujii, Jessica A.; McLeish, Don; Brooks, Andrew J.; Gaskell, John; Houtan, Kyle S. Van (2018-03-28). "Limb-use by foraging marine turtles, an evolutionary perspective". PeerJ. 6: e4565. doi: 10.7717/peerj.4565 . ISSN   2167-8359. PMC   5878658 . PMID   29610708.
  14. ↑ "Sea turtles use flippers to manipulate food". ScienceDaily. Retrieved 2019-12-21.
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