Pinniped
Pinnipeds | |
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Clockwise from top left: New Zealand fur seal (Arctocephalus forsteri), southern elephant seal (Mirounga leonina), Steller sea lion (Eumetopias jubatus), walrus (Odobenus rosmarus) and grey seal (Halichoerus grypus)
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Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Class: | Mammalia |
Order: | Carnivora |
Clade: | Pinnipedimorpha |
Clade: | Pinnipediformes |
Clade: | Pinnipedia Illiger, 1811[1] |
Subclades | |
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Range map |
Pinnipeds (pronounced ), having diverged about 50 million years ago.
Seals range in size from the 1 m (3 ft 3 in) and 45 kg (100 lb) Baikal seal to the 5 m (16 ft) and 3,200 kg (7,100 lb) southern elephant seal. Several species exhibit sexual dimorphism. They have streamlined bodies and four limbs that are modified into flippers. Though not as fast in the water as dolphins, seals are more flexible and agile. Otariids primarily use their front limbs to propel themselves through the water, while phocids and walruses primarily use their hind limbs for this purpose. Otariids and walruses have hind limbs that can be pulled under the body and used as legs on land. By comparison, terrestrial locomotion by phocids is more cumbersome. Otariids have visible external ears, while phocids and walruses lack these. Pinnipeds have well-developed senses—their eyesight and hearing are adapted for both air and water, and they have an advanced tactile system in their whiskers or vibrissae. Some species are well adapted for diving to great depths. They have a layer of fat, or blubber, under the skin to keep warm in cold water, and, other than the walrus, all species are covered in fur.
Although pinnipeds are widespread, most species prefer the colder waters of the Northern and Southern Hemispheres. They spend most of their lives in water, but
The meat, blubber and skin of pinnipeds have traditionally been used by
Etymology
The name "pinniped" derives from the
Taxonomy
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Cladogram showing relationships among the living pinnipeds, found in Berta, Churchill and Boessenecker (2018). The Southern Hemisphere eared seal clade is not fully resolved.[4] |
The German naturalist
Otariids are also known as eared seals due to the presence of
Odobenidae consists of only one living member: the modern
Phocids are known as true or "earless" seals. These animals lack external ear flaps and are incapable of positioning their hind-flippers to move on land, making them more cumbersome. This is because of their massive
Evolution
One popular hypothesis suggested that pinnipeds are
Enaliarctos, a fossil species of late
The ancestors of the Otarioidea and Phocidea diverged around 25 mya.
Anatomy and physiology
Pinnipeds have streamlined, spindle-shaped bodies with small or non-existent ear flaps, rounded heads, short muzzles, flexible necks, limbs modified into flippers and small tails.[24][25][26] The mammary glands and genitals can withdraw into the body.[24] Seals are unique among carnivorans in that their orbital walls are mostly shaped by the maxilla and are not contained by certain facial bones.[13] Compared to land carnivores, pinnipeds have fewer teeth, which are pointed and cone-shaped. They are adapted for holding onto slippery prey rather than shearing meat like the carnassials of other carnivorans. The walrus has unique tusks which are long upper canines.[27]
Pinnipeds range in size from the 1 m (3 ft 3 in) and 45 kg (100 lb)
Almost all pinnipeds have fur coats, the exception being the walrus, which is only sparsely covered. Even some fully furred species (particularly sea lions) are less furry than land mammals. Fur seals have lush coats consisting of an
The simple stomach of pinnipeds is typical of carnivores. Most species have neither a cecum nor a clear demarcation between the small and large intestines; the large intestine is comparatively short and only slightly wider than the small intestine. Small intestine lengths range from 8 times (California sea lion) to 25 times (elephant seal) the body length. The length of the intestine may be an adaptation to frequent deep diving, allowing for more room in the digestive tract for partially digested food. An appendix is absent in seals.[36] As in most marine mammals, the kidneys are divided into lobes and filter out excess salt.[37]
Locomotion
Pinnipeds have two pairs of flippers on the front and back, the fore-flippers and hind-flippers. Their elbows and ankles are not externally visible.
When swimming, otariids rely on their fore-flippers for locomotion in a wing-like manner similar to
Pinnipeds can move around on land, though not as well as terrestrial animals. Otariids and walruses are capable of turning their hind-flippers forward and under the body so they can "walk" on all fours.[47] The fore-flippers move along a transverse plane, rather than the sagittal plane like the limbs of land mammals.[48] Otariids create momentum by laterally swaying their heads and necks.[49][48] Sea lions have been recorded climbing up flights of stairs. Phocids lack the ability to walk on their hind-flippers, and must flop and wriggle their bodies forward as their fore-flippers keep them stable. In some species, the fore-flippers may act like oars pushing against the ground. Phocids can move faster on ice, as they are able to slide.[50]
Senses
The eyes of pinnipeds are relatively large for their size and are positioned near the front of the head. Only the smaller eyes of the walruses are located on each side of the head;
On land, pinnipeds are
The pinniped ear is adapted for hearing underwater, where it can hear sound frequencies of up to 70,000 Hz. In air, hearing is somewhat reduced in pinnipeds compared to many terrestrial mammals. While their airborne hearing sensitivity is generally weaker than humans', they still have a wide frequency range.[60] One study of three species—the harbor seal, California sea lion and northern elephant seal—found that the sea lion was best adapted for airborne hearing, the elephant seal for underwater hearing and the harbor seal was equally adapted for both.[61] Although pinnipeds have a fairly good sense of smell on land,[62] it is useless under water as their nostrils are closed.[63]
Pinnipeds have well-developed
Unlike terrestrial mammals, such as
Diving adaptations
To dive, a pinniped must first exhale much of the air out of its lungs and shut its nostrils and throat cartilages to protect the
The circulatory system of pinnipeds is large and elaborate; retia mirabilia line the inside of the trunk and limbs, allowing for greater oxygen storage during diving.[78] As with other diving mammals, pinnipeds have large amounts of hemoglobin and myoglobin stored in their blood and muscles. This allows them to stay submerged for long periods of time while still having enough oxygen. Deep-diving species such as elephant seals have blood volumes that represent up to 20% of their body weight. When diving, they reduce their heart rate, and blood flow is mostly restricted to the heart, brain and lungs. To keep their blood pressure stable, phocids have an elastic aorta that dissipates some of the energy of each heartbeat.[77]
Thermoregulation
Pinnipeds keep warm by having large, thick bodies, insulating blubber and fur, and quick-burning metabolism.[79] In addition, the blood vessels in their flippers are adapted for countercurrent exchange; small veins surround arteries transporting blood from the body core, capturing heat from them.[80] While blubber and fur keep the seal warm in water, they can also overheat the animal when it is on land. To counteract overheating, many species cool off by covering themselves in sand. Monk seals may even dig up the cooler layers. The northern fur seal cools off by panting.[81]
Sleep
Pinnipeds spend many months at a time at sea, so they must sleep in the water. Scientists have recorded them sleeping for minutes at a time while slowly drifting downward in a belly-up orientation.[82] Like other marine mammals, seals sleep in water with half of their brain awake so that they can detect and escape from predators, as well as surface for air without fully waking. When they are asleep on land, both sides of their brain go into sleep mode.[83]
Distribution and habitat
Living pinnipeds are widespread in cold oceanic waters; particularly in the North Atlantic, the North Pacific and the
As a whole, pinnipeds can be found in a variety of aquatic habitats, mostly
Behavior and life history
Pinnipeds have an
Pinnipeds may dive during foraging or to avoid predators. When foraging, for example, the Weddell seal typically dives for no more than 15 minutes and 400 m (1,300 ft) deep, but can dive for as long as 73 minutes and reach 600 m (2,000 ft) deep. Northern elephant seals often dive 350–650 m (1,100–2,100 ft) for as long as 20 minutes. They can also dive 1,500 m (4,900 ft) and for over an hour. The dives of otariids tend to be shorter and less deep. They typically last 5–7 minutes with average depths to 30–45 m (100–150 ft). However, the New Zealand sea lion has been recorded diving to a maximum of 460 m (1,510 ft) and have submerged for as long as 12 minutes.[92] The diet of walruses does not require them to dive very deep or very long. Pinnipeds generally live 25–30 years.[93]
Foraging and predation
All pinnipeds are
Pinnipeds may hunt solitarily or
Seals typically swallow their food whole, and will rip apart prey that is too big.
Pinnipeds themselves are subject to predation. Most species are preyed on by the
Pinnipeds lessen the chance of predation by gathering in groups.[103] Some species are capable of inflicting damaging wounds on their attackers with their sharp canines. Adult walruses are particularly risky prey for polar bears.[102] When out at sea, northern elephant seals dive out of the reach of surface-hunting orcas and white sharks.[82] In the Antarctic, which lacks terrestrial predators, pinniped species spend more time on the ice than their Arctic counterparts.[104]
Interspecific predation among pinnipeds does occur. The leopard seal is known to prey on numerous other species, especially the crabeater seal. Leopard seals typically target crabeater pups, particularly from November to January. Older crabeater seals commonly bear scars from failed leopard seal attacks; a 1977 study found that 75% of a sample of 85 individual crabeaters had these scars.[102][105] Walruses, despite being specialized for feeding on bottom-dwelling invertebrates, occasionally prey on Arctic seals. They kill their prey with their long tusks and eat their blubber and skin. Steller sea lions have been recorded eating harbor seals, northern fur seals and California sea lions, particularly pups and small adults. New Zealand sea lions feed on pups of some fur seal species, and the South American sea lion may prey on South American fur seals.[102]
Reproductive behavior
The
Other seals, like the walrus and most phocids, breed on ice and copulate in the water—a few land-breeding species also mate in water.[28][106] Females of these species tend to be more spaced out and there is less site fidelity, since ice is less stable than solid land. Hence polygyny tends to be weaker in ice-breeding species. An exception to this is the walrus, whose distribution of food forces females closer together. Pinnipeds that breed on fast ice tend to cluster together more than those that breed on drift ice.[110] Seals that breed on ice tend to have little or no sexual dimorphism. In Antarctic seals, there is some size bias in favor of females. Walruses and hooded seals are unique among ice-breeding species in that they have pronounced sexual dimorphism in favor of males.[28][111]
Adult male pinnipeds have several strategies to ensure reproductive success. Otariid males gain access to females by establishing territories where females can bask and give birth and contain valuable resources such as shade, tide pools or access to water. Territories are usually marked by natural barriers,[112] and some may be fully or partially underwater.[113] Males defend their territorial boundaries with threatening vocalizations and postures, but physical fights are usually not very violent, and are mostly limited to early in the season.[114] Individuals also return to the same territorial site each breeding season. In certain species, like the Steller sea lion and northern fur seal, a dominant male can maintain a territory for as long as 2–3 months. Females can usually move freely between territories and males are unable to coerce them, but in some species such as the northern fur seal, South American sea lion and Australian sea lion, males can successfully keep females in their territories with threatening displays and even violence. In some phocid species, like the harbor seal, Weddell seal and bearded seal, the males establish "maritories" and patrol and defend the waters bordering female haul-out areas, waiting for a female to enter.[112] These are also maintained by vocalizations.[115] The maritories of Weddell seal males include entries to female breathing holes in the ice.[116]
Younger or subdominant male pinnipeds may attempt to achieve reproductive success in other ways including sneakiness, harassment of females or even coordinated disruption of the colony. Female pinnipeds do appear to have some choice in mates, particularly in lek-breeding species like the walrus, but also in elephant seals where the males try to dominate all the females that they want to mate with.[122] When a female elephant seal or grey seal is mounted by an unwanted male, she tries to resist and get away. This commotion attracts other males to the scene, and the most dominant will take over and mate with female himself.[123][124] Dominant female elephant seals stay in the center of the colony where they in the domain of a more dominant male, while marginal females are left with subordinates.[125] Female Steller sea lions may solicit their territorial males for mating.[126]
Birth and parenting
With the exception of the walrus, which has five- to six-year gaps between births, female pinnipeds enter
Pinniped milk has "little to no lactose".[133] Mother pinnipeds have different strategies for maternal care and lactation. Phocids such as elephant seals, grey seals and hooded seals have a lactation period that lasts days or weeks, during which they fast and nurse their pups on land or ice. The milk of these species consists of up to 60% fat, allowing the young to grow fairly quickly. Each day until they are weaned, northern elephant seal pups gain 4 kg (9 lb). Some pups gain weight more quickly than others by stealing extra milk from other mothers. Alloparenting occurs in these fasting species;[134] while most northern elephant seal mothers nurse their own pups and reject nursings from alien pups, some do accept alien pups with their own.[135]
For otariids and some phocids like the harbor seal, mothers fast and nurse their pups for a few days at a time. In between nursing bouts, the females forage at sea while the young stay behind onshore. If there is enough food close to shore, a female can be gone for as little as a day, but otherwise may be at sea for as long as three weeks.[136] Lactation in otariids may last 6–11 months; in the Galápagos fur seal it can last up to 3 years. Pups of these species are weaned at heavier weights than their phocid counterparts, but the latter grow quicker.[137] Walruses are unique in that mothers nurse their young at sea.[138] Young pinnipeds typically start swimming on their own and some species can even swim as newborns. Young may wait days or weeks before entering the water; elephant seals start swimming weeks after weaning.[139]
Male pinnipeds generally play little role in raising the young.[140] Male walruses may help inexperienced young as they learn to swim, and have even been recorded caring for orphans. When a group is threatened, all the adults may protect the young.[141] Male California sea lions have been observed to help shield swimming pups from predators.[142] Males can also pose threats to the safety of pups, particularly during fights.[140] Pups of some species may be abducted, assaulted and killed by males.[143]
Communication
Pinnipeds can produce a number of vocalizations. While most vocals are audible to the human ear, Weddell seals have been recorded in Antarctica making
The Weddell seal has perhaps the most extensive vocal repertoire, producing both airborne and underwater sounds. Trilling, gluping, chirping, chugging and knocking are some examples of the calls produced under water. When warning other seals, the calls may be pronounced by "prefixes" and "suffixes".[115] The underwater vocals of Weddell seals can last 70 seconds, which is long for a marine mammal call. Some calls have around seven rhythm patterns and could be categorized as "songs".[148] Similar calls have been recorded in other Antarctic seals[149] and in bearded seals. In some pinniped species, there appear to be regional dialects or even individual variations in vocalizations. These differences are likely important for territorial males becoming accustomed to their neighbors (dear enemy effect) and mothers and pups who need to remain in contact on crowded beaches. Female seals emit a "pulsed, bawling" contact call, while pups respond by squawking. Contact calls are particularly important for otariid mothers returning from sea.[150] Other vocalizations produced by seals include grunts, rasps, rattles, creaks, warbles, clicks and whistles.[115]
Non-vocal communication is not as common in pinnipeds as in cetaceans. Nevertheless, when they feel threatened, hauled-out harbor seals and Baikal seals may slap themselves with their flippers to create a warning sound. Teeth chattering, hisses and exhalations are also made as aggressive warnings by pinnipeds. Visual displays also occur: Ross seals resting on the ice will show the stripes on their chests and bare their teeth to a perceived threat, while swimming Weddell seals will make an S-shaped posture to intimidate rivals under the ice.[115] Male hooded seals use their inflatable nasal membranes to display to and attract females.[29]
Intelligence
In a match-to-sample task study, a single California sea lion was able to demonstrate an understanding of symmetry, transitivity and equivalence; a second seal was unable to complete the tasks.[151] They demonstrate the ability to understand simple syntax and commands when taught an artificial sign language, though they only rarely used the signs semantically or logically.[152] In 2011, a captive California sea lion named Ronan was recorded bobbing its head in synchrony to musical rhythms. This "rhythmic entrainment" was previously seen only in humans, parrots and other birds possessing vocal mimicry.[153] Adult male elephant seals can recognize each other's vocalizations by remembering the rhythm and timbre.[154] In the 1970s, a captive harbor seal named Hoover was trained to imitate human speech and laughter.[155]
For sea lions used in entertainment, trainers toss a ball at the animal or simply place the object on its nose, so it will eventually understand the behavior desired. A sea lion may need a year of training before it can publicly perform. Its long-term memory allows it to perform a trick after as much as three months of non-performance.[142]
Human relations
In culture
Various human cultures have for millennia depicted pinnipeds. In
In captivity
Pinnipeds can be found in facilities around the world, as their size and playfulness make them popular attractions.[160] Seals have been kept in captivity since at least Ancient Rome and their trainability was noticed by Pliny the Elder.[c] Zoologist Georges Cuvier noted during the 19th century that wild seals show considerable affection for humans and stated that they are second only to some monkeys among wild animals in their easy tamability. Francis Galton noted in his seminal work on domestication that seals were a spectacular example of an animal that would most likely never be domesticated, despite their friendliness, survivability and "desire for comfort", because they serve no practical use for humans.[161]
Some modern exhibits have a pool with artificial haul-out sites and a rocky background, while others have seals housed in shelters located above a pool which they can jump into. More elaborate exhibits contain deep pools that can be viewed underwater with rock-mimicking cement as haul-out areas. The most popular captive pinniped is the California sea lion, due to its trainability and adaptability. Other commonly kept species include the grey seal and harbor seal. Larger animals like walruses and Steller sea lions are much less common.[160] Some organizations, such as the Humane Society of the United States and World Animal Protection, object to keeping pinnipeds and other marine mammals in captivity. They state that the exhibits could not be large enough to house animals that have evolved to be migratory, and a pool could never replace the size and biodiversity of the ocean. They also state that the tricks performed for audiences are "exaggerated variations of their natural behaviors" and distract the people from the animal's unnatural environment.[162]
California sea lions are used in
Hunting
Humans have hunted seals since the Stone Age. Originally, seals were merely hit with clubs during haul-out. Eventually, more lethal weapons were used, like spears and harpoons. They were also trapped in nets. The use of firearms in seal hunting during the modern era drastically increased the number of killings. Pinnipeds are typically hunted for their meat and blubber. The skins of fur seals and phocids are made into coats, and the tusks of walruses have been used as ivory.[166] There is a distinction made between the subsistence hunting of seals by indigenous peoples of the Arctic and commercial hunting: subsistence hunters depend on seal products for survival.[167] National and international authorities have given special treatment to aboriginal hunters since their methods of killing are seen as more sustainable and smaller in scope. However indigenous people have recently used more modern technology and are profiting more from seal products in the marketplace. Some anthropologists argue that the term "subsistence" should also apply to these activities, as long as they are local in scale. More than 100,000 phocids (especially ringed seals) as well as around 10,000 walruses are harvested annually by native hunters.[166]
Commercial sealing rivaled whaling as an important industry throughout history. Harvested species included harp seals, hooded seals, Caspian seals, elephant seals, walruses and all species of fur seal.[168] After the 1960s, the harvesting of seals decreased substantially as an industry[166] after the Canadian government implemented measures to protect female seals and restrict the hunting season.[169] Several species that were commercially exploited have rebounded in numbers; for example, Antarctic fur seals may have reached their pre-harvesting numbers. The northern elephant seal nearly went extinct in the late 19th century, with only a small population remaining on Guadalupe Island. It has since recolonized much of its historic range, but has a population bottleneck.[168] Conversely, the Mediterranean monk seal was extirpated from much of the Mediterranean and its current range is still limited.[170]
Several species of pinniped continue to be exploited. The Convention for the Conservation of Antarctic Seals protects species within the Antarctic and surrounding waters, but allows restricted hunting of crabeater seals, leopard seals and Weddell seals. Weddell seal hunting is forbidden between September and February if the animal is older than a year, to ensure healthy population growth.[171] The Government of Canada permits the hunting of harp seals. This has been met with controversy and debate. Proponents of seal hunts insist that the animals are killed humanely and the white-coated pups are not taken, while opponents argue that it is irresponsible to kill harp seals as they are already threatened by declining habitat.[172][173]
The
Conservation issues
As of 2021, the International Union for Conservation of Nature (IUCN) recognizes 36 pinniped species. With the Japanese sea lion and the Caribbean monk seal recently extinct, ten more are considered at risk. They are ranked as:[176]
- "Endangered": Hawaiian monk seal, Mediterranean monk seal, Galápagos fur seal, Australian sea lion, New Zealand sea lion, Caspian seal, and Galápagos sea lion).
- "Vulnerable" (northern fur seal, hooded seal, and walrus).
Pinnipeds face various threats. They are unintentionally caught in
Species that live in polar habitats are vulnerable to the effects of climate change on oceans, particularly declines in sea ice.[179] In 2010 and 2011, sea ice in the Northwest Atlantic was at or near an all-time low and harp seals as well as ringed seals that bred on thin ice saw increased death rates.[180][181] In the Antarctic, the decreased duration and extent of the sea ice and nutrient availability could potentially reduce the survival of Weddell seal pups and may have important implications for population growth rates.[182] Antarctic fur seals in South Georgia in the South Atlantic Ocean saw major decreases over a 20-year study, during which scientists measured increased sea surface temperature anomalies.[183]
Some species have become so numerous that they conflict with local people. In the United States, pinnipeds are protected under the
Notes
- ^ This term typically excludes the walrus in everyday English. In science, it is also sometimes restricted to the "true" seals of the family Phocidae. This article uses it for all pinnipeds.
- ^ Odyssey, book IV, verses 404–413.
- ^ Natural History, book IX, XV:41–43.
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Bibliography
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External links
- "Seal Conservation home page". Seal Conservation Society.
- "Pinnipeds: Seals, sea lions, and walruses". Department of Fisheries. National Oceanographic and Atmospheric Administration. Archived from the original on 19 July 2014.
- "Pinniped Research Lab (PEARL) home page". Pinniped Ecology Applied Research Laboratory. Oregon State University. Archived from the original on 5 July 2019. Retrieved 11 August 2013.
- "Pinniped Laboratory home page". Pinniped Cognition & Sensory Systems Laboratory. University of California, Santa Cruz.