Pinniped

Seals are a group of carnivorous, fin-footed, semiaquatic marine mammals that are widely spread and diversified. They include the extant groups Odobenidae (walruses), Otariidae (eared seals: sea lions and fur seals), and Phocidae (sea lions and fur seals). Pinnipedia are found in 33 extant species and much more than 50 extinct species have been identified from fossils. Whereas seals were once assumed to be descended from two ancestral lines, molecular evidence now shows that they are a single lineage (descended from one ancestral line). Bears and the superfamily of musteloids (weasels, skunks, raccoons, and red pandas) are the closest extant cousins of pinnipeds, having separated around 50 million years ago.

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Seals vary in size from the 1 m (3 ft 3 in) Baikal seal with 45 kg (99 lb) to the 5 m (16 ft) southern elephant seal male with 3,200 kg (7,100 lb), and is also the largest member of the order Carnivora. Sexual dimorphism can be found in a variety of organisms. Their bodies appear streamlined, and their four limbs have been converted into flippers. Seals are much more flexible and agile than dolphins, while not being as swift in the water. Otariids move themselves via the water with their front limbs, whereas phocids and walruses utilise their rear limbs. The rear limbs of otariids and walruses could be dragged under the body and utilised as legs on land. In comparison, phocids' terrestrial locomotion is more difficult.

External ears are noticeable in otariids, but not on phocids or walruses. Pinnipedia have very good senses—their eyesight and hearing are suited for both air and water, and their whiskers, or vibrissae, carry an advanced tactile system. Some creatures have evolved to dive to extreme depths. To remain warm in the freezing water, they have a layer of fat beneath their skin called blubber, as well as all species are coated with fur save the walrus. Despite their vast distribution, pinnipeds favor the colder waters of the Northern and Southern Hemispheres.

Anatomy and Physiology

Pinnipeds possess streamlined, spindle-shaped bodies featuring reduced or absent external ear flaps, flexible necks, rounded heads, flipper-like limbs, and short tails. Pinnipeds have big eye orbits, small snouts, and a narrow interorbital region in their skulls. They are the only carnivorans whose orbital walls are fashioned mostly by the maxilla and are not constrained by facial bones. They have fewer teeth (particularly incisors and rear molars) than other carnivorans, and their teeth are sharp and cone-shaped, with no carnassials. The walrus possesses enlarged upper canines which resemble tusks. Pinnipeds' mammary glands and genitals could retract within the body.

The Baikal seal is 1 m (3 ft 3 in) tall and weighs 45 kg (99 lb), whereas the southern elephant seal is 5 m (16 ft) tall and weighs 3,200 kg (7,100 lb). They are often larger than other carnivorans, with the southern elephant seal being the largest. Male-biased sexual dimorphism is found in many species and is related to the degree of polygyny throughout the species: extremely polygynous species, such as elephant seals, are significantly sexually dimorphic, whereas less polygynous species include males and females which are similar in size. Females of lobodontine seals are considerably larger in size. Secondary sex features, including elephant seals' broad proboscis, hooded seals' inflatable crimson nasal membrane, and otariids' thick necks and manes, are common in males of sexually dimorphic species. Despite the fact that there is a link between size dimorphism and the level of polygyny, certain evidence indicates that size variations between the sexes developed earlier to the development of polygyny because of ecological factors.

Locomotion

The fore-flippers and hind-flippers are two sets of flippers on the front and back of pinnipeds. Inside the body, the elbows and ankles are contained. Pinnipeds are slower swimmers than cetaceans, averaging 5–15 kn (9–28 km/h; 6–17 mph) compared to around 20 kn (37 km/h; 23 mph) for numerous dolphin species. Seals are more nimble and flexible, and several otariids, like the California sea lion, can extend their necks backwards very far to approach their hind-flippers, permitting them to turn dorsally. Pinnipeds have a number of adaptations that help them reduce drag. They feature smooth systems of muscle bundles in their epidermis, which might promote laminar flow and simplify things for them to glide through water, in contrast to their streamlined bodies. They additionally lack arrector pili, which allows them to swim with their fur streamlined.

Senses

Pinnipeds' eyes are enormous for their size, and they are located near the front of the head. The walrus is an exception, as it has smaller eyes on the edges of its head. This is due to the fact that it feeds on immobile bottom-dwelling mollusks and thus does not require keen vision. 

The eye of a seal is designed to see both underwater and in the air. Almost all of the retina is equidistant from the lens centre, and the lens is largely spherical. In both water and air, the cornea does have a flattened centre where refraction is almost equal. Pinnipeds' irises are similarly incredibly muscular and vascularized. The animals' well-developed dilator muscle allows them to dilate their pupils to a wide range. The pupil of a pear-shaped seal is normally pear-shaped when constricted, but the bearded seal's seems to be more diagonal. Dilation varies slightly in shallow-water animals like harbour seals and California sea lions, whereas it changes significantly in deep-diving elephant seals.

Thermoregulation

With their enormous and compact body size, protective fat and fur, and high metabolism, pinnipeds retain heat. Furthermore, the blood arteries in their flippers have been modified to allow for countercurrent exchange. The arteries that carry warm blood from the center of the body are surrounded by veins that carry cool blood from the body's extremities. Heat is transported from arterial blood to blood vessels, which further recirculate blood back to the heart. When you're out of the water, the same adaptations that keep you warm in the water continue to hold you warm when you're not. Numerous species cool off by flicking sand onto their backs, adding another layer of cool, damp sand which promotes heat escape and helps them avoid overheating. Northern fur seals wear pyjamas to keep cool, whereas monk seals burrow holes throughout the sand to reveal cooler layers.

Sleep

Pinnipeds need to sleep in the water since they devote months at a time at sea. Scientists had also captured them snoozing for minutes in a belly-up position as they slowly fell downhill. Seals, like other marine mammals, sleep in the water keeping half of their brain alert to identify and flea predators. Both halves of their brain get in a sleep state when they rest on land.

Distribution and Habitat

Pinnipeds live mostly in polar and subpolar environments, such as the North Pacific, North Atlantic, and Southern Ocean. Indomalayan waters are completely devoid of them. Tropical and subtropical waters are home to monk seals and several otariids. Seals prefer chilly, nutrient-rich waters with temperatures below 20 degrees Celsius (68 degrees Fahrenheit). Even those who reside in warm or tropical climes are affected by current patterns, which cause areas to become chilly and nutrient-rich. Monk seals are the only seals that dwell in seas that aren't normally chilly or nutrient-rich. Large landlocked bodies of water are home to the Caspian and Baikal seals (the Caspian Sea and Lake Baikal respectively).

Pinnipeds live in a wide range of aquatic environments, comprising coastal water, brackish water, open ocean, and even freshwater lakes and rivers. The majority of species live along the coast, however others migrate offshore to feed in deep seas near oceanic islands. Even though some ringed seals reside in freshwater lakes near the Baltic Sea in Russia, the Baikal seal seems to be the only freshwater species. Harbor seals also frequent estuaries, lakes, and rivers, and can stay for up to a year. California sea lions and South American sea lions are two more species that have been documented to enter freshwater. Pinnipeds exploit a variety of terrestrial habitats and surfaces, including continental and island environments.

They congregate on sandy and pebble beaches, shoals, rocky coasts, tide pools, mud flats, and sea caves in temperate and tropical locations. Man-made structures including piers, buoys, jetties, and oil platforms are also used by some animals to rest. Pinnipeds may go inland, resting in sand dunes or vegetation, and sometimes even scaling cliffs. Polar-dwelling creatures congregate on both fast and drift ice.

Behavior and Life History

Pinnipeds are amphibious, meaning they invest the majority of their lives in the water but come out to breed, produce young, moult, sleep, thermoregulate, or flee from aquatic predators. Numerous species have been known to travel long distances in reaction to dramatic environmental changes such as El Nio or ice cover shifts. Elephant seals spend 8–10 months of the year at sea, migrating between breeding and moulting grounds. At 18,000–21,000 kilometres (11,000–13,000 mi), the northern elephant seal does have one of the greatest reported migratory ranges for a mammal. Phocids have a higher migration rate than otariids.

Seals would have to use a variety of environmental features to get to their objective, such as geomagnetic fields, wind and water currents, the location of the sun and moon, and the flavor and water temperature.

Foraging and Predation

Pinnipeds all seem to be predatory and carnivorous. Fish and cephalopods are by far the most common prey, preceded by zooplankton, crabs and bivalves, and endothermic ("warm-blooded") animals like sea birds. Whereas the majority of species are opportunistic and generalist feeders, a handful are specialists. The crabeater seal, that eats mostly krill, the ringed seal, which feeds mostly crustaceans, the Ross seal and southern elephant seal, that consume squid, and the bearded seal and walrus, that consume clams as well as other bottom-dwelling invertebrates, are just a few examples.

Pinnipeds can either hunt alone or in groups. When chasing non-schooling fish, slow-moving or stationary invertebrates, or endothermic prey, the former is the more common activity. Coastal seas, bays, and rivers are often used by solitary foraging species. Northern elephant seals, that graze on fish at deep depths inside the open ocean, are an exception to this rule. Furthermore, walruses feed alone but are frequently found in small or large groups, which may surface and dive in unison. Pinnipeds, including certain otariids, hunt collaboratively in huge groups when vast schools of fish or squid are available, identifying and herding their food. Some sea lions, such as those found in California and South America, might interact with cetaceans and marine birds when foraging.

Reproductive Behavior

Pinnipeds' mating pattern ranges from severe polygyny to serial monogamy. Twenty of the 33 species breed on land, while the other 13 breed on ice. Females assemble in huge aggregations, and males are ready to bond with them and also protect them from competition, making land-breeding species polygynous. Elephant seals, grey seals, and the majority of otariids are polygynous. Pinnipeds that spawn on land prefer to mate on islands with fewer terrestrial predators. There are very few islands which are suitable for breeding, and those that are usually crowded. Females travel to the very same breeding places for several years because the land they nest on is fixed. Males migrate early in the season and wait for females to arrive. Males remain on land and attempt to mate with just as many females as possible; many will just fast. If a male departs the shore to feed, he would most certainly lose his dominance and mating possibilities.

Birth and Parenting

Female pinnipeds enter estrous immediately after giving birth, except for the walrus, which has a five- to six-year inter-birth gap. Delay implantation occurs in all species, in which the embryo stays in a state of stopped development for weeks or months before being implanted in the uterus. Delayed implantation means the birth of the young is postponed until the mother hauls out on ground or the birthing situations are good. Seals go through a year of gestation (including delayed implantation). The majority of species give birth throughout the spring and summer months. Single puppies are usually born; twins are rare and have a high death rate. Precocial pups are born in the majority of animals.

Pinniped milk contains little to no lactose, unlike that of terrestrial animals. Mother pinnipeds have quite a variety of maternal care and nursing techniques. Throughout their fairly short breastfeeding period–four days for the hooded seal and five weeks for elephant seals–phocids like grey seals, elephant seals, and hooded seals stay on ground or ice and fast. The milk of such species contains up to 60% fat, which allows the young to grow rapidly. Northern elephant seal pups, in particular, acquire 4 kg (9 lb) per day until they are weaned. A few puppies might try to steal more milk from other nursing mothers in order to acquire weight faster.

Communication

Barks, creaks, rasps, chirps, grunts, clicks, growls, rattles, warbles, trills, chugs, and whistles are among the vocalisations produced by pinnipeds. Whereas most vocals are audible to the human ear, an underwater recording of a captive leopard seal emitting ultrasonic sounds has been made. Furthermore, northern elephant seal vocals might cause infrasonic vibrations. Both air and water are used to generate vocals. On land, otariids seem to be more vocal, whereas in water, phocids are much more vocal. Owing to the unavailability of terrestrial and pagophilic predators such as the polar bear, Antarctic seals are more noisy on land or ice than Arctic seals. Male vocals are often lower in frequency than female vocals.

During the breeding season, vocalisations are especially crucial. With "clap-threats" and loud drum-like noises which may be adjusted by the proboscis, dominant male elephant seals proclaim their rank and threaten rivals. Otariid males have powerful barks, roars, growls, and "whickers." When trying to attract females, male walruses are reported to make peculiar gong-like cries. With their expanded throats, they can also make melodic sounds.

Intelligence

A single California sea lion had been able to exhibit a comprehension of symmetry, transitivity, and equivalence in a match-to-sample task study; a second seal has been unable to finish the research. When given an artificial sign language, they display the ability to comprehend similar interface and commands, albeit they rarely utilise the signs semantically or logically. Ronan, a caged California sea lion, was videotaped bobbing his head in time to musical rhythms in 2011. Formerly, this "rhythmic entertainment" had only been observed in parrots, humans, as well as other birds with vocal mimicry. Hoover, a captive harbour seal, was taught to imitate human words, phrases, and laughing in 1971.

Ronan, a caged California sea lion, was videotaped bobbing his head in time to musical rhythms in 2011. Formerly, this "rhythmic entertainment" had only been observed in parrots, humans, as well as other birds with vocal mimicry. Hoover, a captive harbour seal, was taught to imitate human words, phrases, and laughing in 1971.

In these starving species, alloparenting happens; whereas most northern elephant seal mothers nurse their own pups and reject alien pups, few adopt alien pups with their own.