SEALS AND PINNIPEDS
Seals and sea lions are carnivorous pinnipeds while manatees and dugongs are called plant-eating sirenians. The difference between a sea lion and a seal is that the former had has flaps over its ears and large front flippers that allow it to maneuver around on land more easily while the latter doesn't. Seals and sea lions have many similarities with land carnivores like dogs and cats such as a long snout and sharp teeth and are thought to have developed from land mammals about 20 million to 25 million years ago. [Source: Roger Gentry, National Geographic, April 1987]
There are 33 species of pinniped, all of which mate and give birth on land and feed at sea. They fall into two categories: true seals and eared seals. True seals (phocids) have no external ear and use their fishtail-like rear flippers for swimming. They are not very mobile on land. They hunch along caterpillar fashion using their whole bodies. Their clawed foreflippers do not offer much help getting around on land.
Sea lions and fur seals are eared seal (otariids). They have visible earflaps and use all four limbs for walking on land and use their front flippers for swimming. Their rear flippers, which are short and turn forward and backward, are most useful for getting around on land. Eared seals can move much faster on land than seals. The creatures you see doing tricks in marine mammal shows are generally sea lions not seals. The name sea lion is derived from the male’s mane.
A male seal is called a bull. A female is called a cow. Young are called pups. A group is called a herd, rookery or harem. Pinniped means “fin-footed.” Walruses (odobenids) are neither seals nor sea lions and have no external ear flap but can walk on all fours. Odobenids are the third grouping of pinnipeds. Seals have fairly long lifespans; most live to 20 or more. A captive seal has lived to the age of 46.
The origin of seals and sea lions is a matter of some debate. Some say that they evolved separately, with sea lions descending from a common ancestor of weasel. Other say that seals and sea lions evolved together from otter-like creatures. In any case they descended from creatures that once lived full time on land. This is also believed to be the case with whales. Of seal lions and seals, sea lions — with their external ear flaps and more mobile limbs — are most closely related to their land-living ancestors.
Websites and Resources: Animal Diversity Web (ADW) animaldiversity.org; National Oceanic and Atmospheric Administration (NOAA) noaa.gov; Fishbase fishbase.se; Encyclopedia of Life eol.org; Smithsonian Oceans Portal ocean.si.edu/ocean-life-ecosystems ; Monterey Bay Aquarium montereybayaquarium.org ; MarineBio marinebio.org/oceans/creatures
Difference Between Seals and Sea Lions
Seals and sea lions, together walruses, are pinnipeds. So how are seals and se lion different? First, seals' furry, generally stubby front feet — thinly webbed flippers, actually, with a claw on each small toe — seem petite in comparison to the mostly skin-covered, elongated fore flippers that sea lions possess. Secondly, sea lions have small flaps for outer ears. The "earless" or "true" seals lack external ears altogether. You have to get very close to see the tiny holes on the sides of a seal’s sleek head. Third, sea lions are noisy. Seals are quieter, vocalizing via soft grunts. [Source: NOAA]
Fourth, while both species spend time both in and out of the water, seals are better adapted to live in the water than on land. Though their bodies can appear chubby, seals are generally smaller and more aquadynamic than sea lions. At the same time, their hind flippers angle backward and don't rotate. This makes them fast in the water but basic belly crawlers on terra firma. Sea lions, on the other hand, are able to "walk" on land by rotating their hind flippers forward and underneath their big bodies. This is why they are more likely to be employed in aquaria and marine shows.
Finally, seals are less social than their sea-lion cousins. They spend more time in the water than sea lions do and often lead solitary lives in the wild, coming ashore together only once a year to meet and mate. Sea lions congregate in gregarious groups called herds or rafts that can reach upwards of 1,500 individuals. It's common for scores of them to haul out together and loll about in the sand, comprising an amorphous pile in the noonday sun.
To sum up, seals and sea lions differ in physical characteristics and adaptations. Sea lions are brown, bark loudly, "walk" on land using their large flippers and have visible ear flaps. Seals have small flippers, wriggle on their bellies on land, and lack visible ear
Phocidae — True Seals
True seals— earless seals, phocids — make the family Phocidae, one of the three main groups of mammals within the seal lineage, Pinnipedia. True seals are different from fur seals and sea lions which belong of the family Otariidae. [Source: Wikipedia]
The true seals group is a diverse group containing 19 species in 10 genera. Phocids are broadly distributed along coastlines north of 30º N latitude and south of 50º S latitude. Some species are also found at intermediate tropical localities, and in a few freshwater lakes and rivers.[Source: Phil Myers, Animal Diversity Web (ADW) /=]
Members of this family vary greatly in size, from small ringed seals, which weigh around 90 kilograms (200 pounds), to enormous elephant seals, the males of which weigh up to 3600 kilograms (almost four US tons) and are the largest of the pinnipeds. Most seals feed on fish, squid, octopus, and shellfish, but some eat plankton, and one species — the leopard seal — catches penguins and small seals. /=\
Seals are warm blooded creatures. What keeps them warm in cold water where many of them live is their thick layers of fat, called blubber, under their skin. To keep warm in deep seawater and on land seals and sea lions need blubber. Sea lions have relatively thin layers of blubber but have more hair. Seals have a thin layer of coarse hair and more blubber. The blubber on seals that live in polar regions is typically between (3 and 5 centimeters (1.6 and 2.4 inches) thick.
Seals are not as fully adapted to sea life as whales. They still retain their legs and their heads are similar in shape to those of land mammals. Seals have four limbs as opposed to whales and dolphins which have two. The arm and leg bones of seals are relatively short and are contained within the body. The hand and foot bones are elongated and webbed and extend out of the body to form the flippers. The rear flippers can rotate, which allow the animals to move on land and swim with maximum efficiency.
There are two kinds of pinniped back limb positions: 1) sea lions and fur seals have forward-pointing back limbs, which allow them to get around better on land. 2) true seals have back limbs pointing backward, which are better adapted for swimming in the open sea. Flippers allow pinnipeds to maneuver around in both water and land. Seals have flippers at the front and backs of their bodies. Those at the front mover freely and allow them to pull themselves out of the sea and get around, albeit slowly and laboriously, on land. Seals move on land like caterpillars by arching their back and drawing the hindquarter forward and then lunging forward with the front part of the body.
True seals have streamlined "fusiform" (spindl-shaped) bodies and lack external ear. According to Animal Diversity Web: Forelimbs are relatively short, less than 25 percent of the length of the body and smaller than the hind flippers. They have well developed claws. The large hind flippers extend straight backward and cannot be brought under the body. On land, earless seals are awkward, moving by a combination of sliding and flexing their spines from side to side. Even so, some species are capable of moving faster than a human. Phocids have a short, stubby tail, and males have a well-developed baculum. [Source: Phil Myers, Animal Diversity Web (ADW) /=]
Young of many phocids are covered with dense, soft, often white coats. In adults, the fur is often stiff and short, without an appreciable undercoat. A few species are nearly naked. Some have spotted or banded color patterns. A thick, insulating layer of blubber lies beneath the skin; the weight of the blubber may amount to more than 25 percent of the entire weight of the animal. /=\
The skulls of phocids nearly or completely lack postorbital processes, and the alisphenoid canal is also absent. The bullae are somewhat inflated. The dental formula is 2-3/1-2, 1/1, 4/4, 0-2/0-2 = 26-36. The upper incisors have simple, pointed crowns. The canines are long and pointed, and the cheek teeth usually have three cusps, but their structure varies considerably among species. In crab-eating seals, for example, the cheek teeth have complex cusps that make them into sort of a sieve, used for straining plankton. /=\
Seals can close their eyes and ears underwater. In dim light, thanks to their large eyes, they can see underwater better than humans can on land. The seals’ sharp vision is essential for finding food at great depths. They can also discern higher pitched noises than humans and sometimes make clicking sounds and long sweeping trills. Seals have very sensitive whiskers which they use to track prey. Dolphins use echolocation (emitting sound waves and sensing their reflections to determine the location of objects) to find their prey and navigate their surroundings but researchers had been unable to find evidence that pinnipeds, the group that includes seals, had anything similar. Little is known about a seal's sense of taste. Researchers believe that seals have an acute sense of smell on land. This sense may be important for mothers identifying their pups.
Seals have a well-developed sense of hearing in the water. Research indicates that under water, harbor seals respond to sounds from 1 to 180 kHz with a peak sensitivity of 32 kHz. In the air, hearing ability is greatly reduced; seals respond to sounds from 1 to 22.5 kHz, with a peak sensitivity of 12 kHz. (The average hearing range for humans is 0.02 to 20 kHz.). [Source: Sea World]
Seals have large eyes. Their vision under water is better than a human's, but human’s is better on land. Lenses are enlarged and almost round, adapted for focusing on light that is refracted upon entering water. The lenses are not as well-adapted for sight in air. Mucus continually washes over the eyes to protect them. Unlike most land mammals, pinnipeds lack a duct for draining eye fluids into the nasal passages. When a seal is out of the water, mucus surrounding the eyes gives them a wet, "tear-rimmed" look. Good vision does not seem to be essential to seal survival; scientists have found blind but otherwise healthy individuals, including mothers with pups, at sea. Seals probably do not have color vision.
Seals' eyes are adapted for sight in dark and murky water. Like the eyes of other pinnipeds, seals' eyes contain high numbers of rod cells — photoreceptor cells that are sensitive to low light levels. Seals have a well-developed tapetum lucidum, a layer of reflecting plates behind the retina. These plates act as mirrors to reflect light back through the retina a second time, increasing the light-gathering ability of the rod cells. (The tapetum lucidum is the same structure that makes a cat's eyes appear to "glow" when reflecting light at night.) Under water, the pupils dilate (expand) into a wide circle to let in as much light as possible. In bright light, the pupils constrict to a slit.
Seal Vibrissae (Whiskers)
A seal uses its sensitive vibrissae (whiskers) to find food, especially in dark, deep waters, or at night. A substantial nerve system transmits tactile information from the vibrissae to the brain. Each vibrissa can move independently. Under water, a seal thrusts its vibrissae to and fro in a sweeping movement by pushing its mobile upper lip in and out.
Vibrissae are modified hairs packed with nerves that relay information about direction, velocity, and the physical environment to the brain. Bret Stetka wrote in The Atlantic, Nearly all mammals sports them at some stage of life. Humans don't have them though they still have vestigial whisker-moving muscles in their upper lips. Seals’ whiskers are better equipped than many other animals’ to process sensory data: While rat and cat whiskers, for example, each house around 200 nerve endings, seal whiskers each contain up to 1,500. [Source: Bret Stetka, The Atlantic, October 30, 2015]
If you were to blindfold a harbor seal, give a fish a 30-second head start, and then let the seal start swimming — well, things would not end well for that fish. Take the blindfold off and throw a pair of noise-canceling headphones on the seal, and the next fish would fare no better. Even when they’re completely cut off from all visual and auditory input, the blubbery beasts can still home in on their prey with GPS-like accuracy. Rather than rely on sight and sound, the seals use antenna-like whiskers — precise instruments of marine carnage capable of sizing up a herring down to the centimeter.
Reuters reported: Because a harbour seal's whiskers have between 1,000 and 1,600 nerve fibres per hair, they are possibly the most sensitive whiskers in the animal kingdom, said researcher Guido Dehnhardt of Germany's Ruhr-Universitat Bochum. Mr. Dehnhardt and his colleagues enlisted Nick and Henry, two harbour seals from the Cologne Zoo, to help show that seals can use these highly attuned whiskers to track a water disturbance even in very clouded conditions and when the object that disrupted the water is long gone, at distances that could range up to 180 meters. [Source: Reuters, July 6, 2001]
A study published in the journal Science in July 2001 tells how a blind-folded Henry — after being trained to follow a miniature submarine without any sensory restrictions — was put in murky water with headphones on to mask sound. Then, after turning off the motor, researchers removed the headphones and released the seal so it could try and find the little sub. Not only did Henry track the sub 256 out of 326 times, he actually followed the exact path it had tracked 80 percent of the time. Nick, in clear water, showed similar results. The seals were able to track the submarine from as far away as 40 meters. The sub's water-disturbance trail lasts 30 seconds, but fish leave trails that can last three to five minutes, the time it takes a goldfish to swim 50 meters, Mr. Dehnhardt said. When the animals' whiskers were covered with a stocking mask, they failed to find the sub.
Hypersensitivity of Seal Whiskers to Motion
Bret Stetka wrote in The Atlantic: MIT research suggests, the real evolutionary genius behind the seal whisker is how it does — and doesn’t — respond to movement. As a seal glides through the water, its whiskers stay still, unresponsive to self-generated motion, yet remain hypersensitive to external turbulence. Wakes stirred up by moving objects in the water create vortices, or rapidly spinning whirlwinds of water; seal whiskers may sync up with these vortices and vibrate in a back-and-forth, slalom-like manner at the same frequency, allowing them to lock in and track the turbulent water to its source. [Source: Bret Stetka, The Atlantic, October 30, 2015]
Michael Triantafyllou, the MIT engineering professor who led the research in conjunction with his former graduate student Heather Beem, attributes this nuanced biology to the unusual shape of the whiskers. Most mammalian whiskers are circular, with a consistently tapering width. Seal whiskers, on the other hand, are elliptical and fluctuate in size throughout their length, undulating in thickness like a python that’s just feasted on a burrow of mice.
Using a 3-D printer, Triantafyllou and Beem fashioned a large-scale model of a seal whisker that they attached to a moving track above a 30-meter tank of water. Dragging a long cylindrical object through water would ordinarily create vibrations and vortices, but the artificial appendage advanced easily, stirring up very little turbulence. The team then ran a second experiment, dragging a circular rod in the tank in front of the “whisker.” As the rod moved through the pool, it mimicked the vibrations of a fish swimming by; the whisker, in turn, picked up the turbulence and slalomed at precisely the same frequency. “Now we have an idea of how it's possible that seals can find fish that they can't see,” Beem said in a statement. “The geometry of the whisker allows for this phenomenon of being able to move very silently through the water, if the water's calm, and extract energy from the fish's wake in order to vibrate.”
Study Confirms Seals Use Whiskers to Track Prey in Deep Ocean
When they are in the dark ocean depths, seals use their whiskers to track down their prey, a study published in the Proceedings of the National Academy of Science in June 2022 confirms. In the study, led by Taiki Adachi, assistant project scientist of University of California, Santa Cruz, scientists analysed footage from small video cameras with infrared night-vision attached to the animals while they were in in their natural habitat. [Source: Sofia Quaglia, The Guardian, June 13, 2022]
The Guardian reported: Adachi and his team positioned small video cameras with infrared night-vision on the left cheek, lower jaw, back and head of five free-ranging northern elephant seals, in Año Nuevo state park in California. They recorded a total of approximately nine and a half hours of deep sea footage during their seasonal migration. By analysing the videos the scientists noted that diving seals held back their whiskers for the initial part of their dives and, and once they reached a depth suitable for foraging, they rhythmically whisked their whiskers back and forth, hoping to sense any vibration caused by the slightest water movements of swimming prey. (Elephant seals like to snack on squid and fishes, and spend a long time out at sea.) Then, on their swim back to the surface, the whiskers were curled back towards the face again.
For less than a quarter of the time the seals were hunting, they could also see some bioluminescence — the light that some creatures deep underwater can emit thanks to chemicals in their bodies — to track down their meals using sight. But for the remaining 80 percent of their hunting spree, they were presumably just using their whiskers, according to Adachi. This technique isn’t dissimilar to rodents, Adachi noted. It’s just that, since water is much denser than air, the whisking speed is much slower in elephant seals. “This makes sense,” said Sascha Kate Hooker, a pinniped researcher from the Sea Mammal Research Unit at University of St Andrews, who was not involved in the study. “Among the deep-diving marine mammals, the elephant seal reaches the same depths as sperm and beaked whales, often well over a kilometer below the surface.”
Image Sources: Wikimedia Commons, NOAA
Text Sources: Animal Diversity Web (ADW) animaldiversity.org; National Oceanic and Atmospheric Administration (NOAA) noaa.gov; Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.
Last Updated June 2023