One popular hypothesis has suggested that pinnipeds (seals, sea lions, elephant seals and walruses are diphyletic (descended from two ancestral lines), with walruses and otariids (eared seals such as sea lions and fur seals) sharing a recent common ancestor with bears and phocids (earless seals) sharing one with Musteloidea (a superfamily of mammals that includes raccons, skunks and weasels). However, morphological and molecular evidence support a monophyletic origin. A 2021 genetic study found that pinnipeds are more closely related to musteloids. [Source: Wikipedia]
Pinnipeds split from other caniforms ("dog-like" carnivorans) 50 million years ago during the Eocene Period. Their evolutionary link to terrestrial mammals was unknown until 2007, when Puijila was discovered in early Miocene deposits in Nunavut, Canada. Like a modern otter, Puijila had a long tail, short limbs and webbed feet instead of flippers. However, its limbs and shoulders were more robust and Puijila likely had been a quadrupedal swimmer—retaining a form of aquatic locomotion that gave rise to the major swimming types employed by modern pinnipeds.
Enaliarctos, a fossil species of late Oligocene/early Miocene (24–22 million years ago) California, closely resembled modern pinnipeds; it was adapted to an aquatic life with a flexible spine, and limbs modified into flippers. Its teeth were adapted for shearing (like terrestrial carnivorans), and it may have stayed near shore more often than its extant relatives. Enaliarctos was capable of swimming with both the fore-flippers and hind-flippers, but it may have been more specialized as a fore-flipper swimmer.
The ancestors of the Otarioidea and Phocoidea diverged 33 million years ago. Phocids are known to have existed for at least 15 million years, and molecular evidence supports a divergence of the Monachinae and Phocinae lineages 22 million years ago. The fossil monachine Monotherium and phocine Leptophoca were found in southeastern North America. The monochines diversified southward. Monachus emerged in the Mediterranean and migrated to the Caribbean and then the central North Pacific. The two extant elephant seal species diverged close to 4 million years ago after the Panamanian isthmus was formed. The lobodontine lineage emerged around 9 million years ago and colonized the southern ocean in response to glaciation.
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
According to Animal Diversity Web: The social structure of phocids varies from species to species. Some are monogamous or associate in small groups, while elephant seals are highly gregarious and polygamous. Most seals differ from sea lions, however, in that they do not congregate in the huge rookeries. Some species are migratory (make seasonal movements between regions, such as between breeding and wintering grounds). [Source: Phil Myers, Animal Diversity Web (ADW) /=]
Seals that do not migrate may wander as far as 1,600 kilometers (1000 miles) from their home bases in search of food. When they are on land seals prefer to stay on ice, rocky islands or points. They “haul out”, or come ashore, to rest.
Even in dense groups seals do not form social bonds. Each animal acts on its own behalf, and for its own benefit. Some seals are tolerant of humans, even snuggling up next to them, while males occasionally view a human as rival and charge. Bull fur seals weighing 350 pounds have been known to charge people. They have also been know to back off if shouted at loudly enough.
Seals can perform many of the same sophisticated learning tasks as dolphins and apes. After a seal “has learned words and signals for objects and actions," Roger Gentry wrote in National Geographic. "It can carry out very complex commands such as: Take the large white cube over to the small black ball."
Seals are superb divers. Some species can reach great depths and stay under water for prolonged periods but abilities to do this varies a great deal from species to species. One of the best divers is the Weddell seal, This species can reach depths of 600 meters (1970 feet) and to stay submerged for more than an hour. [Source: Phil Myers, Animal Diversity Web (ADW) /=]
Scientists have long wondered exactly how dolphins, whales and seals can stay under water so long. The secret seems to be that they sink rather than swim downwards to conserve energy and do not use up oxygen unnecessarily. Seals stay as still as possible on the way down and use their energy to catch prey and swim back up. Whales, seals and dolphins all seem to use the same strategy. Scientists were able to observe this phenomena by strapping critter cam cameras on bottlenose dolphins, Wendell seals and even a blue whale.
When humans dive deep air taken into their lungs under pressure dissolves in the blood. When the human returns to the surface the air returns in the form of bubbles in the blood vessels, producing a condition called the bends, causing bleeding in vital organs, paralysis and even death. Seals avoid getting the bends because their lungs collapse during deep dives and force air into the windpipe, where nitrogen can't be absorbed into the blood.
The blood of some deep-diving seals contains 3½ times the hemoglobin of humans. Hemoglobin absorbs oxygen and transports it in the blood. Seals also have large amounts of another substance, myoglobin, within their muscles that absorbs oxygen in a similar way. Using the large qualities of hemoglobin and myoglobin they possess, seals can hold more oxygen in their blood and thus don’t need to store it in their lungs or take in more oxygen.
For very deep dives seals conserve their oxygen stores by stopping or slowing blood circulation to all but critical organs. They can also slow their heart rate, sometimes to a mere 10 percent of their normal rate at the surface.
Seals’ muscles are almost black in color because myoglobin has a dark color. The black meat has a gamy flavor. Scientists at research stations in the Antarctic claim that seal brains are a real delicacy.
The larger a seal is generally the deeper it can dive. Animals weighing 100 pounds can go 600 feet and back in five minutes or less. Elephant seals that weigh several tons can dive down to 4,000 feet and stay submerged for two hours.
Seals Sleep As They Sink in the Sea
Stéphan Reebs wrote Natural History magazine, “When northern elephant seals (Mirounga angustirostris) migrate between their breeding and foraging grounds, they spend as long as eight months at a time at sea. They’re almost always underwater, devoting only a few minutes to breathing at the surface between dives — hardly long enough for a nap. After a sip of air, they often sink quickly to 500 feet, then drift farther down in a shallow descent. Some experts have suggested that the drift is when the seals catch their Zs.” [Source: Stéphan Reebs. Natural History magazine. February 26, 2010]
“To find out, a team led by Yoko Mitani of Hokkaido University in Japan fitted six juvenile elephant seals with satellite transmitters and newfangled data loggers capable of recording such information as body position, flipper strokes, and the 3-D path of movement. They tracked the seals for up to eight days off the California coast. The resulting data revealed that drifting seals usually rolled over on their backs, stopped stroking, and spiraled peacefully down for a dozen minutes or so. (The team dubbed it the “falling-leaf phase” of the descent.)
“The belly-up position is consistent with slumber: ventral blubber tends to flip an unresponsive seal’s body. What’s more, a few animals that drifted in shallow areas hit the seafloor without reacting. The initial rapid descent is important, Mitani’s team points out. It takes the seals below the usual cruising depths of their main predators, killer whales and [white sharks. And their slow sinking thereafter makes for a relatively short ascent for air once they awake.”
Seals on average consume eight percent of their body weight in food every day. They eat a variety of fish and invertebrates, feeding on schools of small anchovy-like fish, squid, octopus and sometimes lobster. Some eat seabirds. Leopard seals and some sea lions eat seals. A critter cam attached to a monk seal in Hawaii revealed that it ate on the slopes of atolls not just in shallow reefs and was able to flip over boulders to get at fish that were underneath
Prey moving under water creates vibrations that the seal may detect with its vibrissae (whiskers). Studies have shown that seals are able to detect and follow the hydrodynamic wake of a miniature submarine by using their sensitive vibrissae to sense water movement. Hydrodynamic trail-following is probably a way for a seal to locate and catch fish in low visibility conditions. [Source: Sea World]
Male and Female Seals
It is sometimes difficult to determine the sex of seal. The seal penis and testes are internal. Nipples can be retracted to reduce drag. Size is often the easiest way to tell males and females apart. Sea lion and fur seal males may be five times larger than females by weight. Greater size may give males an advantage in breeding by helping them remain on shore longer where most of the mating takes place. Larger males store more fat, which provides the fuel and water for long fasts. Males from some species of seal have been observed to go without food for 70 days.
During the breeding season, dominant bulls arrive first at the seals’ land gathering area, or colony. They establish territories on sections of beach and keep others away. The dominant bulls force the young bachelor seals to form herds on the periphery of the breeding areas. When the females arrive they gather around the dominant bulls.
sexual dimorphism (in this case size difference)
between male (left) and female (right) California sea lion Male seals fight by charging each other. The fights can last for hours. Small males have little chance of winning and often retreat early to the edge of their territory or a neutral area at the head of the beach. The winners, surrounded by their harem, swagger arrogantly and bellow belligerently.
Dominant bulls end up with a harem of five to 25 cows while defeated males wander the sea alone until the next breeding season when they try again. A particularly large bull may support a harem of 20 or 30 females who are about one third the male's size.
Seal Breeding and Sex
Unlike whales, seals and sea lions have not developed the ability to mate and give birth in the sea. Sea lions and fur seals bear their young on land. Finding a good place to give birth and raise young is not so easy. The place should be smooth and sandy and protected from large waves. The sea floor leading up to it should be sloped to make it easy to come ashore. It should also be safe from land predators either on an island that has no such predator or in an area protected by steep cliffs. Places that meet all these conditions are few and far between, which is why often hundreds or thousands of seals gather at such as places where they do exist.
Most true seals breed on ice rather land. There is always plenty of ice around and so it is not necessary to crowd into one place. Consequently their breeding habits are much different than those of sea lions. Rather than battle males for harems, seal males pursue females individually on the ice.
Crab-eater seals pair off on the ice during the breeding season, each next to a pup. They are not a happy family though with the male pitching in to help his mate protect their young. Rather it is a male waiting for the female to stop suckling their pup so he can mate with her again. The pup is typically born the previous year to a different father. The presence of the new seems to deter challenges. There are rarely battles between males.
Weddell seal baby and mother Most seals copulate out of the water. Harbor seals mate in the water. Copulation takes place just below the water and often it lasts more than an hour. Groups of males wait for the female to enter the water and try to swim out to sea. Males swim near the female and start quivering their neck and producing a rumbling noise that gets louder and louder culminating with a loud crash. It is not clear whether this activity is intended to scare away other males or demonstrate to the female he is strong and powerful.
In any case other male harbor seals often arrive and start singing along with the first-arriving male as if they were part of an a capella group. It is not known why they do this because the first-arriving make usually establishes himself as the dominant male. What the other males get out of it is perplexing.
Female seals often do not get pregnant right away after mating. The fertilized egg remains dormant for three months before development begins. The gestation period is nine months. This way female seals can synchronize mating with giving birth, freeing both males and females to spend the rest of the year at sea gorging themselves. The females stay close to the colony where they nurse their young.
Often the cows give birth to a single pup a few weeks after the bulls return from the open sea. The young seal lion are delivered in the territory of the dominant male. A week or so later the male begins mating with females in his harem.
The area where females give birth to their pups is very specific. Some produce pups within ten meters of the same spot year after year, which often times is where they themselves were born. Usually only one pup is born, probably because the young must be fairly large to survive in cold ocean water. Most seals can swim on the day they are born.
Hooded seal milk is 61 percent fat. An eight ounce glass of it contains about 1,400 calories. Seal pups nurse for only four days but gain 45 pounds in that time and build up a layer of fat for protection in cold water. In contrast, rhino milk has two thirds the calories of human milk and is only 0.2 percent fat, one of the lowest of any species. Young rhinos and horses get their energy from sugars rather than fat.
baby Weddell seal After pups are born mothers spend much of their time at sea feeding to support themselves and produce enough milk for their young. Mother seals who return from fishing can tell their pups among the hundreds of other by smell. Most females nurse their young for only short time, sometimes only ten days.
After about two of three months on land the young are taught how to swim by their mothers. A mother nuzzles her pup into the water and lets it struggle until it become exhausted and then puts her flipper under it and to let the pup rest. After about two weeks the pups learn to swim. After that the mothers teach them how to hunt. At the end of the summer the pups head off to sea with their mothers and the bulls go their own way.
On average one in every four seals born in Cape Cross Namibia dies. Sometimes they starve to death when their mother is eaten by a shark. They also die after being born prematurely or trampled to death.
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