Sea lions are large marine mammals in the family Otariidae, sub-order Pinnipedia, order Carnivora, found now along the Pacific and South Atlantic coasts and on many islands of the southern hemisphere. Sea lions may have appeared first on the Pacific shores during the Lower Miocene. They are less fully adapted to aquatic life than are the true seals (family Phocidae of the same sub-order Pinnipedia) and are believed to be evolutionarily more primitive than the seals.
Large male sea lions are about 8.2 ft (2.5 m) long, weigh about 1,144 lb (520 kg), and have a mane on the neck reaching the shoulders. Females are usually less than 6.6 ft (2 m) long and lack a mane. Adults are darker than the young, especially after the third year of life, although some are known to be gray, even pale gold or dull yellow. Newborn sea lions, on the other hand, are brown or dark brown. The fur of sea lions consists of one layer of coarse hair, with little undercoat fur, although a few underhairs may be present. For this reason the pelts of sea lions are valued for leather, not for fur.
Sea lions are often mistaken for seals when seen in zoos or in circuses. Sea lions have small external ears (which are absent in seals) and a short tail (which seals lack). The hind limbs of sea lions can be turned forward to aid with locomotion on land (which seals cannot do). In the water, sea lions use the front flippers for low-speed swimming and the hind flippers to swim faster.
Sea lions have a total of 34-38 teeth. The first and second upper incisors are small and divided by a deep groove into two cusps, and the third, outer, upper incisor is canine-like. The canine teeth are large, conical, pointed, and recurved. The premolars and molars are similar, with one main cup. The number of upper molars varies within and among the different genera of the otarids. The skull is somewhat elongated and rounded, but quite bear-like.
Sea lion eyes are protected from blowing sand by the third eyelid (nictitating membrane). Sea lions lack tear ducts, and their tears may be seen running down their face. The whiskers of sea lions are particularly sensitive.
The best known species of sea lion include Zalophus californianus, of which there are three isolated populations along the coast of California and in Japan. Otaria byronia is a species found in South America; Neophoca cinerea is a species of Australian sea lion confined to the waters west of Adelaide, while N. hookeri is found around the coast of New Zealand. Eumetopias jubatus is the northern or Steller sea lion found from northern California to Alaska.
The diet of sea lions has been studied by observing feeding directly by examining the stomach contents, regurgitated food, and feces. California sea lions feed mostly on fish such as hake or herring as well as on squid and octopus. The less common Steller sea lion on the coast of northern California and Oregon eats flatfish and rockfish, but in Alaskan waters it also eats sculpin and occasionally salmon. Fragments of crabs from the Pribilof Islands were found in stomachs of sea lions from that area, together with shrimp and common bivalve mollusks. Sea lions are known to accumulate as much as 35 lb (16 kg) of food in their stomach. The New Zealand sea lion (Neophoca hookeri) was reported to feed on penguins. Harem bulls (except perhaps those of the genus Neophoca) do not feed at all during the breeding season.
California sea lions are the trained animals of circuses and old time vaudeville. The feeding of sea lions have a great fascination for zoo visitors, but many sea lions fall victim to objects dropped into their pools, which they tend to swallow. A documented sea lion deaths was attributed to swallowing many stones weighing a total of 60 lb (27.3 kg). Other deaths were due to swallowing fallen leaves, which the animal could not digest. Although a few stones in a sea lion's stomach are not abnormal, animals kept within narrow confines may experience serious problems. A California sea lion born in a zoo was unable to feed itself at the age of ten months and it had to be captured each day to be fed. As a consequence it suffered a torn diaphragm and a fatal pleuroperitoneal hemorrhage. In recent years great progress has been made in the management of zoological parks and public aquariums which permits sea lions and other marine mammals to live for many years.
Outside of the breeding season, sea lions live in large apparently unorganized herds, but with the approach of summer they separate into breeding and non-breeding herds. The breeding herd consists of harem bulls, sexually mature cows, and newborn pups. Cows usually mature sexually about the end of the fourth year. The average harem consists of one bull with nine females. Bulls identify themselves by barking, advertise their location, declaring social status, or warning potential intruders.
The non-breeding sites where sea lions come out of the water are called hauling grounds. California sea lions gather in breeding sites, called rookeries, in May and August. Adult females stay most of the year at the breeding sites.
Copulation occurs predominantly on land. Gestation takes about 330 days and the females breed soon after the young (usually one, rarely two) are born. California and Stellar sea lion pups may suckle beyond their first year. Only the mother cares for the young which usually cannot swim for about two weeks.
The pups at birth are 30 in (76.9 cm) long and weigh 12.5 lb (5.7 kg). At six months they weigh 60 lb (27.3 kg). California sea lion milk has 35% fat and 13% protein, compared with cow's milk which has 3.45% fat and 3.3% protein. When the pup is born the mother makes loud trumpeting barks, and the pup answers with tiny bleats. They repeat and learn each other's sounds. After four days the mother goes to sea to find food and when she returns she calls and finds her own pup: they touch, sniff, rub noses, and recognize each other by their odor. Newborn sea lions have temporary teeth, which are replaced at four months. The teeth of sea lions are not used to chew food, which is swallowed whole. Estimates of the age of sea lions in the wild are based on the condition and size of their teeth. It is believed that in their natural habitat sea lions live about 15 years, while in captivity they may live up to 30 years. Food needs are relatively high: a one-yearold eats 5.1-9.9 lb (2.3-4.5 kg) of food daily, and an adult female consumes from 25-60 lb (11.4-27.2 kg).
Research on the social behavior of sea lions has been carried out both in their natural habitat and under laboratory conditions. California and Steller sea lions, especially the younger ones, display social interactions characterized by playful activities which take up about one third of their time. Otherwise they rest, often in contact with four or five larger animals. Young California sea lions exhibit manipulative play, tossing and retrieving small rocks or bits of debris. In the process they produce a variety of sounds, including barks, clicks, bangs, buzzes, and growls. All these sounds appear to have a social function. Sometimes they relate to a dominant-subordinate relation with a larger male who may be chasing, intimidating, and restricting the movement of a smaller male, especially when there is an incentive such as food, resting position, swimming pool space, or females. Aerial barking is typical of larger males to achieve dominance over the younger ones. Dominant or alpha animals occur in a sea lion group, and dominant and agonistic behavior has been extensively studied. Sea lions were also the first animals studied to determine the characteristics by which zoo animals recognize their keepers; this research was done in the early 1930s.
The diving performance of sea lions has been studied extensively. Diving vertebrates are known to exhibit bradycardia (a distinct slowing of the heart rate) during rapid submersion. At the moment of diving the nostrils are shut, and bradycardia can be produced even on land, without diving, by closing the nostrils. California sea lions have been trained to retrieve underwater rings placed at different depths and attached to a buoy in such a way as to determine that the animal reached the target. They were also trained to push signal arrays, and have returned in answer to the signal of a small waterproof strobe light. With appropriate training, taking only a few months, any lake, river, and even open sea are suitable test sites.
California sea lions may swim at speeds of 11-24 MPH (17.7-38.7 km/h) and dive to the depth of 1,300 ft (396.2 m). They may stay submerged for 10-15 minutes at a time. When they dive their heart beat may slow from 85 beats per minute on land to only 10 beats per minute. At such time the blood flow is reduced to all parts of the body, except to the brain. These are very important and valuable adaptations. In addition, thick body fat, called blubber, keeps sea lions warm in the cold seas. On land sea lions keep cool in hot weather by lying on wet sand.
Since 1972 the Marine Mammal Protection Act has protected sea lions along the cost of the United States in their breeding sites. There are about 100,000 sea lions in California. In the oceans the chief predators of sea lions are sharks and killer whales. Steller sea lions in the Arctic are also hunted by polar bears. In the United States, sea lions that are injured or ill are taken to Marine Mammal Centers to recover from injury, illness, or malnutrition. When ready to return to their natural habitat, a tag of the National Marine Fisheries Service is attached to one of back flippers. These tags help identify sea lions when rescued again, and to monitor their movements and activities.
See also Walruses.
Evans, Phyllis R. The Sea World Book of Seals and Sea Lions. New York: Harcourt Brace, 1986.
Ridgway, S.H., and R. Harrison, eds. Handbook of Marine Mammals. Vol. 1. The Walrus, Sea Lions, Fur Seals, and Sea Otter. London: Academic Press, 1981.
Riedman, Marianne. The Pinnipeds: Seals, Sea Lions, and Walruses. Berkeley: University of California Press, 1990. Seals and Sea Lions of the World. New York: Facts on File, 1994.