Otariidae

(vertebrate zoology) The sea lions, a family of carnivorous mammals in the superfamily Canoidea.

(Otariidae)

Class: Mammalia

Order: Carnivora

Suborder: Pinnipedia

Family: Otariidae

Thumbnail description
Medium- to large-sized pinnipeds that have large front flippers for underwater propulsion, a dog-like head, and the ability to walk or run on all fours on land

Size
Males range from 154 lb (70 kg) (Galápagos fur seal) to 2,469 lb (1,120 kg) (Steller sea lion); females range from 77 lb (35 kg) (Galápagos fur seal) to 772 lb (350 kg) (Steller sea lion); neonates range from 11 to 51 lb (5–23 kg)

Number of genera, species
7 genera; 15 species

Habitat
Feed in coastal or deep ocean areas, and they breed on sandy and rocky substrate, mostly on islands

Conservation status
Endangered: 1 species; Vulnerable: 5 species

Distribution
Bering Sea, north and south Pacific Oceans, south Atlantic Ocean, southern Indian Ocean, Southern Ocean; they occupy subpolar, temperate, and equatorial waters

Evolution and systematics

The most recent view is that all pinnipeds (walruses, true seals, and eared seals) had a single evolutionary origin. However, this should be considered a provisional view because the evolutionary sequences for these three groups are still incompletely known. Pinnipeds are most closely related to the arctoid carnivores, especially bear-like and racoon-like mammals. The current debate is whether walruses are more closely related to the true seals or the eared seals. The eared seals arose in the late early Miocene (20–15 million years ago) in the North Pacific. From there they moved south and diversified in the Southern Hemisphere where most species now live. Fur seals are considered to be older than sea lions. Callorhinus (northern fur seal) is the genus having the oldest lineage. Otariids are not perfectly separable into sea lions and fur seals; the Afro-Australian fur seal is intermediate between the two in behavior and anatomy.

Physical characteristics

All eared seals are sexually dimorphic. Males are two to four times larger, have proportionately larger heads, necks, and chests (related to fighting), and may have a wider range of colors than females. Otariids as a group are somewhat smaller than true seals as a group. The head is dog-like, and both sexes have sharp, dog-like canine teeth. The eyes are large, irises are brown, and the pupils usually close to a pinhole in bright light. The postcanine teeth, a series of intermeshing points, are adapted for seizing prey, not for shearing or chewing. All species have small, cone-like external ears (hence the scientific name), unlike the true seals and walrus which have only an external auditory meatus (passageway). The vibrissae are well developed, white in adults, and 3–18 in (8–45 cm) long. The flippers are black and either hairless or have sparse hair. The pectoral muscles are well developed in both sexes and provide fore-flipper propulsion. Sea lions (Otariinae five genera, six species) have a pelage composed only of coarse guard hairs. The females are usually brown to light tan, whereas the males may vary from almost white to black. Fur seals (Arctocephalinae, two genera, nine species, two subspecies) have a pelage composed of guard hairs emerging from a dense, fine, brown, usually unseen underfur. Fur seal females are usually grizzled gray (black when wet) with light bellies, and males vary from white to reddish to black, depending on the species. Pups are usually black. Females make loud, prolonged calls related to finding their young. Males make a variety of calls, including repeated pulses (like barks), and prolonged calls.

Distribution

Four genera of otariids still inhabit the ancestral home of this family, the north Pacific Ocean (northern fur seal, Callorhinus ursinus; Steller sea lion, Eumetopias jubatus; California sea lion, Zalophus californianus; Guadalupe fur seal, Arctocephalus townsendi). All four co-occur only at San Miguel Island, California. Two species (Galápagos fur seal, A. galapagoensis, and sea lion, Z. wollebaeki) live at the equator, and two (South American sea lion, Otaria byronia, and fur seal, A. australis) are found along the Pacific and Atlantic coasts of South America and associated islands. One species (Juan Fernández fur seal, A. philipii) is confined to the Juan Fernández Islands off Chile. One species (Cape fur seal, A. pusillus pusillus) occurs around the tip of southern Africa, and a subspecies (Australian fur seal, A. p. doriferus) occurs only in Australia. New Zealand and Australia have one species of sea lion each (Hooker's [Phocarctos hookeri] and Australian sea lion, respectively), and the New Zealand fur seal (A. forsteri) is found in both places. The subantarctic fur seal (A. tropicalis) breeds on a series of islands from the latitude of New Zealand to the Antarctic Convergence. Finally, the Antarctic fur seal (A. gazella) breeds south of the convergence in the Atlantic and Indian Ocean sectors of the Southern Ocean, and extends as far south as the Antarctic Peninsula. The extent to which the distributions of modern otariids reflect the depredations of nineteenth-century sealing, which nearly exterminated some species, is not known.

The at-sea distributions of otariids are poorly known because the main source of data has been satellite transmitters attached to foraging mothers. The two Galápagos species are believed to forage locally. But species like the subantarctic, Juan Fernández, and Guadalupe fur seals are known to forage over moderately large 580 mi² (1,500 km²) areas. The northern fur seal makes an annual migratory loop from breeding colonies as far north as the Bering Sea, south over the open ocean to about 35°N latitude, then north to the breeding islands again along the continental shelf break. The Antarctic fur seal may also migrate, but its pathway is less well known because it was never hunted at sea as the northern fur seal was.

Habitat

All otariids bear young and suckle on land. All mate on land, but at least four species also mate to a small degree in the water. They prefer islands, possibly because these sites afford more freedom from land predators, winds and spray that provide better cooling, and a shorter transit time to offshore feeding areas (or a combination of these benefits) than mainland sites do. Nevertheless, large mainland breeding sites now exist in Africa, Argentina, and Peru, and historically existed along the Pacific Coast of North America. Animals prefer breeding on sand or rock, and tend to avoid mud. Most species breed on beaches or headlands, but when the northern fur seal is numerous its colonies extend several hundred yards (meters) up hillsides. Hooker's sea lion breeds under trees on at least two islands, and often suckles its young under dense brush. The Antarctic and northern fur seals can tolerate snow, but do not habitually breed on ice. Where cold water is available, several species live in very hot climates. Otariids can climb nearly vertical surfaces, an ability that gives them access to land areas that true seals cannot use.

Otariids tend to use very few of the many land sites available to them. All species have a long (more than four month) period of neonate dependency, which forces mothers to alternate between feeding themselves and suckling. Land sites must be located where the feeding/nursing pattern is energetically feasible, and this may reduce the number of sites they can use. True seals wean young at a few days to weeks of age, and suspend feeding while nursing. Thus, they are not constrained to breed on sites that permit commuting to feeding areas, and, as a group, tend to breed in smaller groups at more sites than eared seals.

Fur seals tend to feed in the open ocean; sea lions tend to forage on the continental shelf.

Behavior

In most species, males arrive at breeding sites first and establish territories in habitat that females will later use (usually land but also water areas in hot climates). They maintain territories by fighting and by vocal and visual threat displays. The frequency and intensity of aggression depends largely on the turnover rate of adult males. It is most intense when males first come together as neighbors, and when they are not separated by rocks or ledges. In almost all species, males defend space, not females. In established colonies, territory holders are the largest, most mature males. But in colonization situations, subadult males may defend territories and do all the mating. No evidence has been found for hierarchies, but a lek system has been suggested for the California sea lion. All male otariids fast while defending territories. Northern fur seal males have been recorded fasting for 80 successive days with no food at all.

Reproductive females arrive on shore a day or so before giving birth and settle on a parturition (birthing) site, sometimes the same one used in previous years. Females of all species are gregarious but do not form social bonds or hierarchies that denote permanent status. Sea lion females tend to lie in body contact with each other, but fur seal females usually do not. Females regulate space by making low-level threats. As parturition nears, females of some species may become more aggressive, separate from the group, and appear to hold a territory (Steller sea lion) until estrus approaches. Fostering has been seen in four of the otariid species; the others suckle only their own young.

The attempt by males to herd or control females varies by species. Northern fur seal and South American sea lion males may bite and injure females while herding them, but Steller and Hooker's sea lion males rarely touch or interfere with females. The term "harem" is an inappropriate term to describe social organization in most otariids because it implies an exclusivity in mating access that usually does not exist. Males interact with females frequently as estrus approaches, but may simply approach and mount when the female is fully receptive.

Females mate usually once per estrus, and usually depart on their first foraging trip soon after. When they return the lack of a formal social structure allows them to rejoin the group without aggression. Females interact with their young intensely after birth and after foraging absences, but play with them very little, and protect them only moderately from other females.

The young are precocial. They can swim on the day of birth if forced, and they spontaneously show components of adult behavior, such as copulation and fighting, in the first weeks of life. They gather in groups during the mothers' absence.

Juvenile males gather on the fringes of breeding groups, but juvenile females may join those groups, before or after the peak of mating.

Feeding ecology and diet

As stated previously, fur seals tend to forage on the high seas, and sea lions tend to forage near the coast. Fur seals are fairly small (females average less than 110 lb [50 kg]), and can thrive on large numbers of small fish (like myctophids) and squid that rise to the surface at night in association with the deep scattering layer. The Antarctic fur seal exploits euphausids (krill) that undergo similar vertical migrations. Fur seal females can dive to 656 ft (200 m) or so, which enables the largest of them to feed on the bottom where the continental shelf is broad and productive.

Sea lion females may dive to 1,312 ft (400 m) or so, and generally take small numbers of larger fish and squid that are part of the continental shelf fauna. Unlike fur seals, which suspend feeding in the middle of the day when their prey are at maximum depth, sea lions may forage night and day without stopping. Their greater energy needs may preclude them from exploiting the smaller organisms of the deep scattering layer. Both groups are capable of exploiting silvery schooling fish, such as herring, anchovy, or sardines, wherever they are encountered.

The length of the mothers' feeding absence depends on the foraging environment they use, and therefore tends to be longer for fur seals than for sea lions. The shortest trips are made by the equatorial species that feed locally (a few hours to one day). The longest trips are made by the temperate fur seals (Guadalupe, Juan Fernández, subantarctic, and some populations of the New Zealand fur seal) that feed on organisms of the deep scattering layer. Pup fasting ability varies accordingly.

Most sea lion species (males only) have been observed preying on other species of seals, but fur seals have not. Both groups occasionally eat birds. Otariids are visual feeders; echolocation has not been demonstrated for any species, although researchers have looked for this ability in the laboratory.

Reproductive biology

Otariids are strict annual breeders except for the Australian sea lion which, for reasons yet unknown, follows an unusual 17.5 month cycle. Females of all species bear a single, large pup per season. Twins are rare (1 in 10,000 births) in some species. The uterus is bicornuate (has two sections), which allows females to undergo a postpartum estrus. All otariids, including the Australian sea lion, undergo an embryonic diapause (delayed implantation) for about four months before the embryo implants and begins active growth. Diapause seems to function as a timing mechanism, insuring that births occur near particular dates. The timing can be quite precise; in northern and Antarctic fur seals, individual females give birth within three to four days of the same date (specific to each female) in successive years.

All otariids are polygynous; adult sex ratios of 60 females per adult male have been observed in populations subjected to sealing, but undisturbed sex ratios are typically between 2:1 and 10:1. Polygyny is an apparent result of several factors, including breeding at a few large colonies instead of many small ones, gathering at a predictable time and place, higher agespecific mortality in males than in females, length of the breeding season, male competition rate, and a postpartum estrus. Males may mate 100 or more times during a breeding season, depending on their location relative to females.

Females have a single estrus that occurs less than 10 days postpartum (except in the California sea lion where it is 30 days). Most females mate only once per estrus, rarely twice. If they fail to mate, estrus may last less than two days (36 hours in the northern fur seal). If they mate, estrus may be terminated by coitus (the Whitten effect). Females of most species mate indiscriminately with whichever male is nearest when they enter estrus. Females usually do not mate with juvenile or peripheral males because adult males exclude them from the breeding sites. However, in colonization situations, when males intercept females on their way to feeding (South American and Hooker's sea lions), or in captivity, females of some species readily mate with juvenile or peripheral males. Pregnancy rates may exceed 90% in some age classes of females.

The duration of the breeding season varies with latitude. The interval in which 90% of the females enter estrus varies from 21 days in the Antarctic fur seal to 70 days in the Galápagos fur seal. Many aspects of the social system change when the breeding season is long lasting.

Conservation status

Most otariids were exploited by nineteenth-century sealers to some extent. Fur seals were exploited most heavily because their pelts, with guard hairs removed, were prized for clothing. Sea lions were taken mostly for oil, hides, vibrissae, and organs. After a very long lag time (50–60 years) fur seals are showing better recovery from sealing than sea lions, and at present seem to be thriving somewhat better. The most spectacular recovery was made by the Antarctic fur seal, which was once believed to be extinct and now numbers 3–4 million animals. All fur seals are presently increasing except the northern fur seal. This species recovered from nineteenth-century sealing and reached a peak in 1956, but has been declining ever since. Because the reason for this decline is unknown, there is as much concern for this species as there is for those that have smaller total numbers (Guadalupe, Juan Fernández, South American, subantarctic fur seals) but a good growth rate. The Galápagos fur seal recovered from sealing, but its total numbers appear to be limited now by periodic El Niño events that depress adult survival. The Galápagos, Juan Fernández, Guadalupe, and northern fur seal are all listed as Vulnerable by the IUCN.

Sea lions are harder to summarize. California and South American sea lions are increasing. The Galápagos sea lion seems to be limited by El Niño events like Galápagos fur seals. Hooker's sea lions are of concern because, while never present in large numbers, they have been stable for 20 years and are facing competition with a squid fishery. The Steller sea lion has declined by more than 90% in the last 20 years and may be impacted by a commercial fishery. The Stellar sea lion is listed as Endangered, and Hooker's sea lion is Vulnerable.

Significance to humans

Fur seals were formerly a source of pelts from which clothing was made. From 1760 to about 1900, they were the subjects of intense international trade, treaties, and even small wars. A more controlled form of trade continued until 1985 when the last commercial sealing ended. Since the 1970s otariids have been increasingly seen as competitors of commercial fishing operations, or a problem in fisheries bycatch. All pinnipeds are used as scapegoats to explain declining fisheries catches.

Species accounts

Resources

Berta, A., and L. Sumich. Marine Mammals: Evolutionary Biology. San Diego: Academic Press, 1999.

Gentry, R. L. Behavior and Ecology of the Northern Fur Seal. Princeton: Princeton University Press, 1998.

Perrin, W. F., B. Würsig, and J. G. M. Thewissen. Encyclopedia of Marine Mammals. San Diego: Academic Press, 2002.

Riedman, M. The Pinnipeds: Seals, Sea Lions and Walruses. Berkeley: University of California Press, 1990.

Trillmich, F., and K. A. Ono, eds. Pinnipeds and El Niño. Berlin: Springer-Verlag, 1991.

[Article by: Roger Gentry, PhD]