True Seals (Phocidae)

(vertebrate zoology) The seals, a pinniped family of carnivoran mammals in the superfamily Canoidea.

(Phocidae)

Class: Mammalia

Order: Carnivora

Suborder: Pinnipedia

Family: Phocidae

Thumbnail description
Large marine mammals that are fusiform in shape; crawls on its stomach on land or ice using either a caterpillar movement (on land) or swishing its rear end from side to side (on ice); propels itself in water by thrusting rear flippers from side to side, fanning the leading flipper and closing the trailing one; foreflippers are short and not used as rudders much in aquatic or terrestrial movement, other than to push off the ground on land; coloration varies from solid color to mottling or spotted with dark on light background or the reverse

Size
Approximately 3–15 ft (1–5 m); approximately 100–5,700 lb (45–2,600 kg)

Number of genera, species
13 genera; 19 species

Habitat
Shallow and deep marine areas, polynyas (open water within large ice masses), drifting ice floes, and island and mainland beaches (sand, cobble and rock)

Conservation status
Extinct: 1 species; Critically Endangered: 1 species; Endangered: 1 species; Vulnerable: 1 species; Near Threatened: 1 species; Data Deficient: 1 species

Distribution
Circumpolar and occupying temperate latitude waters in the Northern and Southern Hemisphere; subtropical for one species in the Pacific Ocean

Evolution and systematics

The family Phocidae is the oldest of the modern day pinnipeds, with the oldest fossil phocid reported from the late Oligocene or about 25–30 million years ago. The current phocids originated in the North Atlantic and consists of two subfamilies, the Monachinae and the Phocinae. The Phocinae contains five genera and the Monchinae six genera. The phocids along with the other two families of current pinnipeds, the otariids (fur seals and sea lions) and odobenids (walrus), appear to have evolved from a single lineage of either a bear-like, otter-like, or raccoon-like ancestor. Most evidence points toward bear or otter-like ancestry but there is conflicting evidence.

Physical characteristics

Phocid seals range from moderate to large size. Most are fusiform in shape, but the leopard seal is almost tubular looking, being longer and more slender. All phocids are characterized by having relatively short hair that lies smooth along the body, thick skin and a thick subcutaneous layer of blubber that can be 5–6 in (11–13 cm) thick to insulate the animal and provide energy stores when not foraging. The foreflippers are small while the rear flippers are large and contain webbing between the digits that expands them into paddles, which provide thrust for swimming. All phocids appear to have no neck; the head blends into the trunk of the body. They have no external pinnae (ear), minimizing structures that would produce drag during swimming. Males of some species such as elephant seals (Mirounga spp.), hooded seals (Cystophora cristata), and gray seals (Halichoerus grypus) have enhanced (elongated or inflatable) snouts that are used in competitive social contexts.

Distribution

The majority (14 species) of phocids are found in the Northern Hemisphere although five species occur south of the equator. In both hemispheres many species are circumpolar or subpolar, there are a few temperate species, such as gray seals, elephant seals, and harbor seals (Phoca vitulina). Three species of monk seals (one of which has gone extinct in the past century) inhabit subtropical or tropical regions of the Pacific, Mediterranean and Caribbean waters. Virtually all major oceans, except the Indian Ocean, contain phocid species. There are even two species found in large inland lakes, the Baikal seal (Phoca sibirica) of Lake Baikal in Siberia and two subspecies of ring seal (Phoca hispida) found in Lake Ladoga and Lake Saimaa of Russia and Finland, respectively. There may also be a subspecies of harbor seal that inhabits inland lakes in the Ungava Peninsula of Quebec, although it is unclear if these animals are completely isolated.

Habitat

All species forage at sea but use either land or ice to produce their pups, to molt, and to rest. Mating occurs on land in a few species and in the water in most; occasional mating on ice has been seen but usually seals that give birth on ice mate in the water.

Land habitats used are variable, including sand, cobble and boulder beaches, rocky outcroppings, and caves (gray seals and Mediterranean monk seals [Monachus monachus]). Some species like gray and harbor seals may use all or most of these different land habitats as well as ice in some locations.

Ice habitats used by phocids include both floe ice and land fast ice. The former is free floating and the size of ice pans is highly variable. The latter consists of large ice masses affixed to land masses and are usually more stable.

Behavior

Most phocid species are gregarious during an annual molting period, when they haul out on land or ice to shed their hair and acquire a new coat. Many species form colonies during an annual breeding period, with species like elephant seals forming large dense harems of over a hundred females and a few males, all lying in contact with one another, to dispersed male-female pairs of hooded or crab-eater seals (Lobodon carcinophagus) a half mile (0.8 km) or more apart on floating ice pans.

Males and females of many species migrate from breeding areas to separate areas used for foraging. In some cases these migrations are associated with seasonal changes in ice patterns. Other species remain in the vicinity of the seasonal breeding areas for feeding or disperse in a more random pattern rather than migrating.

Researchers are only beginning to learn more details about foraging tactics as a result of a few studies that have attached cameras to free-living seals to record the behavior of the seals during foraging. One such study of harbor seals shows that some males use a tactic of digging in the bottom substrate for species that bury themselves in sand while others find and follow schools of fish, picking off individual fish that leave the school and go to the bottom to avoid the seal.

Feeding ecology and diet

There is a strong link between the breeding pattern, body size, and feeding ecology in phocid seals. The relatively large size of phocids allows them to build body stores of fat that fuel breeding without foraging for periods of time (a phenomenon known as capital breeding). Some of the smaller phocids, however, such as the harbor seal, may have to forage some during the breeding period to rear their young successfully because they cannot store enough fat. The ability to store fat and fast during breeding means that foraging grounds can be separated by considerable distances from breeding grounds in these species. Northern elephant seals (Mirounga angustirostris), for example, are known to leave breeding and resting areas to travel about 12,000 mi (20,000 km) during two major foraging periods, after breeding and after molting.

The diets of phocid seals may be varied both within and among species, including varying seasonally as food resources change or as the location of seals change seasonally. Crabeater seals, for example, eat krill (small shrimp-like organisms) almost exclusively, whereas leopard seals eat penguins, other seals, fish, krill, squid, octopus, and other invertebrates. Fish of one sort or another is the primary diet of most phocid species.

To obtain the energy-rich food and dense patches of fish, phocids often have to dive to considerable depths. Elephant seals will dive to depths as deep as about 5,000 ft (1,500 m), staying underwater from 20 to 60 minutes while they forage for squid. Other species, like harbor seals in some locations, regularly forage only at depths of 80–200 ft (25–60 m), whereas at other locations may forage more often at about 650–825 ft (200–250 m). The shallower dives are usually much shorter in duration, lasting more in the range of three to five minutes.

Reproductive biology

All phocids have an annual reproductive cycle in which females give birth during a fairly distinct breeding period. They are particularly noted for short lactation periods, lasting from as little as four days in the hooded seal to as long as two to three months in a couple of species. Females become receptive near the end of lactation or shortly thereafter in all species. Females of all species exhibit delayed implantation or embryonic diapause during which the fertilized ovum suspends development and remains in the uterus without implanting. This is thought to help synchronize parturition among females and produce the highly synchronous breeding seasons found in these species.

The short lactation periods of phocids are associated with fasting or feeding little during this time. Also associated with this is the buildup of extensive blubber layers by females to provide the major nutrients to produce milk for the young. Milk fat content is highest in those species with the shortest lactations, as is pup mass gain. Females may lose over 40% of their mass at the beginning of the breeding season and between 60% and 80% of that loss is in the milk transferred to the pup. Males do not participate in the care of offspring in any species.

Males of all phocids fast or feed little during the period when receptive females are available. Consequently, they too build up extensive blubber layers prior to the mating season to fuel the energy-intense competition for access females. Mass loss as a percentage of initial body mass in males, however, is not as great as it is in females. Among several species for which there are such data, the average percentage loss is between 15% and 35% compared to the values given above for females.

Mating patterns in phocids is best known among the three species that mate on land: gray seals and the northern and southern elephant seals. In these cases, the primary mating tactic is one of defending females directly. Recent studies, which use genetic paternity analyses, show that there may be alternative tactics that are more opportunistic but are also successful, even if to a lesser degree. For example, gray seal males that defend and mate with females before they depart only fertilize about 22% of them and males that capture and mate with departing females, fertilize about 9% of the females they capture. A few studies of a couple of species that mate at sea suggest that males may be more likely to engage in efforts to attract females by displaying than guarding them directly; such a mating system is known as a lek-type system. Females may actually choose their mates in this system, although the evidence for female choice is not strong for any of these species yet.

Females begin producing a single young each year from four to seven years of age, whereas males become sexually mature a couple of years later than females in many species. Furthermore, males do not become socially competitive for several additional years; they may be 10 years old or more before they succeed in mating. Research seems to indicate that both males and females are potentially reproductively active until they die. There is some evidence in a few species to suggest older females may not perform as well as middle-aged females in rearing fat, healthy pups. The youngest and inexperienced mothers may also perform poorly.

Conservation status

The main factor leading to extinction, endangered, or threatened status of phocids has been hunting pressures from humans within the past century or two. More recently, the Hawaiian monk seal (Monachus schauinslandi), which was showing signs of coming back in the mid-1900s, was probably disturbed by military operations on the small islands used for breeding and molting, and produced poor recruitment into the breeding population. Subsequently, signs of environmental regime shifts that have caused a change in carrying capacity may be inhibiting the return of this species. It is currently listed as Endangered. Likewise the Mediterranean monk seal is Critically Endangered, and continuing to decline to near extinction (fewer than 500 seals remaining) despite protection, because of degradation in its environment. Recent disease problems producing mass die-offs can probably be attributed to the poor state of the environment. One species has gone extinct in the past 100 years, the West Indian monk seal (Monachus tropicalis). It is the cousin to the other two monk seals. The cause of this extinction was likely hunting pressure. Additionally, the Caspian seal is listed as Vulnerable, and the Baikal seal is listed as Lower Risk/Near Threatened by the IUCN.

Significance to humans

True seals have been hunted for hundreds of years, serving as a source of food, oil, and hides or furs. In recent years, these products have been boycotted as a result of public pressure against clubbing pups, and the market for these items has nearly vanished. Subsistence hunting of some species occurs on a very small scale. Commercial hunting of at least two species, harp and hooded seals, continues annually, but at a very reduced level. These hunts take many fewer pups than in the past and have a small market for the products. One offshoot of the pressure against hunting has been the evolution of a small eco-tourism industry centered on trips to the pack ice during the harp seal breeding season. As these trips are expensive, the industry is likely to remain small for species such as harp seals. However, for species like the northern elephant seal, which now breeds in accessible mainland colonies, controlled nature programs have become successful educational experiences.

Species accounts

Resources

Berta, Annalisa, and James L. Sumich. Marine Mammals: Evolutionary Biology. New York: Academic Press, 1999.

Blix, Arnoldus S., Lars Walløe, and Øyvind Ultang. Whales, Seals, Fish and Man. Amsterdam: Elsevier, 1995.

Boness, Daryl J. "Life History and Reproductive Strategies." In Marine Mammal Biology, an Evolutionary Approach, edited by A. Rus Hoelzel. Oxford: Blackwell Science, 2002.

King, Judith E. Seals of the World. Ithaca, NY: Comstock, 1983.

Lavigne, David M., and Kit M. Kovacs. Harps & Hoods. Waterloo: University of Waterloo Press, 1988.

Le Boeuf, Burney J. and Richard M. Laws. Elephant Seals. Berkeley: University of California Press, 1994.

Perrin, William F., Bernd Wursig, and J. G. M. Thewissen. The Encyclopedia of Marine Mammals. San Diego: Academic Press, 2002.

Reynolds, John E., III, and Sentiel A. Rommel. Biology of Marine Mammals. Washington, DC: Smithsonian Institution Press, 1994.

Rice, Dale W. Marine Mammals of the World. Lawrence, KS: Allen Press, 1998.

Riedman, Marianne. The Pinnipeds. Berkeley: University of California Press, 1990.

Boness, D. J. "The Evolution of Maternal Care in Pinnipeds." Bio-Science 46 (1996): 1–10.

Kovacs, K. M., and D. M. Lavigne. "Hooded Seal, Cystophora cristata." Mammalian Species 258 (1986): 1–9.

Stewart, B., and Hubner, H. R. "Northern Elephant Seal, Mirounga angustirostris." Mammalian Species 449 (1993): 1–10.

Thomas, J. "Baikal Seal, Phoca sibirica." Mammalian Species 188 (1982): 1–6.

[Article by: Daryl J. Boness, PhD]