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Lobodon carcinophagacrabeater seal

By Kristin Batty

Geographic Range

The crabeater seal, Lobodon carcinophaga, is primarily found on the coast and pack ice of Antarctica. In the winter months, it may be found on the shores of South America, Australia, South Africa, Tasmania, New Zealand, and various islands surrounding Antarctica. In the winter its range covers about 22 million sq km. (Kooyman 1981, Nowak 1997). (Kooyman, 1981; Nowak, 1997)

Habitat

The crabeater seal lives on the pack ice and the near freezing water surrounding Antarctica.

  • Terrestrial Biomes
  • icecap

Physical Description

After the summer moult, the crabeater seal is dark brown dorsally and grades to blonde ventrally. It has darker brown markings on the back and sides over the paler brown pelage. The flippers are the darkest parts of the body. Its fur slowly changes to blonde throughout the year and it is almost entirely blonde by the summer. In fact it has been called "the white Antarctic seal" (Kooyman 1981). It has a long snout and a fairly slim body compared to other seals. Females are slightly larger on average than males with a length from 216 cm to 241 cm. Males range from 203 cm to 241 cm.

The crabeater seal often has long scars running along the sides of its body. These are most likely inflicted by its major predator, the leopard seal, Hydrurga leptonyx (Siniff and Bengston 1977).

Its teeth are very distinct and have been called "the most complex of any carnivore" (Kooyman 1981). There are several tubercles on each tooth with spaces between them that cut deeply into the tooth. The main cusps of upper and lower teeth fit perfectly together. When the crabeater seal closes its mouth, the only spaces are those between the tubercles. This arrangement probably serves as a sieve through which to strain krill, their primary food source. (Kooyman 1981, Nowak 1997). (Kooyman, 1981; Nowak, 1997; Siniff, et al., 1979)

  • Sexual Dimorphism
  • female larger
  • Range mass
    200 to 300 kg
    440.53 to 660.79 lb
  • Range length
    203 to 241 cm
    79.92 to 94.88 in

Reproduction

Reproduction in the crabeater seal probably takes place on the pack ice surrounding Antarctica in the austral spring, from October to December (Kooyman 1981). Starting in September, a pregnant female occupies a space on the ice floe in which she gives birth and cares for her single pup. A male joins the female in her chosen area just before or just after parturition. He defends the female and the newborn pup. He is, in all likelihood, not the father of the pups. Females come into estrus just after weaning and Siniff et al. (1979) report that the male's only apparent interest is in waiting for the female to be sexually receptive. Males aggressively defend females from other intruding males. It is not clear if the males join the females because of a female cue such as scent or because of the pup. (Kooyman, 1981; Siniff, et al., 1979)

Pups are born weighing approximately 20 kg and gain weight while nursing at about 4.2 kg/day (Shaughnessy and Kerry 1989). Physical contact between the mother and pup during this period is necessary. If either the pup or the mother strays, the other immediately follows. Pups are weaned at about 3 weeks old. It is unclear if physical mechanisms in the mother, such as reduced milk production, cause the weaning or if the defending male drives the pup and mother apart. Throughout the lactation period the male is aggressive towards the female. She defends herself by biting him on the neck and sides. By the end of lactation her body weight may be reduced by half, so she would be unable to defend herself adequately. She becomes sexually receptive shortly after weaning and, unlike most seals, copulation appears to occur on the ice floes instead of in the water (Shaughnessy and Kerry 1981; Siniff et al. 1979)

Gestation lasts about 11 months and probably includes a period of delayed implantation (Nowak 1997). Crabeater seals become sexually mature between 3 and 4 years of age and females may have successful pregnancies between 5 and 25 years old (Bengston and Siniff 1981). (Bengston and Stewart, 1992; Nowak, 1997; Shaugnessy and Kerry, January 1989; Siniff, et al., 1979)

  • Breeding season
    Reproduction in the crabeater seal probably takes place on the pack ice surrounding Antarctica in the austral spring, from October to December.
  • Average number of offspring
    1
  • Average gestation period
    11 months
  • Average weaning age
    3 weeks
  • Average time to independence
    3 weeks
  • Range age at sexual or reproductive maturity (female)
    3 to 4 years
  • Range age at sexual or reproductive maturity (male)
    3 to 4 years
  • Parental Investment
  • precocial
  • pre-fertilization
    • provisioning
    • protecting
      • female
  • pre-hatching/birth
    • provisioning
      • female
    • protecting
      • female
  • pre-weaning/fledging
    • provisioning
      • female
    • protecting
      • female

Behavior

Crabeater seals may be found in large aggregations of up to 1,000 animals, but are usually solitary or in small groups (Siniff 1979). They dive primarily at night and are reported to average 143 daily dives in late February (Nordoy et al. 1995). Once in the water, diving occurs nearly continually for approximately 16 hours. Bengston and Stewart (1992) report several types of diving: foraging dives, traveling dives, and exploratory dives. Most dives are for traveling and are less than a minute long and less than 10 m deep. Foraging dives are slightly deeper, up to 30 m, and appear to very throughout the day, with crepuscular dives being deeper. This is most likely in response to krill distribution. Exploratory dives are the deepest and presumably for navigation as they usually occur just before a traveling or foraging dive. Crabeater seals may use breathing holes created by Weddell seals (Leptonychotes weddellii). Young Weddell seals may even be chased from breathing holes by adult crabeater seals.

At the end of the summer, crabeater seals disperse northward as the ice freezes. It appears that some seals, usually young ones, become disoriented and head farther south over the pack ice. As they are particularly mobile on land for pinnipeds, they may travel hundreds of kilometers inland. These seals almost always die and are well preserved as "mummies" in the ice throughout Antarctica (Stirling and Kooyman 1971). Most seals, however, successfully travel north to oceanic islands, Australia, South America, and even South Africa. (Nordoy, et al., 1995; Siniff, et al., 1979; Stirling and Kooyman, February 1971)

The crabeater seal is probably the fastest pinniped on land, reaching speeds of up to 25 km/hour. When sprinting, it lifts its head high and swings its head from side to side in synchrony with its pelvis. Its foreflippers move alternately across the snow and its hind flippers are lifted off the ground and held together. (Siniff, et al., 1979)

Communication and Perception

Food Habits

The crabeater seal appears to be a misnomer as there is no evidence that it eats crabs. Its primary food is krill, Euphausia superba. It probably also eats other invertebrates. The crabeater seal feeds by swimming through a school of krill with its mouth open, sucking them in and then sieving the water out through its specialized dentition (Kooyman 1981, Nowak 1997). Klages and Cockcroft (1990) report that a captive crabeater seal was able to suck small fish into its mouth at distances of up to 50 cm. They note that this prey is much larger than the krill that it would consume in the wild, and suggest that it could probably suck krill in from a much greater distance. The seal preferred fish smaller than 12 cm and swallowed everything whole, in contrast to many seals which tear their food up with their teeth before swallowing. It was often observed exploring the bottom of its pool and sucking up debris. Klages and Cockcroft suggest that this is an adaption to winter feeding on krill in the Antarctic. At this time of year, krill is mainly found in crevices and caverns. The seal may be able to suck the krill out from these unreachable areas. Feeding probably occurs prinicipally at night (Nowak 1997). (Klages and Cockcroft, 1990; Kooyman, 1981; Nowak, 1997)

  • Primary Diet
  • carnivore
    • eats non-insect arthropods
  • Animal Foods
  • aquatic crustaceans
  • other marine invertebrates

Predation

When the crabeater seal is approached it snorts, hisses, and bares its teeth. If caught, it rolls over several times. This is probably an avoidance tactic developed for its primary predators, the killer whale (Orcinus orca) and the leopard seal (Hydrurga leptonyx) (Stirling and Kooyman 1971). (Stirling and Kooyman, February 1971)

Economic Importance for Humans: Positive

Since crabeater seals occupy a habitat that is fairly inaccessible to humans, there has been very little contact between the two species. There is a report, however, that a young crabeater seal found on the coast of South Africa was easily tamed and trained (Klages and Cockcroft 1990). (Klages and Cockcroft, 1990)

Economic Importance for Humans: Negative

Commercial fisheries have expressed interest in exploiting Antarctica's krill resources. As this is the primary food of the crabeater seal, there are bound to be negative consequences associated with the crabeater seal in this budding industry (Nowak 1997). (Nowak, 1997)

Conservation Status

The crabeater seal is the most numerous species of pinniped in the world, with a population estimated at between 15 and 40 million. Since its habitat is remote, the only concerns for conservation are indirect. Trace amounts of chemicals such as DDT have been found in populations of the crabeater seal, and if the fishing industry decides to use the krill in the Antarctic seas, the major food source of these seals may be severly depleted (Kooyman 1981). Now, however, its numbers appear to be stable. (Kooyman, 1981)

Other Comments

The large population of crabeater seals has been associated with the near-extinction of the large baleen whales. This is because the whales, like the crabeater seal, eat krill. Now there is probably more krill available to the crabeater seal. It is hypothesized that changes in sexual maturity and other reproductive characteristics correlate with large ecological changes such as this. Bengston and Siniff (1981) have shown that the age of sexual maturity of the crabeater has significantly decreased since the 1950s. (Siniff and Bengston, August 1977)

Contributors

Tanya Dewey (editor), Animal Diversity Web.

Kristin Batty (author), University of Michigan-Ann Arbor, Phil Myers (editor), Museum of Zoology, University of Michigan-Ann Arbor.

Glossary

Antarctica

lives on Antarctica, the southernmost continent which sits astride the southern pole.

Atlantic Ocean

the body of water between Africa, Europe, the southern ocean (above 60 degrees south latitude), and the western hemisphere. It is the second largest ocean in the world after the Pacific Ocean.

World Map

Australian

Living in Australia, New Zealand, Tasmania, New Guinea and associated islands.

World Map

Ethiopian

living in sub-Saharan Africa (south of 30 degrees north) and Madagascar.

World Map

Neotropical

living in the southern part of the New World. In other words, Central and South America.

World Map

Pacific Ocean

body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.

World Map

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

carnivore

an animal that mainly eats meat

chemical

uses smells or other chemicals to communicate

coastal

the nearshore aquatic habitats near a coast, or shoreline.

delayed implantation

in mammals, a condition in which a fertilized egg reaches the uterus but delays its implantation in the uterine lining, sometimes for several months.

endothermic

animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

migratory

makes seasonal movements between breeding and wintering grounds

monogamous

Having one mate at a time.

motile

having the capacity to move from one place to another.

natatorial

specialized for swimming

native range

the area in which the animal is naturally found, the region in which it is endemic.

nocturnal

active during the night

oceanic islands

islands that are not part of continental shelf areas, they are not, and have never been, connected to a continental land mass, most typically these are volcanic islands.

pelagic

An aquatic biome consisting of the open ocean, far from land, does not include sea bottom (benthic zone).

polar

the regions of the earth that surround the north and south poles, from the north pole to 60 degrees north and from the south pole to 60 degrees south.

saltwater or marine

mainly lives in oceans, seas, or other bodies of salt water.

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

solitary

lives alone

tactile

uses touch to communicate

viviparous

reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.

young precocial

young are relatively well-developed when born

References

Bengston, J., D. Siniff. 1981. Reproductive aspects of female crabeater seals (Lobodon carcinophagus) along the Antarctic Peninsula. Canadian Journal of Zoology, 59: 92-102.

Bengston, J., B. Stewart. 1992. Diving and haulout behavior of crabeater seals in the Weddell Sea, Antarctica, during March 1986.

Klages, N., V. Cockcroft. 1990. Feeding behaviour of a captive crabeater seal. Polar Biology, 10: 403-404.

Kooyman, G. 1981. Crabeater Seal Lobodon carcinophagus. Pp. vol 2: 221-235 in S Ridgeway, R Harrison, eds. Handbook of Marine Mammals. London: Academic Press.

Nordoy, B., L. Folkow, A. Blix. 1995. Distribution and diving behaviour of crabeater seals (Lobodon carcinophagus) off Queen Maud Land. Polar Biology, 15: 261-268.

Nowak, R. 1997. "Walker's Mammals of the World 5.1 (online): John's Hopkins University Press" (On-line). Accessed May 31, 2002 at http://www.press.jhu.edu/books/walker/.

Shaugnessy, P., K. Kerry. January 1989. Crabeater Seals Lobodon carcinophagus during the breeding season: observations on five groups near enderby land, Antarctica. Marine Mammal Science, 5(1): 68-77.

Siniff, D., J. Bengston. August 1977. Observations and hypotheses concerning the interactions among craveater seals, leopard seals, and killer whales. Journal of Mammalogy, 58 No.3: 414-416.

Siniff, D., I. Stirling, J. Bengston, R. Reichle. 1979. Social and reproductive behavior of crabeater seals (Lobodon carcinophagus) during the austral spring. Canadian Journal of Zoology, 57: 2243-2256.

Stirling, I., G. Kooyman. February 1971. The crabeater seal (Lobodon carcinophagus) in McMurdo Sound, Antarctica, and the origin of mummified seals. Journal of Mammalogy, 52 No.1: 175-180.

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