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Ommatophoca rossiiRoss seal

By Barbara Lundrigan and Amy Kamarainen

Geographic Range

Ross Seals are unique in that they are the only Antarctic seal whose range is restricted to the Antarctic seas, and they have never been documented in extra-polar regions (Allen 1942). These seals are circumpolar, with most individuals found on the pack ice off the shores of Antarctica, with their range extending no farther than 60° S latitude (King 1990). (Allen, 1942; King, 1990)

Habitat

Ross seals are associated with areas of medium to dense pack ice (Spettstoesser et al. 2000). The areas in which they dwell are often remote and hard to navigate. This leads to a lack of information about the specific habitats they are prone to utilize. Observational accounts recorded in Spettstoesser et al. (2000) make an initial attempt to determine specific habitat use, however data remain vague and anecdotal. (Splettstoesser, et al., 2000)

  • Terrestrial Biomes
  • icecap

Physical Description

Ross seals are the smallest seals of the Antarctic region, with a thick neck and a slender body. Members of this species have short body hairs, with the shortest hair and vibrissae of any phocid. They are dark brown on their dorsal surface and their ventral surface is silvery; spots and streaks frequently mark the head, neck, and flank. During the summer, unmoulted seals are tan to brownish, with moult occurring in January. Males average smaller than females, from 168 to 208 cm long and weighting 129 to 216 kg. Females measure from 190 to 250 cm long and weigh between 159 and 204 kg. Ross seals can easily be distinguished from closely related seals by their disproportionately large eyes (70mm in diameter). The large eye sockets in the skull are a good character by which a Ross seal can be identified (King 1990).

  • Sexual Dimorphism
  • female larger
  • Range mass
    150 to 215 kg
    330.40 to 473.57 lb
  • Range length
    168 to 250 cm
    66.14 to 98.43 in

Reproduction

Little is known about mating in Ross seals.

Females become sexually mature at 2 to 4 years of age, while males can reproduce for the first time between ages 3 and 4. Ross seals mate in early December, but implantation is delayed until early March. Pupping season occurs in early November, after a 9 month gestation period. A typical male weighs 16.5 kg at birth and nurses for 4 to 6 weeks. Weaning is complete around mid-December, approximately 6 weeks after birth (Skinner 1984). After 15 days of nursing pups reach a weight of about 75 kg.

  • Breeding season
    Breeding occurs in December, pups are born in early November.
  • Average number of offspring
    1
  • Average number of offspring
    1
    AnAge
  • Average gestation period
    9 months
  • Average gestation period
    228 days
    AnAge
  • Range weaning age
    4 to 6 weeks
  • Range age at sexual or reproductive maturity (female)
    2 to 4 years
  • Range age at sexual or reproductive maturity (male)
    2 to 4 years

Young Ross seals develop quickly once born, gaining weight rapidly from their mother's rich milk. Once they are weaned they become independent from their mother.

  • Parental Investment
  • precocial
  • pre-fertilization
    • provisioning
    • protecting
      • female
  • pre-hatching/birth
    • provisioning
      • female
    • protecting
      • female
  • pre-weaning/fledging
    • provisioning
      • female

Lifespan/Longevity

Ross seal males have been known to reach 21 years, while the oldest female known was 19 years old (King 1990).

  • Range lifespan
    Status: wild
    21 (high) years

Behavior

Ross seals are thought to be solitary and sparsely distributed across pack ice habitats (Spettstoesser 2000). This is because only 3% - 9% are seen in pairs. However, it is thought that they may be more social than they appear because lone seals on ice are often associated with diving seals beneath the surface of the ice (King 1990).

Ross seals may establish territories underwater through vocalizations (Nowak, 1991). (Splettstoesser, et al., 2000)

Communication and Perception

Ross seals use vocalizations to communicate with other seals. (Nowak, 1991)

Food Habits

Squid beaks and fish remains have been found in the guts of Ross seals (Skinner 1984). Studies have shown the diet to consist of approximately 64% cephalopods, 22% fish, and 14% other invertebrates (Oritsland 1977). (Oritsland, 1977; Skinner, 1984)

  • Animal Foods
  • fish
  • mollusks
  • aquatic crustaceans

Predation

Ross seals are thought to have no predators since typical seal predators, such as killer whales and leopard seals, are rarely found in habitats utilized by Ross seals (Skinner 1984). (Skinner, 1984)

Ecosystem Roles

The role of Ross seals in the ecosystem has not yet been determined, however, they are important predators on fish and cephalopods.

Economic Importance for Humans: Positive

Aside from their role in healthy Antarctic ecosystems, there is no established positive economic importance for humans.

Economic Importance for Humans: Negative

Habitat occupied by Ross seals is accessible only by ice breaker or aircraft, therefore they have little direct economic importance. Also, the remote location of their habitat makes them a poor potential tourist attraction.

Conservation Status

Ross seals are thought to be the least abundant seal in Antarctica and recent estimates suggest that the population may be approximately 220,000. However, these estimates are little more than guesses since so much is still unknown about Ross seal distribution and behavioral patterns. Exploitation of this rare seal species is not likely due to the remoteness of its preferred habitat, yet Ross seals are protected under the Convention for the Conservation of Antarctic Seals (King 1990).

Contributors

Barbara Lundrigan (author), Michigan State University, Amy Kamarainen (author), Michigan State University.

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

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

acoustic

uses sound to communicate

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

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.

fertilization

union of egg and spermatozoan

internal fertilization

fertilization takes place within the female's body

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).

molluscivore

eats mollusks, members of Phylum Mollusca

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.

nomadic

generally wanders from place to place, usually within a well-defined range.

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.

seasonal breeding

breeding is confined to a particular season

sexual

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

solitary

lives alone

tactile

uses touch to communicate

territorial

defends an area within the home range, occupied by a single animals or group of animals of the same species and held through overt defense, display, or advertisement

visual

uses sight 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

Allen, G. 1942. Extinct and vanishing mammals of the western hemisphere. American Committee for International Wild Life Protection.

King, C. 1990. The Handbook of New Zealand Mammals. Oxford University Press.

Nowak, R. 1991. Walker's Mammals of the World. Baltimore: The Johns Hopkins University Press.

Oritsland, T. 1977. Food Consumption of Seals in the Antarctic Pack Ice. Pp. 749-768 in G Llano, ed. Adaptation within Antarctic Ecosystems. Washington, D.C.: Smithsonian Institution.

Skinner, J. 1984. Research on the Ross Seal, *Ommatophoca rossii*, in the King Haakon VII Sea, Antarctica. South African Journal of Science, 80: 30-31.

Splettstoesser, J., M. Gavrilo, C. Field, C. Field, P. Harrison. 2000. Notes on Antarctic wildlife: Ross Seals, *Ommatophoca rossii*, and emperor penguins, *Aptenodytes forsteri*. New Zealand Journal of Zoology, 27: 137-142.

Wartzok, D., D. Ketten. 1999. Marine Mammal Sensory Systems. Pp. 131 in J Reynolds, S Rommel, eds. Biology of Marine Mammals. Washington, D.C.: Smithsonian Institution Press.

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