Marine Vertebrates: Lecture 8 February 6, 2008 Focus: (Sub)Order Pinnipedia




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Marine Vertebrates: Lecture 8

February 6, 2008


  1. Focus: (Sub)Order Pinnipedia

    1. Physical comparison of the three groups:

      1. Overview: trade-off of terrestrial vs. marine existence

      2. Family Phocidae: true seals

        1. no external ear

        2. short foreflippers

        3. hindlimbs held parallel, work together for propulsion

        4. relatively short neck (although still much more than a cetacean, which have highly compressed and fused cervical vertebrae)

        5. Sex organs retractable; testes internal

        6. Best divers are in this group

          • Elephant seals: deepest

      3. Family Otariidae: sea lions, fur seals

        1. small external ear

        2. long foreflippers used for standing (on land), and for propulsion

        3. hindlimbs can be splayed apart, turned under body for support on land

        4. Sex organs retractable; testes external

        5. Flexible neck: predation

        6. Porpoising: less resistance of air than water may increase cruising speed.

          • Possibly also for visual sighting on prey

      4. Family Odobenidae: walruses: 1 species, circumpolar

        1. No external ear

        2. Foreflippers short in comparison to otariids but can stand (sort of) on them

        3. Can turn hindflippers under body

        4. Sex organs retractable; testes internal

        5. Tusks=enlarged upper canines

          • Functions?

    2. Feeding

      1. Major food resources of seals

        1. zooplankton

        2. fish and cephalopods

        3. slow-moving/sessile invertebrates (molluscs and crustaceans)

        4. birds

        5. other pinnipeds

      2. Diets may shift seasonally

      3. Young often eat a different food source, possibly because it is easier to catch.

      4. Individual adults often specialize on a narrower range of food types,

      5. Focus: Crabeater seals

        1. >90% of diet is krill (mostly E. superba)

      6. Focus: Leopard seals

        1. Also have specialized tricuspids for krill-feeding

        2. Probably have the broadest diet/trophic position of any seal, from zooplankton to fish and squid to other seals

        3. Mature and subadult males are responsible for most hunting of other mammals and birds

      7. Focus: Birds and mammals as prey (continued from leopard seal)

        1. The leopard seal is the only phocid to feed on birds and mammals; however…

        2. The walrus is the only pinniped known to feed on whales (narwhals)

      8. Focus: walruses

        1. Feed on muddy bottoms, primarily for clams

          • How do they detect and capture them?

      9. Cooperative feeding with other types of animals (birds, dolphins) often seen.

      10. Gastroliths: rock eating (hypotheses)

    3. Reproduction

      1. All otariids and most phocids mate on land; walruses and some phocids mate in the water

      2. All: delayed implantation

      3. Focus: Phocidae

        1. Usually give birth to one offspring per year after maturation

        2. Females and males may fast or feed little during periods of mating and lactation

        3. Milk is rich in fat, sometimes close to 50%

        4. Young can grow and wean quickly

      4. Focus: Otariidae

        1. Sometimes annual births, sometimes every other year (single pup)

        2. Females feed during lactation

        3. Milk is less fatty

        4. Pups grow more slowly

      5. Focus: Odobenidae

        1. Births every other year or even longer intervals (single pup)

        2. Females and males feed throughout breeding and lactation, but reduced during northward migration

        3. Milk tends to be less fatty

        4. Slow growth rate of pups


Key references
Hoelzel AR, ed., 2002. Marine Mammal Biology: An Evolutionary Approach. Blackwell Publishing: University of Durham, UK, 432 pages.
Reynolds JE III, Rommel SA, 1999. Biology of Marine Mammals. Smithsonian Institution Press: Washington D.C., 578 pages.
Riedman M, 1990. The Pinnipeds: Seals, Sea lions and Walruses. University of California Press: Berkeley, 439 pages.

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