Dugongs and manatees
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Dugongs and Manatees. Sirenia. Two Families, Four E xtant Species. Dugongidae Dugong Steller’s Sea Cow (hunted to extinction) Trichechidae Amazonian Manatee West Indian Manatee West African Manatee. Conservation. All Sirenian populations declining over past 200 years

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Dugongs and Manatees


Two Families, Four Extant Species

  • Dugongidae

    • Dugong

    • Steller’s Sea Cow

      (hunted to extinction)

  • Trichechidae

    • Amazonian Manatee

    • West Indian Manatee

    • West African Manatee

  • Conservation

    • All Sirenian populations declining over past 200 years

    • Primary cause of decline is humans- boat collisions, fishing gear

    • Florida Manatee special concern in U.S.

    Aquatic Adaptations

    • Forelimbs modified as flippers

    • Hindlimbs reduced to vestigial pelvis

    • Paddle-like tail

    • Pachyostosis: thick, heavy bones

    • Nostrils closed by valves

    • Large, bristly lips

    Diet and Distribution

    • Feed on seagrass (low quality)

      • “sea cows”- think of them as underwater grazers

  • Energy expenditure needs to be low in order to profit from seagrass

    • Slow moving

  • Low metabolism needed- unable to compensate for heat loss in cold waters

  • Restricts Sirenia to tropical waters

  • Benefits of Large Size

    • Greater mobility

      • Blimp-like

      • Inertia- holding up to water currents

  • Process large quantities of low quality food

  • Reduce surface to volume ratio

    • Reduces heat loss

  • Relationships

    Sirenians belong to a group of mammals called “subungulates”:

    1) Sirenia

    2) Proboscidea (elephants)

    3) Hyracoidea (hyraxes)


    • Unique dental characteristics

      • Teeth replaced horizontally in Sirenia and Proboscidea

    • No clavicle, claws, or hooves

    • Herbivores

    • Non-ruminating, hindgut fermentation

    Sirenians are more closely related to elephants than to whales and dolphins

    Encephalization Quotients and Life-history Traits in the Sirenia

    • What is the relative brain size in recent (modern) Sirenia compared to other mammals?

    • How are life history traits linked to evolution of brain size?


    • Relative brain size calculated for:

      • Florida manatee

      • Dugong

      • Steller’s sea cow

  • Values compared to those of other mammal species previously calculated

  • Results

    • Sirenians have an exceptionally small brain to body ratio- among the lowest of recent mammals

    • However, Sirenians have many life history traits associated with large brains

      • Long gestation (1 yr)

      • Sexual maturity (2-5 yrs)

      • Long-lived (50-60 yrs)

    • Brain size usually associated with ecological and behavioral traits in mammals:

      • Exceptions, not black and white

  • Then why do Sirenians have small brains?

  • Hypotheses

    • Low metabolic rates may explain small brain

      • Less energy available during fetal development

      • Brain is expensive

  • Selection for genes that affect body size, but not brain size at certain growth stages

    • Prolonged postnatal growth (brain matures much earlier)

    • Pleiotrophic effects no longer present

  • Three Most Important Ideas

    1. Evolutionary history

    2. Relationship between diet, size, and distribution

    3. Association between brain size and life history traits and how Sirenia differs from what we might expect


    • Folkens, P.A., R.R. Reeves, B.S. Stewart, P.J. Clapham, and J.A. Powell. 2008. Guide to Marine Mammals of the World. Alfred A. Knopf, Inc., New York, USA and Random House of Canada, Limited, Toronto, Canada.

    • Myers, P. 2000. "Sirenia" (On-line), Animal Diversity Web. Accessed November 23, 2010 at http://animaldiversity.ummz.umich. edu/site/accounts/information/Sirenia.html.

    • O’Shea, T.J., and R.L. Reep. 1990. Encephalization quotients and life-history traits in the Sirenia. Journal of Mammalogy 71:534-543.

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