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By Phil Myers
Four living species placed in two families make up this order. Sirenians, which are sometimes called sea cows, are large mammals that spend their entire lives in water. Their forelimbs are modified to form flippers, their hindlimbs are reduced to nothing more than a vestigial pelvis, and their tail is enlarged and flattended horizontally to form a fluke or paddle. Sirenians are massive, sometimes weighing over 1150 kg. Their body is streamlined and mostly nearly hairless. Their ears have no pinnae. Their eyes lack obvious eyelids, but are closed by a sphincter-like mechanism. Their bones are unusually dense, a condition called pachyostosis; the extra mass probably helps them remain suspended at or below the surface of the water. Their nostrils are located on top of their snouts and closed by valves. The lips are large and mobile, and they are covered with stiff bristles.
The 
skull of sirenians is unmistakable. The premaxillae are large and deflected downward. Nasals are reduced or absent, and the nasal opening extends posteriorly nearly to the orbits. The dentary is very broad. In the region of the ear, the tympanic bone is semicircular, and the petrosal is massive and only loosely bound to the basicranium. The teeth of manatees and dugongs are unusual, but they are very different from each other and are described in the accounts of each family.
Sirenians are members of the group known as subungulates, thought to be distantly related to hyraxes, elephants, and perhaps, artiodactyls and perissodactyls. Their fossil record goes back to the Eocene, but at that time both families were distinct and specialized for aquatic life, so their origin is likely to have been considerably earlier.
A fifth species of sirenian is extinct. Steller's sea cows (genus Hydrodamalis) were huge sirenians (probably over 6000 kg, the size of African elephants!) related to dugongs. They lived in the Bering sea, where they fed on seaweed (no other mammal feeds exclusively on seaweed). Steller's sea cows were exterminated by sailors in the mid 1700's, shortly after their discovery. The remaining sirenians, manatees and dugongs, are seriously threatened by hunting, habitat degradation, and in the case of manatees, collisions with boats in the shallow coastal areas they prefer.
Sirenians are vegetarians, feeding on a variety of marine algae and higher plants. Members of both families are social, occuring in large aggregations and interacting frequently with one another.
Literature and references cited
Feldhamer, G. A., L. C. Drickamer, S. H. Vessey, and J. F. Merritt. 1999. Mammalogy. Adaptation, Diversity, and Ecology. WCB McGraw-Hill, Boston. xii+563pp.
Rathbun, G. B. 1984. Sirenians. Pp. 537-547 in Anderson, S. and J. K. Jones, Jr. (eds). Orders and Families of Recent Mammals of the World. John Wiley and Sons, N.Y. xii+686 pp.
Savage, R. J. G. and M. R. Long. 1986. Mammal Evolution, an Illustrated Guide. Facts of File Publications, New York. 259 pp.
Vaughan, T. A. 1986. Mammalogy. Third Edition. Saunders College Publishing, Fort Worth. vii+576 pp.
Vaughan, T. A., J. M. Ryan, N. J. Czaplewski. 2000. Mammalogy. Fourth Edition. Saunders College Publishing, Philadelphia. vii+565pp.
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Contributors
Phil Myers (author), Museum of Zoology, University of Michigan.
