We thank M. Johnson, H. Marsh, and S. Searle, Sea World Orlando for the sirenian sera, J.L. Patton for permission to sample specimens in the Museum of Vertebrate Zoology, G. Scheuenstuhl for technical assistance, A. Wilson for laboratory facilities, and D. Domning for comments on the sirenian fossil record. The work was supported in part by grants to JML from the Wenner-Gren Foundation for Anthropological Research and NSF.