Manatee (Sea cow): Sirenians

Family Trichechidae

Introduction

The Sirenia are commonly referred to as sea-cows or sirenians, and are an order of aquatic herbivorous mammals that are found living in rivers, estuaries, marine wetlands, coastal marine waters and swamps. The Sirenia currently comprise two distinct families: Dugongidae (the dugong and the now extinct Steller's sea cow) and Trichechidae (manatees, namely the Amazonian manatee, West Indian manatee, and West African manatee) with a total of four species.

Sirenians evolved in the Eocene which is approximately 50 million years ago and there are two extinct families the Protosirenidae (Eocene sirenians) and Prorastomidae (terrestrial sirenians).

Sirenians are strictly herbivorous, and probably arose during the Eocene (50 Ma) near the ancient Tethys Sea between Africa and Europe.  They have large stout bodies, down-turned snouts, short, rounded paddle-like flippers, and a horizontal tail fluke. Sirenians are classified in the clade Paenungulata, and are more closely related to elephants and hyraxes than to other marine mammals. The closest living relatives of the Sirenians are the elephants (Proboscidea) and there is strong molecular evidence that Sirenians belong to the clade Afrotheria (Rainey et al., 1984; Seiffert, 2007).

The Sirenians show a number of sensory and motor adaptations to their diet of sea-grasses.  These include exquisitely sensitive peri-oral bristles (vibrissae) which can be used to explore the sensory environment below the large snout (Reep et al., 2001).  Facial muscles can move the peri-oral bristles to bring them into contact with the feeding substrate and grasp plants (oripulation).  Taste receptors are numerous along the lateral margins of the tongue, but olfaction is poor.  The ciliary muscle of the eye is poorly developed, so focussing on near objects is limited and vision is far-sighted, but colour vision is dichromatic.  Manatee use vocalizations in social interactions (Bullock et al., 1980).