Dolphins are also strictly aquatic and therefore very similar to sea cows in their ecology and swimming mode of locomotion. However, they are very distantly related to sea cows.

The shape of the osseous moulding of the inner ear, the organ of hearing and balance (represented here by a mouse deer ear, with landmarks that capture the shape). Inner ear shape was compared among a total of 40 mammals in the study by Grunstra et al. Sea cows, golden moles, tenrecs and tree hyraxes all belong to Afrotheria, a morphologically and ecologically very diverse group of mammals. The shape of the inner ear for a given afrotherian species is more similar to distantly related non-afrotherian mammals sharing the same environment (for example, sea cows and dolphins, which are both strictly aquatic) than to more closely related species (for example, sea cows and elephants).