Retention of Sweet Taste in Sirenians Reflects Ecological Adaptation in Marine Mammals.

Taste perception is vital for animal survival. Among marine mammals, whales and pinnipeds have lost all sweet and umami taste receptor genes, whereas sirenians (manatees and dugongs) have retained them. The leading hypotheses for taste loss in marine mammals involve feeding behavior, dietary shifts, and high sodium concentrations. To characterize the evolution of sweet taste in sirenians, we analyzed the complete repertoire of sweet taste receptor genes (Tas1r2 and Tas1r3) and their coding sequences in 53 mammal species. Among marine mammals, only sirenians possessed intact sequences for both genes. Sequence analyses showed that both genes in sirenians are under strong purifying selection, comparable to that in most terrestrial mammals. Functional assays revealed that sirenians respond to natural sugars, but in species-specific ways: Manatee receptors responded to both sucrose and fructose, whereas dugong receptors responded only to sucrose with lower sensitivity. Our findings provide the first evidence for the retention of natural sugar detection in sirenians and that this ability differs between species. Thus, sirenians are the only marine mammals known to have functional sweet taste, indicating that taste loss in other marine mammals is more likely linked to feeding behavior and diet rather than high sodium concentrations. The functional differences between manatees and dugongs likely reflect dietary divergence and ecological niche separation.