Steller sea lion
Two other marine vertebrates, the Steller sea lion and the loggerhead turtle, display a rather low number of pseudogenes with respect to cetaceans. The preservation of their OR genes is correlated with their semi-adaptation to an aquatic habitat, since these organisms still perform many functions on land (e.g. mating, basking). Cetaceans do not depend on terrestrial environments, and have thusly lost a large fraction of their OR repertoire. Echolocation was presumable evolved to compensate for this loss.