The evolution of cost efficient swimming in marine mammals: limits to energetic optimization

Mammals re-entered the oceans less than 60 million years ago. The transitio n from a terrestrial to an aquatic lifestyle required extreme morphological and behavioural modifications concomitant with fundamentally different loc omotor mechanisms for moving on land and through water. Energetic transport costs typically reflect such different locomotor modes, but can not be dis cerned from the fossil record. In this study the energetic challenges assoc iated with changing from terrestrial to aquatic locomotion in primitive mar ine mammals are examined by comparing the transport, maintenance and locomo tor costs of extant mammals varying in degree of aquatic specialization. Th e results indicate that running and swimming specialists have converged on an energetic optimum for locomotion An allometric expression, COTTOT=7.79 m ass(-0.29) (r(2)=0.83, n=6 species), describes the total cost of transport in J kg(-1) m(-1) for swimming marine mammals ranging in size from 21 kg to 15 000 kg. This relation is indistinguishable from that describing total t ransport costs in running mammals. In contrast, the transitional lifestyle of semi-aquatic mammals, similar to that of ancestral marine mammals, incur s costs that are 2.4-5.1 times higher than locomotor specialists. These pat terns suggest that primitive marine mammals confronted an energetic hurdle before returning to costs reminiscent of their terrestrial ancestry, and ma y have reached an evolutionary limit for energetic optimization during swim ming.