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

Abstract

Mammals re-entered the oceans less than 60 million years ago. The transition from a terrestrial to an aquatic lifestyle required extreme morphological and behavioural modi¢cations concomitant with fundamentally di¡erent locomotor mechanisms for moving on land and through water. Energetic transport costs typically re£ect such di¡erent locomotor modes, but can not be discerned from the fossil record. In this study the energetic challenges associated with changing from terrestrial to aquatic locomotion in primitive marine mammals are examined by comparing the transport, maintenance and locomotor costs of extant mammals varying in degree of aquatic specialization. The results indicate that running and swimming specialists have converged on an energetic optimum for locomotion. An allometric expression, COT TOT 7.79 mass À0X29 (r 2 0.83, n 6 species), describes the total cost of transport in J kg À1 m À1 for swimming marine mammals ranging in size from 21kg to 15 000 kg. This relation is indistinguishable from that describing total transport costs in running mammals. In contrast, the transitional lifestyle of semi-aquatic mammals, similar to that of ancestral marine mammals, incurs costs that are 2.4^5.1 times higher than locomotor specialists. These patterns suggest that primitive marine mammals confronted an energetic hurdle before returning to costs reminiscent of their terrestrial ancestry, and may have reached an evolutionary limit for energetic optimization during swimming.

Exercise at depth alters bradycardia and incidence of cardiac anomalies in deep-diving marine mammals

“…What is surprising from our study is the frequency of occurrence of these cardiac anomalies and the similarity of the response to those of submerged terrestrial mammals by two different evolutionary lineages of marine mammals that are otherwise adapted for underwater activity. Our study shows that 50 million years of physiological evolution along the land-to-sea transition of marine mammals 32 may not have completely solved the problem of balancing cardiac responses for underwater exercise. Rather, the results suggest that the primary adaptation for diving by marine mammals supports normal cardiac function during species-specific routine dive depths.…”

The rate of metabolism in marine animals: environmental constraints, ecological demands and energetic opportunities

“…Such convergence has been thoroughly described between various groups of fish ( Farrell 1991;Dickson 1995Dickson , 1996Bernal et al 2001;Block & Stevens 2001;Donley et al 2004) as well as between fish and cephalopods (O'Dor & Webber 1986;Packard 1972;Pö rtner 2002;Webber et al 2000). Although less data exist, a case could also be made for convergence towards high metabolic and locomotory capacity in marine mammals ( Williams 1999), pelagic decapod crustaceans (Quetin et al 1994;Childress 1995) and marine reptiles (Humphries & Ruxton 2002).…”

Dolphin social intelligence: complex alliance relationships in bottlenose dolphins and a consideration of selective environments for extreme brain size evolution in mammals

“…Two factors might mitigate this problem for dolphins. First, dolphin travel costs are very low compared with terrestrial mammals (Williams 1999), so the costs of grouping should be substantially less (Connor et al 1998;Connor 2000). More subtly, slight increases in grouping costs might be offset by a slight increase in some benefit of grouping, producing a benefit/cost ratio that is similar across a range of group sizes .…”