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7434076 
Journal Article 
Metabolism and Thermoregulation 
Davis, RW; , 
2019 
Springer International Publishing 
Cham 
Marine Mammals 
57-87 
Marine mammals are endothermic homeotherms that spend all or much of their time in water, which has a thermal conductivity that is 23-fold greater than air and with temperatures that can range from −1.6 to 33 °C. Enhanced thermogenesis resulting in an elevated basal metabolic rate (BMR) may be an adaptation to offset heat loss in water, at least in smaller marine mammals. The most important and consistent factor that affects BMR is body mass. Other than manatees, marine mammals have an average BMR that is 2.3-fold higher, although it may approach the allometric prediction for large whales due to the small surface-to-volume ratio. To maintain a stable core body temperature, endothermic mammals must balance heat loss to the environment with heat production. Marine mammals rely on two types of thermal insulation: blubber and fur. Young pinnipeds, especially those living in polar climates, may experience cold stress and exhibit a degree of heterothermy. In many seasonal breeders, the molt and reproductive cycles are interrelated but variable among species, by age classes, and geographically. Marine mammals that use blubber may have the broadest thermal neutral zone and lowest lower critical temperature. For Cetacea and Sirenia, the only behavioral responses available for thermoregulation are to alter activity level or migrate to a different body of water.