Aquatic AnimalsNow let’s turn to thermoregulation by aquatic endotherms, wherethe aquatic environment limits the possible ways organismscan regulate their body temperatures. Why is that? First, as wehave seen, the capacity of water to absorb heat energy withoutchanging temperature is about 3,000 times that of air. Second,116 Section II Adaptations to the Environmentconductive and convective heat losses to water are much morerapid than to air, over 20 times faster in still water and up to100 times faster in moving water. Thus, the aquatic organism issurrounded by a vast heat sink. The potential for heat loss to thisheat sink is very great, particularly for gill-breathing species thatmust expose a large respiratory surface in order to extract suf-ficient oxygen from water. In the face of these environmental dif-ficulties, only a few aquatic species are truly endothermic.Aquatic birds and mammals, such as penguins, seals,and whales, can be endothermic in an aquatic environmentfor two major reasons: First, they are all air breathers anddo not expose a large respiratory surface to the surroundingwater. Second, many endothermic aquatic animals, includingpenguins, seals, and whales, are well insulated from the heat-sapping external environment by a thick layer of fat whileothers, such as the sea otter, are insulated by a layer of fur thattraps air. The parts of these animals that are not well insulated,principally appendages, are outfitted with countercurrent heatexchangers, vascular structures that reduce the rate of heatloss to the surrounding aquatic environment. Figure 5.25 dia-grams the structure and functioning of a countercurrent heatexchanger in the flipper of a dolphin.The lateral swimming muscles of endothermic fish, suchas tunas and white sharks, are also well supplied with bloodvessels that function as countercurrent heat exchangers. Theseheat exchangers heat cool arterial blood as it carries oxygento the lateral swimming muscles, and by the time this blooddelivers its supply of oxygen and nutrients it has been heatedto the same temperature as the active muscles. On the returntrip the heat in this warm blood is used to heat the newly arriv-ing blood and so, when blood exits the swimming muscles, itis again approximately the same temperature as the surround-ing water. The countercurrent heat exchangers of tuna areefficient enough at conserving heat that these fish can elevatethe temperature of their swimming muscles up to 148 C abovethe temperature of the surrounding water. The anatomy of thecountercurrent heat exchanger in bluefin tuna muscles is pre-sented in figure 5.26 .Francis Carey and his colleagues (Carey 1973) implanteddevices that would measure and transmit the temperature ofthe muscles of bluefin tuna and of the surrounding water.Their tracking boat could usually follow a released fish car-rying a temperature-sensing implant for a few hours, whichprovided enough time to collect data that revealed a great dealabout their temperature relations. As one of the monitored fishswam through water varying in temperature from 78 to 148 C,the temperature of its swimming muscles remained a constant24 8C. These results, shown in figure 5.27, demonstrate thata bluefin tuna can maintain a remarkably constant muscletemperature even in the face of substantial variation in watertemperature. More recent work has shown that other organs,such as the stomach, of bluefin tuna vary in temperature muchmore than do the swimming muscles (Stevens, Kanwisher,and Carey 2000).Now, let’s move from the sea and the giant bluefin tuna,which can reach up to 1,000 kg, to land, where we find someof the smallest endotherm