Body Size

The influence of environmental factors is size dependent. Small animals have relatively more surface area and relatively less metabolically active tissue with which to generate heat. Therefore, we would expect a small animal, when challenged with cold, to lose heat more rapidly than a larger animal. If such an animal attempted to generate heat to offset the heat loss and maintain Tb, it would require a relatively higher rate of energy utilization. Small endothermic homeotherms, by virtue of their higher metabolic requirements, are therefore dependent on a reliable supply of energy-yielding foods. Exposure to extreme cold and/or a food shortage would place a small endothermic homeotherm under profound metabolic stress. Obvious energetic savings are to be made under such conditions by lowering the thermal set-point for Tb regulation and depressing metabolism (i.e., torpor, Figure 8). Examples of torpor are found in all three infraclasses of mammals and some groups of birds.

On the other hand, large animals have relatively small surface areas and cool slowly if placed in a cold environment. Once warmed larger insects take longer to cool than small organisms of equivalent taxa. Subsequently, they can successfully forage in a cool microhabitat provided that the foraging bout was less than the equilibration time. Large body size and the resulting thermal inertia together with improved ability for heat retention (insulative layers of fat, counter-current heat exchangers, or physiological adjustments to the circulation that limit peripheral heat loss) and behavioral thermoregulation (basking) enable the largest reptiles, the leatherback turtle and estuarine crocodile (both up to 1000 kg), to achieve a high degree of thermal stability (Figure 9). Large estuarine crocodiles, whose large size is also associated with an increased Tb, actually risk overheating from solar radiation during the day. To minimize this risk they spend much of the day in the water and at night are frequently observed on land. Leatherback turtles also have elevated Tb's, from a few degrees above the warm oceanic waters at low latitudes, to as much as 18 °C above environmental temperatures when diving into high-latitude subsurface waters as cold as 0.4 ° C. In the case of leatherback turtles it would appear

Time (days)

Figure 9 Body temperature (red line) and operative temperature (blue line, temperature calculated at the body surface and based on heat inputs) for a 520 kg estuarine crocodile. The large body size of the crocodile together with behavioral adjustments permits a high degree of thermal stability in comparison with daily changes in operative temperature. Modified from Grigg GC, Seebacher F, Beard LA, and Morris D (1998) Thermal relations of very large crocodiles, Crocodylus porosus, free-ranging in a naturalistic situation. Proceedings of the Royal Society of London B 265: 1793-1799.

Time (days)

Figure 9 Body temperature (red line) and operative temperature (blue line, temperature calculated at the body surface and based on heat inputs) for a 520 kg estuarine crocodile. The large body size of the crocodile together with behavioral adjustments permits a high degree of thermal stability in comparison with daily changes in operative temperature. Modified from Grigg GC, Seebacher F, Beard LA, and Morris D (1998) Thermal relations of very large crocodiles, Crocodylus porosus, free-ranging in a naturalistic situation. Proceedings of the Royal Society of London B 265: 1793-1799.

that metabolic rate is higher than predicted and, in association with relatively warm surface waters and improved ability for heat retention, this is sufficient to maintain warm and fairly stable Tb's when foraging in cold waters. Interestingly, it appears that muscle tissue metabolism is independent of temperature in leatherback turtles, thereby freeing muscle activity somewhat from thermal constraints in the environment.

In large, non-lamnid sharks, such as the blue shark, a thermal hysteresis permits warming more quickly than cooling. Coupled with regular, vertical excursions in the water column that return the fish to warmer surface water, this enables these sharks to maintain their Tb within a reasonably narrow range (14-21 °C) while diving through water from 26 to 7 °C (Figure 3b).

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