Blood flow & heat exchange in reptiles
extant & extinct
Reptiles are ectotherms. Unlike mammals and birds that generate considerable heat to keep their bodies warm, reptiles rely on external sources of heat to warm their bodies.
Reptiles are also poikilotherms, that is their body temperature varies considerably throughout the day, unlike the homeothermic mammals and birds, which keep their body temperatures steady and high.
Reptiles like high steady body temperatures just as mammals and birds do, however, and this means they have sophisticated ways to manage flows of heat between their bodies and the environment.
One common way they do this is to use blood flow within the body to facilitate heat uptake and retard heat loss. A lizard basking in the sun, for example, ramps up its cardiac output. In this way, heat is transferred rapidly from the warm skin, to the body, and the body heats rapidly. Once the lizard has stopped basking, its cardiac output declines, retaining heat within the body so that the body cools slowly. In this way, a lizard can minimize the time it spends soaking up heat, and maximizes the time it can spend doing other essential things, like looking for food and mates or defending territories.
I became interested in this problem when I was a graduate student. Aside from the desparate need for a dissertation project, my interest in this subject arose because our knowledge on this was mostly anecdotal, a "neat story" that we knew little about otherwise: how did it work, how did it evolve, and what did it do for the creatures that employed it?
Appendages are the principal sites for control of blood-borne heat exchange
Blood flow is not effective as a medium of heat transfer everywhere in the body. Body shape also enters into the equation.
Specifically, blood flow to appendages (legs and tail) controls heat exchange more effectively than blood flow to the torso surface. Appendages act as heat exchange fins, in which heat flow is limited mostly by its rate longitudinally along the appendage. At the torso, however, heat flows radially between the torso surface and interior. Change of blood flow alters heat flow along the appendage more than it does within the torso. Thus, blood flow has relatively little effect on heat exchange in the torso, but has a substantial effect at the appendages.
Among other things, this means that legless reptiles, like snakes or legless lizards, generally will not be able to use blood flow to control heat exchange, unless it is along the length of the body. This may be why snakes typically bask by exposing a loop of their bodies to the sun, allowing blood flow to distribute heat to the cooler parts of the body that remain shaded.
J S Turner. 1987. The cardiovascular control of heat exchange: Consequences of body size. American Zoologist 27: 69-79. [pdf]
J S Turner, C R Tracy, B Weigler and T Baynes. 1985. Burst swimming of alligators and the effect of temperature. Journal of Herpetology 19: 450-458. [pdf]
J S Turner and C R Tracy. 1983. Blood flow to appendages and the control of heat exchange in the American alligator. Physiological Zoology 56: 195-200. [pdf]
J S Turner. 1984. Raymond B Cowles and the biology of temperature in reptiles. Journal of Herpetology 18: 421-436. [pdf]
Body size and the control of blood-borne heat exchange
Reptiles as a group span an enormous range of body sizes, and this variation imposes serious constraints on how effectively reptiles can control heat exchange with blood flow.
For many years, the conventional wisdom held that large reptiles would rely ever more strongly on blood-borne heat exchange to control body temperature. This was thought to be dictated by the physics of heat transfer within bodies: if heat had to flow over longer distances, as it would in larger animals, heat borne by blood flow would become an ever larger component of the total heat flow. This dogma led many people to believe that very large reptiles, like large crocodilians or dinosaurs, were especially adept at controlling body temperature with circulation.
This dogma did not account for the complexities of blood-borne heat transfer within the body and how it interacts with heat flow from the surface to the environment, such as how it is limited by boundary layer resistances. Accounting for these complexities produces an optimum body size for the ability to regulate body temperature by blood-borne heat exchange. For animals in air, this optimum size is a little over 5 kg. For animals living in water, the optimum size is much larger, on the order of 100 kg or so.
This may explain why large reptiles today are largely aquatic and terrestrial reptiles are smaller. It also helps expalin the odd appendages like crests and sails that decorated extinct reptiles like Stegosaurus or mammal-like reptiles like Dimetrodon. Dick Tracy and I did calculations to show these structures could act as very effective heat exchange fins, allowing animals with crests to heat their bodies up to high temperatures much faster than animals without them.
J S Turner and C R Tracy. 1986. Body size, homeothermy and the control of heat exchange in mammal-like reptiles. In: N J Hotton III, P D MacLean, J J Roth and E C Roth, eds., The Ecology and Biology of Mammal-Like Reptiles. Smithsonian Institution Press, Washington, D.C. pp. 185-194. [pdf]
C R Tracy, J S Turner and R B Huey. 1986. A biophysical analysis of possible thermoregulatory adaptations in sailed pelycosaurs. In: N J Hotton III, P D MacLean, J J Roth and E C Roth, eds., The Ecology and Biology of Mammal-Like Reptiles. Smithsonian Institution Press, Washington, D.C. pp. 195-206. [pdf]
J S Turner and C R Tracy. 1985. Body size and the control of heat exchange in alligators. Journal of Thermal Biology 10: 9-12. [pdf]
J S Turner and C R Tracy. 1983. Blood flow to appendages and the control of heat exchange in the American alligator. Physiological Zoology 56: 195-200. [pdf]
