An Endothermic Reptile?
We tend to have the incorrect preconception that an animal is either purely endothermic or purely ectothermic. We assume that mammals and birds are endotherms, or warm-blooded animals, and that reptiles, amphibians, and fish are ectotherms, or cold-blooded animals. However, there is a spectrum of thermoregulation strategies ranging from endo- to ectothermic, and many animals lie somewhere in the middle, exhibiting traits of both strategies.
In that case, what does it mean to be endo- or ectothermic? A purely endothermic animal relies only on internal mechanisms to maintain its body temperature, while a purely ectothermic animal relies only on its surroundings. This does not, however, mean that endotherms have constant body temperatures and ectotherms have changing body temperatures. In order to fully describe an animal’s thermoregulatory strategy, the extent to which they allow their body temperature to change must also be taken into account.

 

The term poikilothermic describes an animal that allows its body temperature to vary a great deal. Heterothermic describes an animal whose body temperature remains constant. Endotherms are not necessarily heterothermic, and ectotherms are not necessarily poikilothermic. For example, a squirrel who allows its body temperature to drop when it hibernates is a poikilothermic endotherm, while a fish in sub-zero arctic waters whose surroundings are at a constant temperature is a heterothermic ectotherm.

Loggerhead, photo courtesy of Paula Baldassin

Another way to describe a hibernating squirrel is to say that it is not purely endothermic; it exhibits ectothermic traits during hibernation by allowing its body to cool along with the decrease in environmental temperatures. Sea turtles also exhibit both ectothermic and endothermic traits. A sea turtle’s body temperature is often higher than that the water that surrounds it and thus cannot be purely ectothermic.

Green turtle, photo courtesy of Kirsten Jones

Leatherbacks have been recorded to have body temperatures as great as 18ºC above the temperature of their surroundings (Frair et al, 1972). They are almost always 3ºC above their surroundings, and the difference most often lies in the 10-15ºC range (Mrosovsky and Pritchard, 1971). Green turtles have body temperatures around 2.5ºC above water temperature (Heath and McGinnis, 1980) but have the potential to be as much as 10ºC above water temperature (Mrosovsky and Pritchard, 1971). Hawksbills and Ridleys also have slightly elevated body temperatures (Heath and McGinnis, 1980), with Ridleys having the potential to reach over 3ºC above water temperatures (Mrosovsky and Pritchard, 1971).

A sea turtle’s marine environment presents particular problems to maintaining an elevated body temperature. Water has a high rate of conductance, which means that heat is exchanged rapidly between the turtle and the water that surrounds it. Ocean currents and the constant activity of sea turtles cause a high rate of convection. Heat is exchanged more rapidly than if the turtle were motionless and the water stagnant. And finally, an ocean’s size means that it will remain roughly the same temperature regardless of air temperature and solar radiation. This temperature depends on latitude, but it is consistently below that of a sea turtle’s body temperature.

Sea turtles must have physiological mechanisms that supply this elevation in body temperature.

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This web site was completed by Katie Fitzpatrick in partial fulfillment of the requirements for Dr. Michael Dorcas's Biology 312, Animal Physiology, at Davidson College in Fall Semester 2005.

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Please direct all comments and questions to Katie Fitzpatrick at kafitzpatrick@davidson.edu