Metabolic Rate
Sea turtles may establish an elevated body temperature as a result of a raised metabolic rate. In sea turtles, and specifically leatherbacks, the metabolic rate is higher than that of most reptiles. A leatherback's metabolic rate is as high as one half the rate of a similar sized mammal, which is significant because most reptile's metabolic rates are only tiny fractions of those of similar sized mammals (Paladino, et al, 1990). In addition, a leatherback’s metabolic rate is roughly 86% that of an elephant. Gigantothermy allows the two species to share thermoregulatory traits and a similar metabolic rate (Spotila, et al, 1991).
When a leatherback is exposed to cold temperatures, its breathing and swimming rates increase (Frair, et al, 1972). Whereas the metabolic rate of most ectothermic reptiles decreases with decreasing temperatures, metabolic rate in the leatherback is higher with decreasing temperatures. This increase indicates some degree of endothermy.

 

Leatherbacks have brown adipose tissue, a insulatory fatty tissue typically found in mammals and surrounding regionally endothermic organs in fish (Goff and Stenston, 1988). The presence of this tissue throughout their bodies further suggests that leatherbacks have the abililty to regulate temperature endothermically.

Green turtles also have an elevated active metabolism, which can be as high as ten times their resting metabolic rates and is significantly higher than those of other reptiles (Penick, et al, 1996). Ridleys have been found to have a metabolic rate equal to that of green turtles, despite a large size difference between the two species. Leatherback metabolic rates are slightly lower due to their larger size (Mrosovsky and Pritchard, 1971).

 

Leatherback, photo courtesy of Noelle Rucinski

Elevated aerobic capacity also indicates endothermy in sea turtles. Sea turtles are more active than most other reptiles, which tend to display brief bursts of activity followed by a period of recovery (Frair, et al, 1972; Penick, et al, 1996; Pough, 1978). In order to maintain high levels of activity, animals must depend on aerobic metabolism. Indeed, green turtle metabolism is highly aerobic (Penick, et al, 1996). In fact, all sea turtles have enough of myoglobin in their bloodstream to give their muscles a reddish color and a high content of oxygen-rich blood. Red muscle, common in birds and mammals (Pough, 1978), is a characteristic of endothermy, as it indicates aerobic metabolism.

 

In classic ectotherms, metabolic rate changes with changes in body temperature. In green turtles however, the metabolic rate of the heart is constant from 12.5 to 25.5 ºC. This temperature independence indicates that the green turtle is not purely ectothermic. Green turtles also have significantly more control over their heart rate than most ectothermic reptiles. They increase or decrease heart rate perhaps to control heat production and certainly to control its distribution rate (Smith, et al, 1986). Furthermore, the classic ectothermic traits of low energy requirements and low, anaerobic metabolic rates (Pough, 1978) are inconsistent with the endothermic traits displayed by sea turtles.

Loggerhead, photo courtesy of Alan Rees

 

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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