Hypothermic Stunning


Hypothermic, or cold stunning, occurs when a sea turtle is abruptly exposed to cold water, usually as a result of unusually cold weather or sudden cold fronts (Witherington and Ehrhart, 1989). It can occur at any latitude, but it increases in occurrence as turtles go north, as stunning is most prevalent in waters below 10ºC (Morreale 1992). In hypothermic stunning, the temperature of water surrounding the turtle drops rapidly, and it cannot compensate fast enough to avoid a sudden drop in body temperature. It becomes less active, and eventually its rate of metabolic functions slows. If the turtle does not raise its body temperature, it will die. However, hypothermic stunning does not always result in turtle death (Witherington and Ehrhart, 1989).

Hypothermic stunning results when the turtle’s core body temperature drops below the temperature at which it can function. Surface temperature drops due to conduction and convection between the water surrounding the turtle and the surface of the turtle. The cooled blood at the surface of the turtle is then transported throughout the body and the turtle’s core body temperature lowers.

Olive ridley, photo courtesy of Michael Jensen

Small turtles are more likely to suffer from hypothermic stunning, both across and within species. Ridleys, loggerheads, and green turtles are known to suffer from hypothermic stunning, whereas leatherbacks are not. Smaller ridleys, loggerheads, and green turtles are more likely to suffer from hypothermic stunning in general, and suffer from it earlier in the cold snap. They are thus more likely to die from hypothermic stunning than are larger members of their respective species (Witherington and Ehrhart, 1989; Morreale, et al, 1992).

Smaller turtles become cold faster because they have a higher surface area to volume ratio. They exchange heat with their surroundings faster through conduction and convection, and the cooled blood reaches the core of their bodies sooner. Leatherbacks benefit not only from their large size, but also from thicker, more effective insulation than other sea turtles (Frair, et al, 1972).

In order to avoid hypothermic stunning, turtles emigrate from colder waters before the onset of the cold season or bury themselves in the sediment on the bottom of the ocean to brummate (Witherington and Ehrhart, 1989; Morreale, et al, 1992). However, sea turtles also avoid hypothermic stunning by endothermically maintaining a body temperature higher than that of their surroundings.

Hawksbill, photo courtesy of Caroline Rogers

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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