Cold Turtle Soup?

I'm out of town today, so here's an old shot:

Painted Turtles (Chrysemys picta), Volo Bog State Natural Area, Lake Co, IL 9/26/2012

Here in Chicago, if we see a turtle in the winter, it's a wonderfully warm winter. They normally disappear for several months. But where do they go?

Other animals employ various strategies for surviving the winter, of course. Many birds migrate -- but if you've ever watched a turtle for very long, the thought of them migrating is rather amusing. Annual plants spend the winter as seeds and germinate in the spring, while many insects overwinter as eggs that hatch in the spring. But turtles are very long-lived, with large Alligator Snappers (Macrochelys temminckii) and Galapagos Tortoises (Chelonoidis nigra) being known to surpass the century mark, so that idea's out. Clearly they don't remain active; even if their food supplies remain available, their low metabolism means cold weather basically immobilizes them.

What's left? Well, hibernation, obviously. So where should a turtle hibernate? Box Turtles (Terrapene carolina) hibernate in shallow burrows (1), but the Painted Turtles pictured above spend the winter here:

Painted Turtle, Spring Bluff Forest Preserve, Lake Co, IL 6/29/2012

Well, at the bottom, anyways. The ice would get in the way of this sort of behavior.

Both of these hibernation sites pose problems, of course. Box Turtles are regularly exposed to temperatures below freezing, and they have been shown to actually resist a significant amount of ice in their intracellular fluids -- something that would painfully kill you or me. (1) Hatchling Painted Turtles are capable of this as well, as they overwinter in their below-ground nests. (2) As mentioned above, however, older Painted Turtles spend the winter at the bottom, where the temperatures are maintained just above freezing by the high specific heat of water and by the insulation of the ice at the surface. That ice, however, causes another problem: it keeps the turtles from reaching the surface, where they can actually breathe. So do they actually make it through an entire winter without even breathing?

Well, not quite. Turtles do have exceptionally low metabolic rates to begin with, so they aren't using too much oxygen. They also appear to be very tolerant of both hypoxia and lactic acidosis (lactic acid is produced when vertebrate cells metabolize sugar without oxygen -- a process that we know as fermentation). But some turtles have also been shown to extract oxygen from water in order to stretch things out; in other words, they can breathe underwater! This ability has been demonstrated in both Painted (3) and Loggerhead Musk Turtles (Sternotherus minor) (4) , but not in Red-eared Sliders (Trachemys scripta) (4), so it's not a universal turtle trait.

Turtles are the only terrestrial vertebrates I'm aware of that have regained the ability to breathe water; whales, dolphins, seals, sea snakes, marine iguanas, otters, even hippos all have to breathe air. Oxygen simply doesn't dissolve in water in sufficient quantity to support most terrestrial animals. Turtles can only manage it because of their low metabolic rates, another advantage of life spent in the slow lane.

The schoolboy version of the scientific method lists observation as the first step in the process. But as Sherlock Holmes once pointed out, sometimes the neatest questions come from the things you don't observe, like turtles on a cold winter day.

(1) Costanzo, J. P., & Claussen, D. L. (1990). Natural freeze tolerance in the terrestrial turtle, Terrapene carolina. Journal of Experimental Zoology, 254(2), 228-232.

 

(2) Storey, K. B., Storey, J. M., Brooks, S. P., Churchill, T. A., & Brooks, R. J. (1988). Hatchling turtles survive freezing during winter hibernation. Proceedings of the National Academy of Sciences, 85(21), 8350-8354.

 

(3) St. Clair, R. C., & Gregory, P. T. (1990). Factors affecting the northern range limit of painted turtles (Chrysemys picta): winter acidosis or freezing?. Copeia, 1083-1089.

 

(4) Belkin, D. A. (1968). Aquatic respiration and underwater survival of two freshwater turtle species. Respiration physiology, 4(1), 1-14.