As illustrated in the popular children’s tale, "The Tortoise and the Hare," tortoises are not fast creatures. Becoming even slower at low temperatures, turtles must take drastic measures to survive the winter. Turtles and tortoises of temperate regions hibernate to escape the effects of seasonally cold temperatures. Most aquatic turtles hibernate underwater, but terrestrial species dig or steal burrows from other creatures.
Ectothermic animals derive their energy from external sources; they cannot move quickly at low temperatures. This makes them poor foragers and puts them at high risk of predation. To cope, turtles find a safe, thermally appropriate hiding spot -- or hibernacula -- and reduce their metabolic rate drastically. This reduces their need for food, and causes them to enter a state of hibernation. Herpetologists often use the term brumation to distinguish this from the type of dormancy exhibited by mammals. When warm temperatures return, the turtles exit their hibernacula and resume feeding.
Tropical turtles and tortoises do not hibernate. Bathed in year-round sunshine and warm temperatures, tropical chelonians stay active and feed throughout the winter. However, turtles in some of these areas must cope with oppressively hot, dry conditions during the summer. In a behavior that roughly parallels hibernation, turtles of the drought-stricken tropics dig into the substrate and aestivate until the rains return.
Land Lover Lairs
Terrestrial turtles and tortoises usually retreat underground to hibernate. The depth to which they burrow is dependent on the climate of their region; turtles of northern latitudes must venture further underground to escape freezing temperatures. Some turtles dig their own hibernacula, some use rotted tree stumps and others use burrows dug by other animals.
Most freshwater turtles bury themselves in the mud at the bottom of deep ponds or lakes to survive the winter. While turtles require oxygen to live, they can extract small amounts directly from the water via their throat or cloaca. Additionally, due to the cold temperatures and their low metabolic rate, their oxygen demands are far lower during hibernation than when the turtles are active. Because freshwater reaches its greatest density at 39 degrees Fahrenheit, the bottom layer of water in sufficiently deep ponds will always be about this temperature -- cool enough to allow hibernation, but not cold enough to freeze. Additionally, turtles dissolve small amounts of calcium from their shells into their bloodstream to keep their hearts beating properly and buffer the production of lactic acid.
Snoozing in the Sea
Native Mexican fishermen had long hunted hibernating sea turtles in the Gulf of Mexico, but it wasn’t until the 1970s that herpetologists became aware of, and began to investigate, the phenomenon. Only some sea turtle populations exhibit hibernating behavior; others migrate to warmer oceans. Those who hibernate appear to dig several inches into the mud to do so. Upon emerging from hibernation, the turtles surface to warm themselves, breathe and clean the mud from their bodies.
- Plainfield Animal Hospital: Turtle Hibernation
- The Tortoise Reserve: The Complexities of Turtle Hibernation
- Respiration Physiology: The Physiology of Hibernation Among Painted Turtles: the Midland Painted Turtle (Chrysemys Picta Marginata)
- Biological Conservation: Apparent Hibernation by the Atlantic Loggerhead Turtle (Caretta Caretta) Off the Coast of Cape Canaveral, Florida
- Biology Letters: First Records of Dive Durations for a Hibernating Sea Turtle
- University of Queensland, Australia: Facultative Aestivation in a Tropical Freshwater Turtle Chelodina Rugosa
- Smithsonian National Zoological Park: Box Turtle