“Antifreeze” is the source that keeps things from freezing completely. Turtles and some other reptiles have a natural “antifreeze”. It allows them to survive at temperatures from -3.84 – -1’C. The antifreeze is made from cryoprotectants consists of a combination of carbohydrates, glucose, glycerol, lactate, and amino acid taurine. These combinations are produced when ice forms around the cells (Storey 23). “Such cryoprotectant’s responses, invoke by onset of freezing, mitigate the osmotic stress associated with freezing concentration of cytoplasm, attendant metabolic perturbations and physical damage” (Costanzo pg. 351).
These antifreeze’s protect the turtles from low temperatures below their own blood’s freezing point. A study was done to determine the ice content of the Midland Painted Turtles (Chrysemys picta marginata) when cooled at various temperatures. They were cooled to -3.06’C and remained liquid. When the temperature was lowered to -3.84’C for one day, 53% of the body water froze (Storey 23).
In another study, hatchlings of a similar species were collected and subjected to three days of continuous freezing at -2.5’C, resulting in ice contents of 43.5 +/- 1.0% of total body water. In the Western Painted Turtle, (Chrysemys picta bellii), survival times dropped to 4 – 5 hours when the temperatures was lowered to -4’C, which correlated with ice contents of greater than or equal to 50%.
Chrysemys picta marginata nests in the spring. They showed greater freezing tolerance, with survival rate of eleven days at -2.5’C and a higher ice content of 50.2 +/- 1.2%. It also showed higher super cooling points of 1.07 +/- 0.13’C and dropped within three days to -4.83 +/- 0.83’C. This suggests that the icenucleating agents break down when hibernation ended. The large free amino acid, taurine, had made up 21 – 47% total of the liquids in the organs (Churchill 530).
The key to the reptiles ability to resist freezing relies on their ability to tolerate and avoid super cooling. Turtles utilize two strategies to avoid and tolerate freezing: one is the moister content of hibernaculum. The second is the accumulation of low molecular weight substance which parallels depression of haemolymoph melting points down to -6.5’C. Super cooling point was lowered to -30’C in the winter and enhanced super cooling below -20’C and was closely correlated to depression of haemolmoph melting points. Thermal hysteresis-producing antifreeze proteins were not present (Gehrken 683).
When the turtles were cooled, their muscles had frozen. The muscle tensions and contraction times were strongly temperature dependent during cooling. The leg muscle continued to function for several minutes after ice formation, but they ceased to respond at an average temperatures of -0.6’C. The temperature recovery of seven turtles was slowed and noted as temperatures ranging from 1.9 to 17.4’C (Claussen 91).
In the second study, their heart beats were timed during cooling. The cardiac tensions and contraction times were also temperature dependent. The contractions continued for several more minutes after the onset of ice formation and eventually ceased at -0.62’C. The heartbeat recovery was quick and occurred at an average temperature of -0.4’C. The tensions during warming were higher than the corresponding tensions during cooling (Claussen 91).
The relationship between freeze tolerance and body size revolved around the kinetics of cooling and freezing. The two turtles that had experienced ice contents of approximately 35%, exhibited long-term survival, but rapid thawing may have contributed and the water content that was inversely related to the body mass and pre and post cooling rates scaled with body mass to -0.55 and -.40 power (Claussen 91).
During this study, twenty-three turtles had died at temperatures of -7.7 and -10.8’C. Four out of the twenty-three had died after producing exotherms at temperatures of -11.5 and -12.7’C. The remaining turtles had died at temperatures above -11’C, apparently they had succumbed to freezing when ice propagated across their integument from the frozen soil, but had died from another cause (Packard 147).
Freezing point depression is the process in which a difference in temperature between the freezing points of a solution and of the pure solvent. When 1000 grams of water is frozen, its freezing point depression is 1.86’C.
Ice is lowered below its normal temperature when salt is added to it. This lowered the temperature well below 0.0’C, the freezing point of water (Addison-Wesley pg. 384).
Claussen, DL. Allometry of Cooling, Super Cool, and Freezing in the Freeze-tolerant Turtle Chrysemys Picta.. American Journal of Physiology. V261, 1991, pr626-pr632.
Churchill, TA. Natural Mechanisms of Vertebrate Ectotherms at Subfreezing Temperatures- Applications in Cryomedicine. Faseb Journal. V9, 1995, pr351-pr358.
Packard, GC. Journal of Comparative Physiology b- Biochemical Systemic and Environmental Physiology. V163, 1993, p147-152.
Storey, Janet, M.. Out Cold. The Winter Life of Painted Turtles. pg. 23-25.
Wilbraham, Antony C.. Chemistry. Addison-Wesley Publishing Company, inc. 1987, p384.
Y., Kim. The Effects of Cooling, Freezing, and Thawing on Cardiac and Skeletal- Muscle of the Turtle, Chrysemys-picta. V18, 1993, p91-101.