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A turtle out of water: to freeze or not to freeze: a hatchling's dilemma

Natural History, Nov, 2001 by Peter J. Marchand

It was a sobering sight, one of those encounters that awakens you to the harsher realities of nature. The ground was dug up and strewn with white, leathery shells--remnants of about ten eggs, near as I could tell. From the tracks in the loose soil, I guessed that a skunk had discovered the buried trove about forty yards from the edge of a pond and, in a single feeding spree, had wiped out a snapping turtle's reproductive effort for the year. In a poignant way, this exemplified the perilous trial of aquatic turtles out of water. Superbly adapted to adult life in the pond, they are less well equipped for their start on land. And hungry predators aren't the only obstacle between turtles and the safety of water; many hatchlings in northern climes, emerging in the fall, are faced with the daunting task of surviving their first winter below ground, often exposed to freezing temperatures.

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Had the snapping turtle's eggs escaped predation, the young would have moved to the pond as soon as they emerged from their nest in autumn. But hatchlings of many other turtle species remain at or close to their natal site during their first winter, digging themselves out the following spring and only then migrating to water. Some, such as the yellow mud turtle, burrow deeper into the soil shortly after hatching, finding safety below the frost line. In contrast, the ornate box turtle and northern populations of the painted turtle frequently overwinter in shallower soils and, when unable to avoid freezing, show a remarkable tolerance for the formation of ice in their body tissues.

Subjected to slow cooling, hatchling turtles may begin to freeze at a temperature slightly below 27 [degrees] F, with ice first forming on the outer skin and then advancing slowly toward the body's core, eventually cutting off peripheral blood circulation. At 25 [degrees] F, slightly more than half the water in the turtle's body may be frozen, and all vital signs cease. If the temperature drops no further, the hatchlings will recover completely upon thawing.

Remarkable as this freezing tolerance is, 25 [degrees] F is close to the turtle's limit of survival in the frozen state; in order to withstand lower temperatures in nature, turtles must utilize another tactic. Through a process known as supercooling, they become chilled well below the normal freezing point of their blood plasma without undergoing ice formation. As long as the water in the turtle's body tissues remains liquid, no immediate harm comes of supercooling (the presence of ice in living tissues, rather than low temperature per se, is the primary cause of cell damage). But life is tenuous in this state, for if a supercooled animal is disturbed in any way, ice may form instantaneously, spreading rapidly throughout the body and resulting in a quick death. And disturbance isn't the only risk. The presence of any impurity in the bloodstream or gut of the animal can also "seed" ice by providing a nucleus, or particle around which crystals can form more easily than they can in a pure liquid. Ice in the immediate environment of the animal is a danger, too, for nothing seeds ice crystals as readily as ice itself.

The surest way for a turtle to minimize the chances of flash freezing in the supercooled state is to avoid conditions that might serve to stimulate ice formation in body tissues. Hibernating in a dry place and evacuating the gut, so that neither external ice nor intestinal impurities pose a threat, are two possibilities. While the former is partly a matter of chance, painted turtles, it appears, do systematically empty their guts of soil particles and pieces of eggshell that are accidentally ingested during the hatching process and that might provide a nucleus for ice formation. Through this evacuation alone, hatchlings can greatly enhance their supercooling capacity. Northern populations of painted turtles are ultimately able to resist freezing to temperatures as low as -10 [degrees] F.

Turtle hatchlings were once thought capable of producing ice-promoting, or ice-nucleating, proteins that facilitate harmless ice crystallization at temperatures slightly below 32 [degrees] F. Hatchlings could thereby avoid the risks inherent in supercooling--a benefit if their winter surroundings remain above 25 [degrees] F or so. But while ice-promoting substances are often present in blood samples taken from turtles, they apparently are contaminants and are not produced by the animal. Hatchlings raised in the laboratory, free of ice-nucleating particles, can supercool to much lower temperatures than turtles raised on natural soils. Sugars and alcohols like those that protect insects in winter may likewise be naturally induced in turtles in small quantities during freezing but do not seem to be produced and stored ahead of time in the animals' bodies. While turtle hatchlings in the wild may utilize either supercooling or freeze tolerance to get through winter, whichever strategy is employed seems to be a matter of environmental circumstances. If the hatchlings ingest bits of eggshell or bury themselves in soil containing ice crystals, they will freeze at relatively high temperatures and survive only if subsequent temperatures do not go too low. In the absence of ice nucleators, they will supercool to lower temperatures and, unless somehow disturbed, survive colder winters.

A turtle out of water: to freeze or not to freeze: a hatchling's dilemma

Natural History, Nov, 2001 by Peter J. Marchand

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