Hatchling Turtle Cold Hardiness
Initiated in 1990, one line of research concerns ecophysiological mechanisms of cold hardiness in hatchling turtles. These animals are of special interest to us because the young of many species spend their first winter of life within the natal (terrestrial) nest. In northern regions and at high latitudes, these hatchlings may encounter extreme cold, yet they thrive under these conditions. Although we have studied various species of turtles (see review: Costanzo et al. 2008), much of our work has focused on the ubiquitous painted turtle, Chrysemys picta, the cold-hardy champion among turtles.
Painted turtles inhabit freshwater habitats from coast to coast in the northern United States and southern Canada. These turtles generally overwinter underwater, except that the young, which hatch in late summer, commonly hibernate inside the natal nest, only ~10 cm beneath the ground surface. The cold hardiness of painted turtle hatchlings is remarkable, as many emerge from their nests in spring after being exposed to temperatures that may fall to -11°C or below. Other species of North American turtles may overwinter as hatchlings inside the natal nest, in the soil column beneath the nest chamber, in other terrestrial sites, and under water.
In collaboration with John Iverson (Earlham College), many of our studies have centered on a population of painted turtles inhabiting the Nebraskan Sandhills. In this region, winters are particularly severe and the hatchlings may encounter subzero temperatures from late November through early March. Individual chilling episodes are usually brief (less than 24 hours) and mild (minimum nest temperature is above -4°C); however, turtles sometimes encounter temperatures as low as -12°C and subzero chilling episodes lasting several weeks. We have also studied this species (and others) in northern Indiana, where the winter is not as severe (Costanzo et al. 2004).
We are particularly interested in the microenvironmental conditions to which turtles are exposed during winter. Field and laboratory research has shown that winter survival depends on not only nest temperature, but also on the particular characteristics of the soil within the nest chamber. Turtles often contact ice, which can trigger the freezing of supercooled body fluids. Also, the sandy soil in which the hatchlings overwinter harbors a host of ice nucleating agents (mineral crystals, various microorganisms, etc.) that sharply limit supercooling capacity, increasing the risk of death due to freezing (Costanzo et al. 2000).
The biochemical and physiological adaptations promoting the extreme cold hardiness seen in some turtles are still incompletely understood. With respect to freeze tolerance, there is no involvement of antifreeze (=thermal hysteresis) proteins or special ice-nucleating proteins, but whether or not cryoprotectants are used is as yet unresolved. During freezing, turtles do accumulate small quantities of glucose, lactate, and certain amino acids, although it seems doubtful that these osmolytes could appreciably reduce the body ice content. An innate anoxia tolerance likely helps frozen turtles cope with ischemia, though our studies indicate that the cause of freezing mortality is unrelated to oxygen deprivation (Dinkelacker et al. 2005). Turtles also accrue glucose and lactate while supercooling, but whether this response improves survival is unknown. It is clear, however, that cold acclimatization is crucial to the development of supercooling capacity, inoculation resistance, and freeze tolerance (Costanzo et al. 2000).
Freeze Tolerance or Supercooling?
Painted turtle hatchlings (and the hatchlings of several other species) tolerate a profound freezing of their body fluids and this ability allows individuals to survive under conditions that would kill most other species. In a two-year field study (Costanzo
et al. 2004)
we excavated natural nests during the heart of winter, finding hard-frozen turtles that revived after thawing. Skeptics have questioned whether turtles can recover from freezing at all: we invite you to judge for yourself (link
to video low bandwidth /
Despite the advantages of being freeze-tolerant, the lower thermal limit for survival in the frozen state appears to be about -4°C, so freeze tolerance alone cannot account for the excellent survival observed in severe winters. Painted turtles exhibit a second cold-hardiness strategy—supercooling—which also contributes to their survival.
The supercooling capacity of hatchling painted turtles apparently is the best of any vertebrate, as these turtles (if carefully isolated from the ice nucleating agents naturally found in their nest) may cool to -20°C before spontaneously freezing. They perish at temperatures below -12°C, even though no ice has formed. Still, supercooling provides protection over a broad range of environmental temperatures, and clearly allows turtles to survive at temperatures much lower than could be tolerated in the frozen state.
One drawback to using supercooling (i.e., freeze avoidance) as a cold-hardiness strategy is that supercooled solutions are inherently metastable. Deeply supercooled turtles are subject to spontaneous ice nucleation, with predictably lethal consequences. Furthermore, under certain environmental conditions, painted turtles are highly susceptible to inoculative freezing
et al. 1998), a scenario in which ice or ice nuclei in the immediate environment triggers the freezing of supercooled body fluids. Some authors have advocated that painted turtles survive subzero exposures solely by remaining supercooled, but this view contradicts the wealth of published evidence that these turtles can and do freeze during chilling episodes.
Although freeze tolerance and supercooling are generally regarded as dichotomous strategies for cold tolerance, results of our studies suggest both may be effective survival mechanisms in painted turtle hatchlings. According to this model, supercooling predominates during periods of low environmental water potential, since the risk of ice inoculation is reduced and the turtles may partially dehydrate. Alternatively, exposure to damp (ice-laden) soil promotes ice nucleation via inoculation at a relatively high body temperature, a condition requisite for freezing survival. Analysis of the actual chilling episodes encountered by turtles in nature showed that although frozen turtles may endure many cooling bouts, survival of the extreme temperatures occurring in some nests can only be ascribed to supercooling.