Tolerance of repeated freezing by hatchlings of three northern turtles
In some Emydidae turtles that live at high latitudes, terrestrial overwintering hatchling survival is dependent on freeze tolerance or freeze avoidance. Traditionally freeze tolerance has been studied in laboratory tests evaluating survival from a single freezing session. However, in the field hatchlings are much more likely to experience repeated freeze/thaw cycles. Diamond-backed terrapin (Malaclemys terrapin) (previously shown to be tolerant of a single freezing event), red-bellied turtle (Pseudemys rubriventris) (freeze tolerance unexplored), and common map turtle (Graptemys geographica) (previously shown to not be freeze tolerant) hatchlings incubated at a 29oC were tested to determine whether repeated freezing had an impact on survival. Hatchlings were taken from an incubator at 5 oC and cooled down to -3 oC over 24 hours to induce freezing. Following each freezing hatchlings were gradually warmed and checked for mortality before being cooled again to 5 oC for the next set of freezing trials. Each specimen was subjected to as many as 5 freezing trials over the duration of the experiment.
Hatchling diamond-backed terrapins had the greatest overall freeze tolerance with 60.0% surviving two freezing trials, 5.0% surviving until the fifth freezing trial, and a single individual surviving all freezing trials. Red-bellied turtles were also tolerant of repeated freezing, but none survived past the third freezing. The common map turtles were not to be tolerant to repeated freezing; all specimens only survived a single freezing trial. In previous studies it has been shown that common map turtles may avoid inoculative freezing and supercool instead. Clearly, repeated freezing has different impacts on survival for these three northern turtle species. However, without factoring in repeated exposure it would be possible to draw the erroneous conclusion that all three turtle species are equally freeze tolerant since all survived at least one freezing. This highlights the importance of conducting freeze tolerance experiments that more closely simulate the known field conditions.