The Impact Of Thermal Fluctuations On Ectothermic Animals

Ectothermic animals such as turtles, snakes and lizards rely on external environments for heat and therefore the environment heavily influences the animals cellular, organ and organismal functions. There are many adaptations used by these animals in order to function at low or high temperatures, such as sun bathing in the day, burrowing at night and with extremely low temperatures, the painted turtles use cryoprotections. However, even with such type of adaptations, naturally occurring thermal extremes can still cause cell or organismal death and disrupt physiological function of these ectotherms.

In a different study, some insects exhibit rapid increases in cold hardiness after a brief cold exposure, this is also termed as ‘cold-conditioning’. This cold-conditioning method will be used on painted turtle hatchlings (Chrysemys picta). With this species the hatchings are usually ~5-10 cm underground and overwinter in their natal nest, they are also often subjected to temperatures at -4°C to -12°C that fluctuates daily. The purpose of the experiment is to see whether painted turtles that undergo cold conditioning; either by rapid fluctuation enhancement or by gradual enhancement of low temperatures, would increase cold hardiness in the painted turtles in comparison to the painted turtles in the reference group.

The eggs of the western painted turtle (Chrysemys picta bellii) was collected from Rock island females that were then treated with oxytocin to induce egg laying. The eggs were incubated at 28.5°C in moist soil until hatching. They were then transferred to new soil and put in a dark incubator at 20°C and then winter-acclimated stepwise using the same method as Costanzo et al. (2000). The incubator temperature was decreased to 15, then to 10 and 4°C on the firsts of October, November, and December, respectively. Turtles stayed at 4°C before used in experiments. The turtles were then placed into clean and dry Falcon tubes, the tubes were placed in a refrigerated bath set at -1.0°C. Reference turtles (n=54) were held at 4°C for 5 days, then cooled to 0.0°C. In the first experiment, following Muir et al., 2010, the turtles were cold conditioned gradually from -1.0°C to their target temperatures of -3.5°C (n=29), -7.0°C (n=27) or -10.5°C (n=21) before warming to 0.0°C. In the second experiment, they tried mimicking daily fluctuations these turtles would have naturally to cold condition them. They were cooled from -1.0°C to -3.5°C (n=33). -7.0°C (n=33), or -10.5°C (n=33) for over 8 hours, held at target temperature for 8 hours and warmed to 0.0°C over 8 hours daily before being warmed. The cold-conditioned and reference turtles were then tested for chill tolerance. They were chilled from -1.0°C to -12.7°C over 12 hours, and held at target temperature for another 12 hours, then warmed to -1.0°C over 12 hours. A turtle that survived would have retracted at least one of its limbs in response to a stimulus over the 7-day recovery period.

The results show that there was an increase in survival via daily thermal fluctuation cold-conditioning (CC) after the cold shock in comparison to the gradual CC. For example, CC group via gradual at -7°C had ~45% of turtle survival in comparison to daily fluctuations with ~70% survival. However, the turtles in the reference group did the poorest after a cold shock, with just 9% surviving. Looking at the results, it is clear to say that those in the reference groups had lesser cold hardiness than those in the CC groups, and also that those CC via daily fluctuations have a higher cold hardiness than CC via gradual at temperatures -7 and -10.5°C.

In conclusion, thermal fluctuations are an important factor for these turtles and will be important for many other ectotherms that go through sub-zero temperatures as they act as triggers to rapidly increase their cold hardiness over a short period of time and to therefore increase their probability for survival.