Available at: https://digitalcommons.calpoly.edu/theses/1952
Date of Award
MS in Biological Sciences
Understanding how organisms respond to climatic variability and novel conditions is becoming an increasingly important task for ecologists. For ectotherms in the northern hemisphere, the response to cold is of special interest, considering that poleward range expansion events and increasing variability of temperatures during winter are already being observed as consequences of a warming planet. Though direction of change in physiological variables in response to cold is well studied in ectotherms, the extent to which traits can change and the rate at which they can change is not.
We compared the extent and rate of change in cold tolerance (CTmin) between two long-term captive populations of the Italian wall lizard (Podarcis siculus) during a lab cold-acclimation treatment. Heat tolerance (CTmax), thermal preference (Tpref), temperature dependent rates of oxygen consumption (SMRO2), and temperature dependent rates of water loss (EWL) were also compared between Italian wall lizards previously introduced to Long Island, NY and San Pedro, CA before and after the lab cold acclimation treatment. Because our study coincided with a cold snap during the spring 2018 season for the San Pedro, CA population, we also studied the effects of cold acclimatization on wild lizards from the CA population.
After initial lab acclimation of the lizards to laboratory conditions, SMRO2 at 15°C and EWL at 10°C were higher in NY lizards compared to CA lizards. Lizards from the two populations did not differ in any other variables measured before the cold acclimation treatment. We found that lizards from the NY population experienced an 80% decrease in CTmin following a switch from 20°C:18°C to 17.5°C:16°C (12h light:12h dark) acclimation treatment. Lizards from the CA population did not decrease CTmin in response to the same cold acclimation treatment. Overall, NY lizards decreased CTmin, CTmax, and Tpref following cold acclimation, whereas CA lizards decreased CTmax only. Wild CA lizards decreased CTmax following the cold spring 2018 season in a manner similar to that of lab acclimated NY and CA lizards, suggesting that these lizards do not maintain a high CTmax when the environment is unlikely to expose them to high temperatures. Thermal sensitivity (Q10) of SMRO2 and EWL was lower in NY lizards, suggesting physiological adaptation to fluctuation in diurnal temperatures. The ratio of CO2 produced to O2 consumed (respiratory exchange ratio, RER) measured at 15°C increased in NY lizards following cold acclimation suggesting an increased use of carbohydrates and/or an increased production of lipids in the colder conditions.
These responses in combination with the higher observed plasticity in NY lizards are in accordance with the climatic variability hypothesis, which predicts that organisms from more variable climates will be better adapted to physiologically respond to variable conditions. The higher capacity for physiological plasticity may explain the relatively high success of P. siculus in NY and other northern U.S. states. By describing the rate of change of CTmin during cold acclimation we hope to better understand how these lizards minimize the risk of low temperature exposure during winter. We ultimately hope to incorporate the rate at which cold tolerance can change into predictions of species distributions and hypothesis tests investigating the relationship between climatic variability and the rate at which animals can exhibit plasticity.