Individual and climatic variation: Predicting nesting phenology in a population of painted turtles (Chrysemys picta)

Background/Question/Methods

Phenological shifts in response to climate change are a current topic of interest and concern among ecologists. For organisms with temperature-dependent sex determination (TSD), phenology in nesting may greatly impact resulting sex ratios. Dramatically different sex ratios occur in cooler versus warmer portions of the nesting season which has important implications for population structure and persistence. Individual plasticity in phenology is cited as one way in which TSD species might circumvent negative impacts of climate change.  

Recent studies have demonstrated that painted turtles (Chrysemys picta) exhibit plasticity in first nesting date. Additionally, research suggests that turtle populations begin nesting earlier in response to warmer winter temperatures. Studies have not, however, looked at how individual variation in condition may have additional effects on phenology. To better understand what factors contribute to differences in nesting phenology, we combine climate and individual variables in a hierarchical Bayesian model     

We utilize a dataset that spans 29 years of painted turtle demographic data at a site in Southeast Michigan. Included in this data are demographic information – age, size, weight, and reproductive output as well as the dates of the onset of nesting. Additionally, we integrate local climate data into the model.  

Results/Conclusions

Preliminary results demonstrate the importance of considering both climatic variables and individual variation within a population when predicting nesting phenology in painted turtles. Individuals demonstrated plasticity in terms of climate cues. Climate data, particularly temperature (measured as the number of heating degree days) in January and February had a significant impact on the onset of first nesting day with turtles beginning nesting earlier during warmer years. Precipitation had a limited effect on predicting initial nesting dates and was a non-significant result.    

Age and size, which were strongly correlated, predicted variation in first nesting day as older females (>18 years) both nested earlier and, when doing so, often laid a second clutch 10-21 days later. Weight did not have any significant impact on nesting onset.

These initial results reinforce the importance of considering individuation variation when studying phenology. In light of climate change, these predictions may prove valuable in addressing how plasticity contributes to alleviating some negative aspects of climate change.