Using Bayesian modeling to predict nesting onset in a population of midland painted turtles (Chrysemys picta marginata)

Background/Question/Methods

Phenological shifts due to climate change are of concern in many systems.  For temperature-dependent sex determination (TSD) organisms, changes in nesting behaviors, such as nesting onset may greater impact sex ratios in the nest.  Dramatically different sex ratios occur in cooler versus warmer portions of the nesting season which has important potential 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, a finding that is linked to modern climate change.  It is unclear, however, how individual variation interacts with climate to impact nesting onset.

To understand the relationship between individual characteristics, climate, and nesting onset, we utilize a Bayesian modeling framework in conjunction with a historic 29 year dataset on midland painted turtle (C. picta marginata) demographics.  We couple this dataset with NOAA-generated climate information on temperature and precipitation patterns.  Through this approach, I will explore a) if and how painted turtles shift nesting onset in response to temperature and b) how individual characteristics such as size or age influence levels of response.    

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 February also in April had a significant impact on the onset of first nesting day with turtles beginning nesting earlier during warmer years. Precipitation had a limited impact on predicting initial nesting dates and was largely non-significant with the exception of rain events in early spring.  

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.