The quantification of repeatability has enabled behavioral and evolutionary ecologists to assess the heritable potential of traits. For behavioral traits that vary across life, age-related variation should be accounted for to prevent biasing the microevolutionary estimate of interest. Moreover, to gain a mechanistic understanding of ontogenetic variation in behavior, among- and within-individual variance should be quantified across life. We leveraged a 30-year study of painted turtles (Chrysemys picta) to assess how age contributes to variation in the repeatability of nesting behaviors. We found that four components of nesting behavior were repeatable, and that accounting for age increased the repeatability estimate for maternal choice of canopy cover over nests. We detected canalization (diminished within-individual variance with age) of canopy cover choice in a reduced dataset despite no shift in repeatability. Additionally, random regression analysis revealed that females became more divergent from each other in their choice of canopy cover with age. Thus, properly modeling age-related variance should more precisely estimate heritable potential, and assessing among- and within-individual variance components in addition to repeatability will offer a more mechanistic understanding of behavioral variation across age.

We collected data on the nesting ecology of C. picta from 1989–2018 at the Thomson Causeway Recreation Area along the Mississippi River in northwest Illinois. We created separate datasets for each dependent variable so that females that remained in each dataset had >1 observation for that variable for analysis. Please see the Methods section in the manuscript for more details.  

Date = date of the observation

Year = year of the observation

YearNestNumber = year + nest number, which is a unique identifier for each nest

ID = female identification code for field ID

Nest = nest ID which restarts at one each year

PL = plastron length

SoilType = 0 represents nests that were entirely loam; 1 represents nests that contained some gravel

TrueID = female identification code that has been cleaned back on campus (used for analysis over ID)

Observed = 1 for every time a female was observed

Frequency = the number of observations a female laid a nest and that dependent variable was recorded

Age = age of the female at the time of that observation

ConvertedCanopyOpen = overstory canopy cover from hemiphotos and converted densiometer measures 

CanopyOpenness = overstory canopy cover from hemiphotos only

DTW = distance to water