Terrapin Station: Individual, Sex, and Site Factors Related to Turtle Growth Variability

For many turtle species, life history traits such as body size, age at maturity, and somatic growth rate can vary among individuals and habitats and between the sexes. Therefore, it is important to consider factors that may influence growth when modeling (somatic) growth for turtles. Long-term capture-mark-recapture studies lend themselves to studying somatic growth in turtles due to the repeated measurements of individuals over time. We used a long-term dataset to examine growth patterns of philopatric Diamondback Terrapins (Malaclemys terrapin) on Kiawah Island, South Carolina, USA. We used a hierarchical three-parameter von Bertalanffy model to estimate individual growth of 44 female and 36 male Diamondback Terrapins that were each captured 3-17 times between 1983 and 2019. Sex and site (i.e., tidal creeks) were included as second-level model effects. Mean maximum asymptotic size (plastron length; L-infinity = 173.4 mm for females and L-infinity = 104.4 mm for males) and mean growth coefficients (K = 0.28 for females and K = 0.61 for males) varied between sexes. Growth variability among individuals was high, ranging from 23 to 56% within species for different parameters, suggesting that models not accounting for individual variability would be pooling dissimilar information. Site was a significant covariate for male growth, but not female growth. Understanding how Diamondback Terrapin somatic growth varies within a population may inform habitat quality as well as population health and vulnerability to anthropogenic stressors. Our model can serve as a comparison for other Diamondback Terrapin populations and provide more detailed information for demographic models that can be used in conservation decisions.