Front-loading life histories: the enduring influence of juvenile growth on age, size, and reproduction of primiparous female freshwater turtles

Primary questions: (1) How do juvenile growth rates influence age and body size at maturity of females of three species of freshwater turtles ? (2) Are the patterns similar among species that occupied the same wetlands over the same three decades ? (3) What are the reproductive traits (i.e. clutch size and egg size) of primiparous females (first lifetime reproduction)? (4) Is there evidence that adult growth rates subsequently reduce the initial differences in the body size and reproductive traits of primiparous females ? Secondary questions: We asked several additional questions of Painted Turtles. Are growth rates of older juveniles more similar to growth rates of young juveniles or adults ? What is the earliest age at which juvenile growth rate is detectably correlated with age and body size of primiparous females ? Sample sizes of the other two species were too small to use for these questions. Organisms: Three long-lived freshwater turtle species: Painted Turtles (Chrysemys pieta marginata), Blanding's Turtles (Emydoidea blandingii), and Snapping Turtles (Chelydra serpentina). Field site: University of Michigan, E.S. George Reserve, southeastern Michigan, USA. Methods: We conducted a 33 year mark-recapture study to document juvenile and adult growth rates and age and body size at maturity of females. We used X-radiography to determine clutch size and egg widths of primiparous and older females of all three species. Conclusions: (1) Juvenile growth rate was the most influential trait determining within-population variation in life-history trait values of primiparous females of all three species of long-lived freshwater turtles, and that variation persisted for many years in older adults. (2) Fast-growing juveniles of all three species matured earlier and at larger (or similar) body sizes than slow-growing juveniles. (3) The relationship between juvenile growth rates and age and size at maturity in Painted Turtles was established by age 4 years. (4) Variation in indeterminate (post-maturation) growth was insufficient to reduce differences in reproductive traits within cohorts of females. (5) Similar results from all three turtle species (families Emydidae and Chelydridae) suggest that the relationships between juvenile growth rates and age and size at maturity were established in a common ancestor early in the evolutionary history of turtles.