Spatiotemporal Thermal Variation Modulates Phenotypic Plasticity in Freshwater Turtle Growth, Behavior, and Sex

ABSTRACT

Phenotypic plasticity in life history traits is critical for organisms in variable environments and a key mechanism for persisting in a rapidly changing environment. For ecothermic organisms with temperature‐dependent sex determination, such as many turtle species, temperature influences phenotypic traits, including somatic growth rate, behavior, and sex, that directly impact life history parameters and long‐term population viability. As many turtle species are imperiled and climate change is pervasive, understanding how spatial and temporal thermal variation affects these traits is crucial, yet long‐term studies for exploring these trends are scarce. Using a dataset spanning more than 50 years, we evaluated somatic growth rates, behavior, and the population sex ratio of northern map turtles ( Graptemys geographica )—an ectothermic organism with temperature‐dependent sex determination—in a spring‐fed stream with a significant thermal gradient. We explored variation in instantaneous somatic growth rates and behavior temporally across three time periods (1969–1972, 2004–2007, and 2019–2022) and spatially between two thermally distinct stream reaches in 2005–2007. We evaluated sex ratios for temporal deviation from parity. Somatic growth rates differed temporally, in conjunction with warmer and longer growing seasons, but not spatially, despite different within‐stream spatial thermal profiles. Spatially distinct thermoregulatory behavioral patterns partially offset the influence of varying water temperatures between the study sections. Temporal shifts in thermoregulatory behavior patterns occurred in the warmest study period (2019–2022) and the population sex ratio became significantly female‐biased. Our results revealed that freshwater turtle somatic growth rate, behavior, and population sex ratios were altered in recent years which were characterized by warmer, longer growing seasons. Phenotypic plasticity in life history traits is essential for turtles and other ectotherms to adjust to shifting abiotic conditions. While turtles exhibited phenotypic plasticity that offset some thermal variation, it is unlikely they will be able to keep pace with climate change given projected temperatures may exceed turtles' physiological thermal thresholds without significant intervention.