Podocnemis expansa Turtles Hint to a Unifying Explanation for the Evolution of Temperature-Dependent Sex Determination in Long-Lived and Short-Lived Vertebrates

Abstract

The adaptive significance of temperature-dependent sex determination (TSD) remains elusive for many long-lived reptiles. Various hypotheses proposed potential ecological drivers of TSD. The Charnov-Bull’77 model remains the most robust, and explains the maintenance of TSD in short-lived vertebrates, where sex ratios correlate with seasonal temperatures within years that confer sex-specific fitness (colder springs produce females who grow larger and gain in fecundity, whereas warmer summers produce males who mature at smaller size). Yet, evidence of fitness differentials correlated with incubation temperature is scarce for long-lived taxa. Here I propose that the Charnov-Bull’77 model applies similarly to short-lived taxa, but at a longer temporal scale, by revisiting ecological and genetic data from the long-lived turtle Podocnemis expansa. After ruling out multiple alternatives, I hypothesize that warmer-drier years overproduce females and correlate with optimal resource availability in the flood plains, benefitting daughters more than sons, whereas resources are scarcer during colder-rainier years (due to reduced flowering/fruiting) that overproduce males, whose fitness is less impacted by slower growth rates. New technical advances and collaborative interdisciplinary efforts are delineated that should facilitate testing this hypothesis directly, illuminating our understanding of TSD evolution in P. expansa and other long-lived TSD reptiles.