AbstractThe environment experienced during embryonic development is a rich source of phenotypic variation, as environmental signals have the potential to both inform adaptive plastic responses and disrupt normal developmental programs. Environment-by-embryo interactions are particularly consequential for species with temperature-dependent sex determination, a mode of sex determination common in non-avian reptiles and fish, in which thermal cues during a discrete period of development drive the formation of either an ovary or a testis. Here we examine the impact of thermal variation during incubation in combination with developmental exposure to a common endocrine-disrupting contaminant on fitness-related hatchling traits in the American alligator (Alligator mississippiensis), a species with temperature-dependent sex determination. Using a factorial design, we exposed field-collected eggs to five thermal profiles (three constant temperatures, two fluctuating temperatures) and two environmentally relevant doses of the pesticide metabolite dichlorodiphenyldichloroethylene; and we quantified incubation duration, sex ratios, hatchling morphometric traits, and growth (9-10 days post-hatch). Whereas dichlorodiphenyldichloroethylene exposure did not generally affect hatchling traits, constant and fluctuating temperatures produced diverse phenotypic effects. Thermal fluctuations led to subtle changes in incubation duration and produced shorter hatchlings with smaller heads when compared to the constant temperature control. Warmer, male-promoting incubation temperatures resulted in larger hatchlings with more residual yolk reserves when compared to cooler, female-promoting temperatures. Together, these findings advance our understanding of how complex environmental factors interact with developing organisms to generate phenotypic variation and raise questions regarding the mechanisms connecting variable thermal conditions to responses in hatchling traits and their evolutionary implications for temperature-dependent sex determination.