Recent increases in sea turtle incubation durations on a North Carolina, USA, beach despite a warming climate

Warming temperatures due to climate change are feminizing sea turtle primary sex ratios, reducing hatchling fitness, and, in extreme cases, limiting hatchling production, including for temperate nesting species such as loggerhead turtles (Caretta caretta). Though more females may lead to short-term population growth through increased egg production, long-term gains may be threatened by the scarcity of males for mating and reduced survival rates of early life stages. Beaches near the limits of sea turtle nesting ranges are typically cooler and thus critical for species conservation efforts as they contribute a higher percentage of males to regional breeding grounds both now and in the future. A feminizing trend up to 88% was previously estimated for loggerhead hatchlings through 2015 on Bald Head Island, North Carolina, USA, near the northern extent of the northwest Atlantic loggerhead nesting range. However, despite a continued increasing trend in air temperature, average incubation durations and modeled primary sex ratios over the past 5 years (67% female) are more similar to those from the mid-2000s. Environmental conditions during incubation, behavioral changes in nest site selection and timing, and physiological variables including clutch size and maternal identity were investigated using a generalized additive model to explain this recent reversal. Increased rainfall, alongshore local movement of nest placement toward cooler conditions, and a minor shift in nesting phenology earlier in the year combined to counteract the 1.1°C increase in average air temperature experienced by incubating nests now versus 20 years ago. Behavioral adaptations may thus mitigate some of the projected impacts from climate change but are insufficient on their own as rainfall had a larger effect size than behavior. Without significant behavioral changes or management interventions, sustained future male hatchling production at many nesting beaches will likely depend on increased rainfall—an uncertain projection for many such beaches.