Small differences between primary and breeding sex ratios at the largest loggerhead turtle rookery in the North Pacific

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Elucidating how sex ratios change as a cohort ages may aid with understanding the relative recruitment of male and female adults to a population. In sea turtles, sex is determined by incubation temperature, with males and females produced at low and high temperatures, respectively. At rookeries exposed to high incubation temperatures, the primary sex ratios are thus skewed toward females. However, because very high incubation temperatures also negatively affect the survival-related traits of female hatchlings, the female bias may be mitigated as the cohort matures and the adult sex ratio may be more male-biased than primary sex ratios. Here, we examined whether the sex ratio changes from hatchlings to breeding adults at Yakushima Island, Japan, the largest loggerhead turtle Caretta caretta rookery in the North Pacific, which is not exposed to high incubation temperatures. The mean (±SD) primary sex ratio (% female) during 1975-2022, estimated from incubation durations linked to sand temperatures, was 24.7 ± 13.6%. Based on genotype data at 3 microsatellite loci from 26 females and their 291 offspring sampled in 2020 and 2021, we detected multiple paternity in 5 clutches (19.2%) and estimated 34 unique sires, which provided a breeding sex ratio (% female) of 43.3%. Small differences between primary and breeding sex ratios suggest similar survival rates for male and female offspring. This rookery is unusual in that it has male-biased hatchling production, whereas most sea turtle rookeries have female-biased hatchling production. The Yakushima rookery is likely resilient to climate warming.