Data from: Breeding sex ratio and population size of loggerhead turtles from Southwestern Florida

Species that display temperature-dependent sex determination are at risk as a result of increasing global temperatures. For marine turtles, high incubation temperatures can skew sex ratios towards females. There are concerns that temperature increases may result in highly female-biased offspring sex ratios, which would drive a future sex ratio skew. Studying the sex ratios of adults in the ocean is logistically very difficult because individuals are widely distributed and males are inaccessible because they remain in the ocean. Breeding sex ratios (BSR) are sought as a functional alternative to study adult sex ratios. One way to examine BSR is to determine the number of males that contribute to nests. Our goal was to evaluate the BSR for loggerhead turtles (Caretta caretta) nesting along the eastern Gulf of Mexico in Florida, from 2013-2015, encompassing three nesting seasons. We genotyped 64 nesting females (approximately 28% of all turtles nesting at that time) and up to 20 hatchlings from their nests (n= 989) using 7 polymorphic microsatellite markers. We identified multiple paternal contributions in 70% of the nests analyzed and 126 individual males. The breeding sex ratio was approximately 1 female for every 2.5 males. We did not find repeat males in any of our nests. The sex ratio and lack of repeating males was surprising because of female-biased primary sex ratios. We hypothesize that females mate offshore of their nesting beaches as well as en route. We recommend further comparisons of subsequent nesting events and of other beaches as it is imperative to establish baseline breeding sex ratios to understand how growing populations behave before extreme environmental effects are evident.

具有温度依赖型性别决定（temperature-dependent sex determination）机制的物种，正面临全球气温升高带来的生存风险。对于海龟而言，孵化期高温会使其种群性别比例向雌性一侧倾斜。学界担忧气温升高将导致后代性别比例严重偏向雌性，进而引发成年种群性别比例失衡。然而，海洋中成年海龟的性别比例研究面临诸多实操难题：海龟种群分布广泛，且雄性海龟终生栖息于海洋，难以直接观测取样。繁殖性别比例（breeding sex ratio, BSR）因此成为研究成年种群性别比例的可行替代方案。解析繁殖性别比例的一种有效途径，是统计参与筑巢雌龟繁殖的雄性个体数量。本研究以2013至2015年（涵盖3个繁殖季）在墨西哥湾东部佛罗里达沿岸筑巢的红海龟（Caretta caretta）为对象，旨在评估其繁殖性别比例。研究团队利用7个多态性微卫星标记（microsatellite marker），对64只筑巢雌龟（约占同期筑巢海龟总数的28%）及其每巢最多20只幼龟（总计989只）进行了基因分型。结果显示，70%的被分析巢穴存在多父本贡献现象，共鉴定出126只雄性个体。本次研究测得的繁殖性别比例约为每1只雌性对应2.5只雄性。未在任何巢穴中发现重复参与繁殖的雄性个体。考虑到红海龟后代初始性别比例偏向雌性，这一繁殖性别比例结果以及无重复雄性参与的现象均超出预期。我们据此提出假说：雌龟会在筑巢海滩近岸海域以及迁徙途中完成交配。鉴于在极端环境影响显现前建立繁殖性别比例基线数据对理解种群增长动态至关重要，本研究建议对后续繁殖季以及其他沿岸筑巢区域开展进一步对比研究。