Data from: Detecting elusive aspects of wildlife ecology using drones: new insights on the mating dynamics and operational sex ratios of sea turtles

Offspring and breeding (operational) sex ratios (OSR) are a key component of demographic studies. While offspring sex ratios are often relatively easy to measure, measuring OSRs is often far more problematic. Yet highly skewed OSRs, and a lack of male-female encounters, may be an important extinction driver.
Using loggerhead sea turtles (Caretta caretta) as a case study, we showed the utility of drones, i.e. unmanned aerial vehicles (UAVs) to distinguish adult males and females in a marine breeding area, using a combination of morphological characteristics (tail length) and behavioural differences (active mating, courting and searching by males versus resting by females). Through repeated surveys, we documented seasonal changes in the OSR.
While the number, and ratio, of males and females on the breeding grounds changed massively, the ratio of receptive females (derived from the rate of influx of new individuals to the area) to breeding males remained close to 1:1 for much of the period before nesting commenced. Hence, we show how large imbalances in the number of adult males and females may translate into relatively balanced OSRs.
Our results suggest that the departure of males from the breeding grounds is linked to a decline in female receptivity, with female sea turtles being known to store sperm to ensure high clutch fertility throughout the nesting season.
In conclusion, while we detected up to three times more females than males at the breeding ground, at present, OSRs appear stable. However, because most males breed annually (versus biannually by females), there might only be ˜100 males in the adult population (i.e. adult sex ratio of 1:7.5), which might become further skewed under expected climate change scenarios; thus, we need to identify the minimum number of males required to prevent extinction. Finally, we highlight the use of UAVs for assessing the mating dynamics of other marine, terrestrial or avian species, in which adults might exhibit visually detectable differences, such as sexual dimorphism, external body characteristics or grouping tendencies.

子代性比与繁殖操作性别比（operational sex ratio, OSR）是种群人口统计学研究的核心组成部分。尽管子代性比的测定通常相对简便，但操作性别比的测算往往难度更高。然而，高度偏离的操作性别比以及雌雄个体相遇机会的匮乏，可能是种群灭绝的重要驱动因素。
本研究以红海龟（Caretta caretta）为案例对象，验证了无人机（unmanned aerial vehicle, UAV）在海洋繁殖场区分成年雌雄个体的实用性：结合形态特征（尾长）与行为差异（雄性主动交配、求偶与巡游，雌性静止休憩）开展识别。通过多次重复调查，我们记录了操作性别比的季节动态变化。
尽管繁殖场中雌雄个体的数量与比例波动剧烈，但在产卵季开始前的大部分时段内，可交配雌性（通过新个体迁入该区域的速率估算得到）与繁殖雄性的比例始终维持在接近1:1的水平。因此，本研究阐明了成年雌雄个体数量的巨大失衡如何转化为相对均衡的操作性别比。
我们的研究结果显示，雄性个体从繁殖场的迁出行为与雌性可交配性的下降存在关联——已知雌性海龟会储存精子，以确保整个产卵季内窝卵的受精率维持较高水平。
综上，尽管我们在繁殖场观测到的雌性个体数量最多可达雄性的三倍，但当前操作性别比仍维持稳定。然而，由于多数雄性海龟每年均可繁殖（而雌性为每两年繁殖一次），成年种群中的雄性个体数量可能仅约100只（即成年性比为1:7.5），且在未来气候变化情景下该比例可能进一步偏离；因此，我们亟需明确防止种群灭绝所需的最小雄性个体数量。最后，本研究强调了无人机可用于评估其他存在成体视觉可辨差异的海洋、陆生或鸟类物种的交配动态，这些差异包括雌雄异形、外部躯体特征或集群行为倾向。