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性比（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，但操作性性比的测算往往难度大得多。然而，严重偏倚的操作性性比以及雌雄个体相遇机会的缺失，可能是物种灭绝的重要驱动因素。 本研究以蠵龟（Caretta caretta）为案例对象，证实了无人机（unmanned aerial vehicles, UAVs）在海洋繁殖区域区分成年雌雄个体的实用性，具体通过结合形态特征（尾长）与行为差异（雄性主动交配、求偶、巡游，雌性则静栖）实现。通过多次野外调查，本研究记录了操作性性比的季节动态变化。 尽管繁殖场地内雌雄个体的数量及比例发生了剧烈变化，但在产卵开始前的大部分时段内，发情雌性（通过新个体迁入该区域的速率推算得到）与繁殖雄性的比例始终接近1:1。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性比。 研究结果显示，雄性个体离开繁殖场地的行为与雌性发情状态的下降存在关联——已知雌性海龟会储存精子，以确保整个产卵季的窝卵受精率维持在较高水平。 综上，尽管本研究在繁殖场地观测到的雌性数量最多可达雄性的三倍，但当前操作性性比整体保持稳定。然而，由于多数雄性每年均可繁殖（而雌性繁殖周期为两年一次），成年种群中的雄性个体数量仅约100头（即成年性比为1:7.5），且在预期的气候变化情景下，该比例可能进一步偏倚；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具有可视性雌雄差异的海洋、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倾向等。