Gail Schofield, Martin K. S. Lilley, Graeme C. Hays, Richard D. Reina, Kostas A. Katselidis
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 (vs. biannually by females), there might only be c. 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.
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