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 (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.