Gail Schofield, Kostas A. Katselidis, Martin K. S. Lilley, Richard D. Reina , Graeme C.
Research on sea turtles is strongly focused on collecting information on beaches about nesting females, their eggs and offspring. The sex of sea turtle offspring is dependent on sand temperature: the warmer the sand, the more females are produced, with many more females than males being produced globally. Yet, we do not know how offspring sex ratios translate into those of reproductively active adults at breeding sites. Monitoring sea turtles in the marine environment is difficult, particularly when gathering data over extended periods, wide areas and under variable sea and weather conditions. Here, we used drones i.e. unmanned aerial vehicles (UAVs) to resolve the knowledge gap on this elusive aspect of sea turtle reproductive ecology. We demonstrated that UAVs were up to seven times more effective for documenting the presence of sea turtles compared to parallel surveys from a boat. We also confirmed that the aerial images could be used to distinguish males from females based on morphological characteristics (males have longer tails) and behaviour (mating and courtship). Over a 2.5-month period, we recorded over 326 reproductive events. During April, primarily males were detected, swimming close to shore. Female numbers suddenly increasing in early-May, with mating activity peaking in mid-May. Subsequently, even though female numbers continued to increase, male numbers and mating activity declined, with most males disappearing by late-May. Female sea turtles are only receptive for around 10 days; thus, the effort required for males to search for females that are still receptive, among those that are not, might become too costly. We recorded a maximum of 89 males and 242 females during surveys (i.e. about one male to every three females), supporting that the strong female bias in offspring sex ratio is retained into adulthood. Of interest, when we accounted for the short period of female receptivity, we found that the ratio of reproductively active males and females remained similar throughout the breeding period. Our results confirm the utility of UAVs for monitoring sea turtle populations in the marine environment, reaffirming the importance of learning more about males to improve conservation efforts.
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Image caption: Adult loggerhead sea turtle (Caretta caretta) at the breeding ground of Zakynthos, Greece. Photo credit: Konstantinos Papafitsoros
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