The mystery of bimodal nesting seasons in marine turtles

The seasonality of egg-laying in marine turtles is a well-known phenomenon that has been used to determine population trends based on an integration of the number of female emergences or nests during a season. However, in some cases, several peaks in female emergences are detected during single year. In this paper, we explore three such situations, two involving leatherback turtles laying eggs in French Guiana and in Northwest Coast of Papua, Indonesia and another involving green turtles in the Indian Ocean. In the three cases, we demonstrate the existence of a bimodal nesting season using a new statistical model. With this model, we show that estimates that fail to consider this bimodality into account can provide biased results both in terms of describing the phenology and quantifying the number of nests. The origin of these different groups of females is discussed based on the available information. This new model opens the door to a more rigorous analysis of egglaying seasonality in marine turtles. Such rigor is essential in the current context of climate change, where shifts in seasonality can have a major influence on these species due to their temperature-sensitive sex determination.