One of only two species of marine turtles who nest in mass numbers (Kemp’s ridleys being the other), olive ridley turtles emerge from the sea in their tens of thousands onto the beaches of Ostional to lay their eggs, mainly in the rainy season from August to December. These arribadas (the Spanish word for arrivals) can last over twelve hours, with a steady stream of females crawling slowly out of the water to a free patch of sand beyond the high tide line where they will begin to lay their eggs.
Each individual will lay around one hundred eggs over the course of a few days; collectively several million eggs may be deposited on the shores of Ostional during a single arribada. It is the sheer number of eggs that is the evolutionary reason behind the unusual behaviour of the olive ridleys: the more eggs there are, the better chance the offspring have of survival. With so many eggs and hatchlings for predators to prey on, the likelihood of a hatchling making it out to sea increases dramatically. Despite the mass layings, however, the odds are still stacked overwhelmingly against the young turtles – only one out of three hundred hatchlings from the protected beaches of Ostional will reach adulthood.