Turtles have great taste in beaches. They will only lay their eggs on beaches with the finest sand. The Pacific coast of Nicaragua is one such spot: a string of golden beaches, with sand so soft it squeaks underfoot, rank among the world’s most popular nesting grounds for the olive ridley turtle.
Playa La Flor, in particular, is one of just a handful of beaches worldwide where this solemn maternal quest has morphed into one of nature’s greatest spectacles. Over the course of two to three nights, tens of thousands of females drag themselves onto this one beach to partake in a synchronised birthing event known as an arribada.
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We don’t yet understand the precise trigger for this event – it could be to do with sea currents, offshore winds, pheromones released by the turtles or the lunar cycle – but the sight is extraordinary. An entire beach studded with expectant mothers packed so tightly there’s barely room to step in between and whose tracks churn up the sand like a freshly ploughed field.
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The effect is overwhelming. Not just for any human lucky enough to witness it, but for the turtle’s many predators. Gathering in vast numbers increases the female’s chances of maternal success.
The turtles will all have started their life on one of these beaches, as a little grey hatchling with a one-in-a-thousand chance of survival. After 15 years wandering the world’s oceans, dodging sharks and dining on jellyfish, they reach sexual maturity and return to their natal beach to breed.
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This stunning feat of navigation is achieved through geomagnetic imprinting: while developing, the precise magnetic field of her nest is etched into the hatchling’s brain. As an adult, this magnetic compass is so powerful that it guides her to within a few hundred metres of the spot she hatched to lay her own eggs.
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But first, she must find a mate, or three. Each female copulates with multiple males to maximise her chances of genetic compatibility. This she does in the warm waters offshore, which look like a real-life turtle soup as dozens of olive ridley’s bob about behind the surf.
A few weeks later, she will lay about a hundred leathery, ping-pong-ball-like eggs, each with the potential to be male or female (a turtle’s sex is governed by incubation temperature; below 28°C and the eggs will become male, but above 31°C they become female).
Environmental sex determination is common among reptiles and pre-dates the genetic coordination found in mammals. Turtle conservationist Tim Rusmisel of Vital Actions told me how global warming is playing havoc with this system. Most of the nests now hatch as females and male turtles are going extinct.
