The breach to the beach
Summary: Ostional, Costa Rica, is one of the eight beaches in the world where olive ridley sea turtles arrive in mass to lay their eggs
By Katherine Herrick
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A galaxy of stars shines through the black sheet of the night sky in Ostional, Costa Rica. Even in the morning, the faint outlines of Venus and Jupiter consecutively puncture the dim skyline as the sun begins to rise again. There are few street lights polluting this view along the dusty dirt road just beyond the beach – an intentional design – but not for the benefit of stargazers enjoying the burning streaks of shooting stars. The absence of light is to limit possible distractions for the influx of female olive ridley turtles that cluster on the beach to build nests and lay eggs, guided only by instinct and the subtle reflections of light on the ocean’s surface that help their return to the sea.
In just three to four days, hundreds of thousands of females may make this ascent called the arribadato lay roughly a hundred eggs. Each egg, akin to a ping-pong ball in size and texture, is lubricated by a thick mucus-like layer of discharge excreted by the mother after each new egg drops into the nest. The discharge leaves behind a glossy shine on the white orbs, mirroring the glistening stars hanging above. Despite the large number of eggs each female lays, multiplied by the mass number of turtles joining in the process, relatively few hatch and make it back into the sea. Only one in 1,000 of the hatchlings survive.
Before the mother is ready to form her nest and leave her eggs to hatch, she must first make it up onto the beach – a seemingly simple task, but it can be easy to get stuck between the edge of the water and the beach, particularly when the tide is low. El espejo del mar, the mirror of the sea, is an area of land washed over by the tide but not fully submerged in water, making the consistency of the sand stickier and difficult for the turtles to cross. Their fleshy underbelly can be suctioned to the wet sand in the same way a human foot might get stuck plodding along the oceanline. If stuck, all the female can do is sit and wait until the tide rises enough to help push her along.
Once on the dry sand, the female can find a spot to dig her nest. She first sniffs the sand with deep inhalations, assessing the spot’s humidity to make sure the area isn’t too wet or too dry before flattening the bed of the nest by gently patting her flippers on the ground. She then begins to dig, rounding out a hole roughly 40 to 50 centimeters deep with her back flippers, which work like a scaly set of human hands. Once she cannot feel any more sand below her to excavate, her body switches into a trance-like state as she lays the eggs. One by one, they are gently dropped to join the clutch. After she is finished, she refills the hole and compresses the sand on top of the nest to protect it from predators lurking above. She then thrusts her flippers around, shuffling the sand to camouflage the nest, and she might even move to another spot and rearrange the sand into a small dip, creating a false nest to fool a predator. Each step in the process takes roughly 10 to 15 minutes for a healthy turtle to complete, but it can take much longer for older or disabled mothers. But once she is finished, she heads back to the ocean; she will return again in 4 to 5 months to lay another clutch. This continues for two years with a three-year break after each cycle.
The mother and the clutch must overcome many obstacles to successfully hatch the offspring – predators, poachers, harvesters – and even simply finding a spot for the nest can be a challenge. During the early days of the arribada, the sheer number of turtles on the beach can make space limited. This might seem counterintuitive for the reproduction of the species as some of the nests might be dug up by mothers trying to find a place for their own eggs, but the turtles do this as a survival strategy for the group. The massive crowds laying eggs in such a short time increase the chance that a few of the nests will hatch before getting eaten by predators – it’s like an all-you-can-eat buffet – there are bound to be leftovers if the consumers have to finish it all in one sitting. But in addition to the massive hordes of turtles filling the beach, the environment can limit the available nesting spots depending on the season. In the rainy season (May through November), the high tide shrinks the space where nests could be built, and the estuary farther up on the beach widens, covering some of the nests in its path as it flows down to the ocean. In the dry season (December through April), these types of environmental factors aren’t as much of an issue.
If the egg-laying process goes uninterrupted by predators or other environmental circumstances, the embryos will stick to the shell and begin developing within two days, forming the delicate body of the baby inside. The layer of discharge enveloping the eggs carries genetic information that also helps with the embryos’ development, and the spherical shape of the eggs allows oxygen to flow through the gaps among the shells. Humidity and temperature are both important factors for egg development; oxygen flow in the nest is vital, and the temperature determines the sex of the offspring. Up to 29 degrees Celsius, the eggs will be male; 31 degrees Celsius or higher produces a female. With the global rise in temperature, females are significantly more common, threatening the survival of the already vulnerable species. The females do have the ability to retain sperm within their body for up to two years, which could be an evolutionary adaptation to help the olive ridley turtles continue to reproduce even if they cannot find a mate. The trait might temporarily delay the consequences rising temperatures will have on the species, but higher temperatures continue to be a concern.
For the turtles that do survive inside the nest, they emerge 45 to 55 days after they are laid. It takes two days for them to break through the tough exterior of their shells and another two days to climb out of the nest, using each other and the sand to form a path to the surface before gently poking their noses through the grains of sand. In 10 to 15 years, the female hatchlings will return to repeat the journey up the beach their mothers made. The new generations will use only their instincts to find their way back to the same beach, where they will follow the same path underneath the same galaxy of stars above, continuing the same ethereal cycle of life.
All facts and figures provided by the Asociación de Desarrollo Integral Ostional. Interpretation from the original Spanish by Carla Willoughby.
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