THE beach is totally deserted. Suddenly, and seemingly out of nowhere, reptiles emerge in their thousands from the sea, clambering onto the sand. A few days later, they鈥檙e gone again. It鈥檚 cited as one of the most spectacular manifestations of reptile life anywhere: the arribada.
鈥淩eptiles emerge in their thousands from the sea鈥�
Arribada is a Spanish word meaning 鈥渁rrival by sea鈥� and refers to the mass nesting behaviour exhibited by Kemp鈥檚 Ridley and Olive Ridley sea turtles (Lepidochelys kempii and L. olivacea). Although a few sea turtle species nest collectively, Ridleys nest in much larger numbers and with greater synchronicity than any others.
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As many as half a million females travel thousands of kilometres to mate and lay their eggs together in a single arribada, which usually lasts only a few days. It鈥檚 an astonishing feat 鈥� how do the turtles manage to find a small stretch of sand in such a small window of time?
Very little is known about why or how an arribada occurs. We know that they take place roughly once a month during the six-month arribada season, but not always at the same time. As yet, we have no way of predicting exactly when it will happen, so how do the turtles? Some studies suggest that the moon and the tide have a role to play; others that it鈥檚 an onshore wind. Some researchers think the turtles may send each other underwater messages signalling when the time is right. Arribadas only occur on a small number of beaches around the Pacific, Atlantic and Indian oceans, but the locations don鈥檛 seem to have anything specific in common. For now, only the turtles know how an arribada is organised.
So why do they do it? It鈥檚 thought that so many eggs hatching at one time means that any predators will be quickly satiated, leaving the majority of baby turtles safe. Or it could be that living in such a huge expanse of water makes it difficult to find a mate 鈥� arribadas provide a great opportunity. Or perhaps there isn鈥檛 any advantage at all, and mass nesting is simply the result of females all responding to the same breeding cues.
To add to the mystery, not all female Olive Ridleys engage in arribadas; many nest in solitude. The existence of two types of reproductive behaviour within the one species makes arribadas all the more baffling. The reason is probably a combination of better avoidance of predators and resistance to overcrowding. There鈥檚 an optimum number of females in an arribada below which the predators are left hungry, and above which there are too many nesters and the eggs risk getting squashed.
Want to witness an arribada for yourself? The eastern Pacific arribada season is about to start, and the best place to go is Ostional National Wildlife Refuge in Guanacaste, Costa Rica.
