Though lots of the newly hatched young are taken by predators, when large numbers emerge virtually simultaneously, each individual’s chance of survival is greater when part of a crowd.
Possibly the best-known mass nestings are those of turtles. In fact, only two of the world’s chelonians (turtles, terrapins and tortoises) take part in spectacular mass nesting: the olive ridley and the closely related Kemp’s ridley.
What cause the mass nestings?
So what causes the sudden rush of females? The mass nestings often takes place close to the last quarter of the moon, so the lunar cycle is a strong contender. The tide might be another factor, since the largest arribadas occur at high tide. Offshore winds may also play a part as some experts think that pheromones are released en masse by females.
When do mass nestings take place?
In Costa Rica, olive ridleys nest during the rainy season. Females start gathering off the nesting beach a few weeks before the event, which tends to start slowly at first, with the numbers building rapidly over the next three to seven days.
However, arribadas do not always occur when predicted. In fact, a typical female is more likely to nest on her own than with others, and may choose to alternate between the two strategies.
Synchronised breeding
Synchronised breeding is a better-known phenomenon– any marine animals, from fish to worms, display similar sorts of behaviour.
People in California and Baja California refer to the breeding behaviour of grunion as ‘silver tides’. During large grunion runs the surfline is swamped with fish. The females push their tails into the wet sand to lay their eggs while the males release sperm. Females spawn up to six times each season and can produce betwen 1,600 and 3,600 eggs each time.
Adult red crabs live in the forested interior of Christmas Island and stream to the coast in November and December
to breed – the most spectacular crab migration in the world. Each female releases up to 100,000 eggs into the sea. After hatching, larvae drift in swarms of millions, attracting manta rays and whale sharks.