It is one of biology's enduring mysteries: green - and other sea - turtles reach sexual maturity, mate and return to the very beach at which they came into the world, where they lay their eggs.
Like prodigal sons, green turtles always come home
It is one of biology's enduring mysteries: green - and other sea - turtles reach sexual maturity, mate and return to the very beach at which they came into the world, where they lay their eggs. Their journeys are frequently hundreds of miles and, sometimes, even thousands of miles long. "Nobody has ever been able to explain this homing behaviour," said Dr Abdulaziz al Kindi, professor of biology and a turtle researcher at Sultan Qaboos University in Muscat, Oman. "It is very mysterious."
As many as 30,000 female green turtles nest each year on the beaches of Ras al Jinz on the Omani coast, the easternmost point of the Arabian Peninsula. It is the most important turtle nesting site in the Indian Ocean, second worldwide only to Australia. Green turtles arrive from as far away as South Africa and Yemen to lay eggs there under cover of night. During peak season, which is between June and September, as many as 500 green turtles, the largest of the hard-shell sea turtles at about 150 kilograms, crawl up onto its beige sand beaches.
Watching them nest is like "wrestlemania", said Dr Kindi, who has been conducting research at Ras al Jinz since 1999. "There are so many turtles nesting in that short, one kilometre of beach that they are almost two to three metres away from each other, less than that sometimes." The turtles slowly crawl up onto the dry beach, and comb the sand for a suitable spot. Their goal is warm air and just enough humidity to allow them to dig a metre deep, 25cm-wide pit that will not collapse.
Everything must be perfect. The female turtle will abandon a half-finished nest if she feels something is not right. "It is well-orchestrated, genetically, we think. It's what we call instinctive behaviour. They come from the sea, they emerge. They go about finding suitable ground, then they start digging one metre deep - if they feel at that time the humidity is right, and they can create a nest. If not, they abandon it and look for another location."
The female turtle will produce anywhere between 80 to 120 eggs, Dr Kindi explained. The turtle then covers the eggs with sand, camouflaging her precious deposit with decoy dunes and a fake body pit, he says. The aim is to trick predators such as foxes and hyenas who eat the eggs. "By the end of the four-hour, exhausting, successful nesting, you won't know where the nest is," Dr Kindi said. The female turtle then trudges back to the sea, leaving a trail in the sand resembling that of a tractor wheel. The turtle's tail, dragging through the sand, forms the middle groove. The turtle's right and left flippers, digging in to pull the turtle's torso forward, leave marks like the tread on the edge of a tyre.
The eggs are left to hatch by themselves, the temperature of the nest determining whether the next generation is predominantly male or female. At about 26 degrees Celsius, the hatchlings will be mostly female. Between 21 to 22 degrees Celsius, the hatchlings will be predominantly male. Dr Kindi was able to duplicate those conditions in the lab and get the same results. "The temperature of the incubator, whether natural or in the lab, determines the sex ratio," said Dr Kindi. "That is what is incredible about turtles."
The eggs, if they successfully dodge animal and human predators, must remain undisturbed in the warm sand for about 55 days before hatching. The baby turtles hatch in the sand but wait just below the surface until the air is cool, usually at night. Once they are above ground, the five-centimetre-long turtles scurry towards the sea, their flippers spinning like propellers, using the moonlight as a guide to the ocean.
At Ras al Jinz, close to 99 per cent of all eggs will grow to be hatchlings - a much higher rate than at other beaches. Typically, the hatching rate is between 85 per cent and just above 90 per cent at other nesting beaches. There are a few theories why for thousands of years turtles have been drawn to Ras al Jinz. One regards the shape of the beach. "It is a saucer shape," said Dr Kindi. "The ocean on one side while the mountains guard it on the other side, creating an ideally protected territory."
At the easternmost tip of the Arabian peninsula, it is one of the few spots where cold and warm currents meet. The temperatures are cooler than other beaches in Oman, and hover near the optimum 26 degrees. Also, there is a very fertile feeding ground in the Arabian Sea just off the Omani coast, said Dr Kindi. There is a 300km stretch of weeds and algae for the turtles to feast on. Nevertheless, the survival rate of hatchlings is low. Of the thousands that emerge from their shells, only one in 1,000 survive to adulthood. Those that make it, head out on their long journey, swimming across oceans and seas through adolescence and adulthood, until 33 years later when they return to the beaches of their birth.
Turtles can live as long as 150 years, swimming as much as 65,000km. "We can read from their migration that they almost cover all the globe," says Dr Kindi. "And that is why we call them global ambassadors." This worldly lifestyle has also enabled researchers to use the turtles to test for marine pollution, he says. Dr Kindi tests their eggshells and amniotic fluid during nesting for heavy metals, paint, lead washed out from oil tankers and other unnatural sea travellers.
Humans can also learn much from the turtle's lifestyle as well, Dr Kindi adds. "There is a very good message between their metabolic rate and the way they live," he says. "They are slow, but they are very sure of their destination. And they live longer because of that." firstname.lastname@example.org