The beaches we protect are visited by three species of turtle – the HAWKSBILL, the GREEN and the LEATHERBACK, but of these only the Leatherback nests regularly in great numbers.
The beautiful HAWKSBILL (eretmochelys imbricata), with its carapace that makes ‘tortoise shell’, has long been targeted by poachers and we are unlikely to see more than half a dozen in a season on the Pacuare beach. The Hawksbills in the Caribbean have a carapace length of about 80 cms or half that of the Leatherback. They are omnivorous but prefer crustaceans, molluscs, sea urchins and the like. They lay more eggs than other sea turtles, and 160 per clutch is not unusual.
The GREEN Turtle (chelonia mydas) is the largest of the hard-shell marine turtles and typically grows to a carapace length of a metre with a weight of 110/130 kilos, though individuals up to 185 kilos have been recorded. It feeds mostly on sea grasses and marine algae but is also known to eat crustaceans, jellyfish and sponges. It is known for its long migrations and uncanny navigation. Green turtles that feed along the shores of Brazil unerringly find their way 1400 miles across the Atlantic to nest on the beaches of tiny Ascension Island.
The Green turtles that we see have only come from their feeding grounds off the coast of Nicaragua, which borders Costa Rica to the north.
Green turtle meat and eggs have been a traditional food source for many native populations.
Though now officially protected by law in most countries, including Costa Rica and Panama, it is still harpooned at sea or taken on the beaches. Green turtle meat and eggs are freely available in many restaurants in Limon.
The nesting season for the Green turtle in the Western Caribbean is from June to September/October and the numbers fluctuate wildly from year to year. The beaches at Tortuguero, 50 kms to the north of Pacuare, may count Greens in their thousands one year and have only a fraction of this number the following year. At Pacuare, on a much smaller scale, we sometimes count a few hundred one year and almost none the next, but we always continue our turtle programme until late September to protect the few Greens and to monitor the last of the Leatherback hatchlings.
As one goes further south from Pacuare there are fewer and fewer Green turtles and 100 km down the coast there are almost none.
The LEATHERBACK turtle (dermochelys coriacea) is the critically endangered giant of the turtle world and it is this species which is the main object of all our three projects. It can have an overall length of over two metres and an adult may weigh from 350 to 700 kilos. The heaviest recorded was one which was drowned in a fishing net off the coast of Wales in 1988. It weighed over 900 kilos.
The Leatherback differs from other turtles in two important aspects. It does not have a carapace of shell. Instead, it is of thick black leather with seven longitudinal ridges, or keels. Also, unlike other turtles, it is able to raise its body temperature well above that of the surrounding water. This factor, combined with the Leatherback’s thick and oily covering, enables it to inhabit waters far too cold for other turtles.
Leatherbacks nest on tropical beaches but can cross the Arctic Circle in search of food. They are the world’s champion divers. The deepest recorded depth is 1.2 km (3,900 feet), slightly more than the deepest known dive of a sperm whale.
When you see a Leatherback out of the water it is a very slow and clumsy creature, hardly able to heave itself over the sand, but in the water it is a fast and agile swimmer pushed along by its powerful front flippers. It has poor hearing and eyesight but a keen sense of smell.
Every Leatherback has a pink mark on the top of its head and each one’s mark is different. Like the fingerprints of a human, it is useful for identification.
The principal food of the Leatherback is jellyfish. In fact, some Marine Biologists say that it only eats jellyfish.
The number of Leatherback nests on our beaches has varied greatly over the last fifteen years from 1400 nests in a season down to 500 and more recently between 500 and 1200.
The Leatherbacks we protect are part of the family that nest on the tropical beaches of the western Atlantic from the Caribbean northwards. After nesting, they swim to the North and North-East, moving up the US east coast and then across the Atlantic, well able to withstand the cold sea temperatures. Turtles tagged at Pacuare have been seen later off the coasts of Newfoundland, Spain, the UK and West Africa. After two or three years wandering in this fashion, the females return to the tropics and to the same area of beaches – but not necessarily the same beach – from which they set off. Then comes another laying season during which they will lay 7-9 clutches of eggs at intervals of about 10 days.
The family of Leatherbacks that nest on the south side of the Caribbean, in Surinam and Trinidad, turn eastwards after nesting and head towards West Africa.
Thanks to the development of satellite transmitters, the migration routes are becoming better known. For instance, it has recently been shown that Leatherbacks that lay on the beaches of New Guinea swim all the way across the Pacific to feed on jellyfish in the cold waters off the US West Coast, before returning again to the same tropical beaches.
Almost nothing is known about the first years of a Leatherback’s life. The hatchlings make for the sea after emerging from the nest and they are not seen again until many years later when the adult females, which have survived the countless predators, find a beach on which to lay their first clutch of eggs. The ‘lost years’ in between are a mystery. Nobody ever sees a half-grown leatherback and no reports come from sailors or fishermen of young Leatherbacks being seen at sea.
As Leatherbacks do not survive in captivity it is impossible to know for certain at what age they reach maturity. The experts’ estimate has come down from 30 years to 8-12 years but opinions still differ.
The Leatherback’s nesting takes place at night under cover of darkness and only very rarely does a turtle appear in daylight. The whole nesting process, from climbing the beach to the final return to the sea, can take up to an hour and a half.
After heaving itself over the sand, the turtle will look for a suitable nesting site, moving and turning within a small area. When it has found the right place, it will start to excavate a body-pit, using all four flippers to throw out the sand. In this way it makes a shallow depression for itself. It is then ready to excavate the egg chamber and this is done by the hind flippers only, which take out alternate scoops of sand until the depth of the chamber is exactly the length of the flipper. The last scoops may take out only a few grains of sand.
Once the egg chamber is completed, laying will start, with eggs dropping into the chamber three or four at a time, their soft shells ensuring that they never break. During the actual egg-laying the turtle goes into a semi-trance and so it is then that we can measure it for length and breadth, and, if it is not already tagged, place tags on each of its hind flippers. It will lay 80-90 full-sized eggs. These are the fertile eggs and they are followed by about 20 small eggs, which are infertile but fill the gaps between the larger ones on top, thus ensuring that there is air between the eggs in the nest.
As soon as it has dropped the last egg, the turtle starts to cover the eggs and fill the chamber with sand, using its hind flippers to pack the sand down very firmly. Once the chamber is filled, it begins to disguise the position of the nest by scattering sand with all four flippers, the front ones throwing sand far behind it. As it moves and turns, scattering sand as it goes, the immediate area is ploughed up by the turtle’s massive front flippers. Soon it is impossible to tell where the nest is concealed, though a professional poacher will nearly always find it.
Eventually, the turtle will move towards the sea, often making a tight circle as it goes, finally heaving itself to the water’s edge. It is a moving experience to see the exhausted turtle pause at the first breaking wave, then push off into deeper water and disappear into the darkness.
After roaming the oceans, the male Leatherbacks find their way back to the same stretch of coastline where they hang around off the nesting beaches and mate with the females throughout the nesting season. The female will receive the sperm of several males and a single nest of hatchlings is likely to have more than one father. Nine days after fertilization the female’s eggs are fully formed and ready to be laid. Further clutches will follow at 9-10 day intervals.
As with all reptiles, it is the temperature of the eggs during incubation which decides whether the hatchling will be male or female. In the case of Leatherbacks, the critical temperature is 29.5 Centigrade. Above that temperature there will be more females and vice versa. For this reason, the hatchlings from the top of the nest are more likely to be female as it is hotter nearer the surface. The sex of the turtle is decided between days 20 and 40 of the incubation period. Incubation can take from 60 to 75 days, depending upon the temperature in the nest.
The hatchlings do not leave the nest as soon as they have struggled out of the egg. Some of them are a long way down in the sand and it can take three days for the baby turtles to work their way up through the sand to just below the surface. There they wait until a drop in temperature gives them the signal to leave the nest and flap their way down towards the sea. This is always during the cool of the night when darkness gives them protection against many of the predators which hunt by day. However, night-feeding birds and ghost crabs are always on the prowl. Though no bigger than a baby turtle, a ghost crab can drag a hatchling down into its hole in the sand.
A hatchling’s problems are only just starting when it reaches the sea. A baby turtle is a tasty morsel for any number of fish. Frigate-birds and gulls will pick them off the surface with ease. The survivors swim on out to sea and nobody will see them again until years later when the adult females come back to lay. The males never come ashore again. Their scamper from the nest to the water’s edge is the only time they touch dry land. Nobody knows the survival rate of hatchlings. It may be as low as one in a thousand.