A Green sea turtle. It will take a sea turtle anywhere from twenty to forty years before it becomes an adult that is ready to breed.
Photo: Courtesy of Parks Australia Cocos (Keeling) Islands
September 10, 2002
The Lives of Sea Turtles - Part 1 - "Breeding"
This is Genevieve Johnson speaking to you from the Odyssey.
The crew has spent the past four months researching the waters of the Indian Ocean between Western Australia and the Seychelles Archipelago where we often encountered marine turtles, both inshore and in the open ocean. We always record such 'sightings' in our database, in the hope that such information from remote regions may assist scientists in filling in some of the many gaps related to the behavior and ecology of sea turtles in this ocean.
Today we spoke with Dr. Jeanne Mortimer, a sea turtle ecologist working in the Seychelles.
Dr. Jeanne Mortimer:
My name is Dr. Jeanne Mortimer and I'm a sea turtle biologist. I am working as a consultant to the Ministry of Environment in the Government of Seychelles.
In the world today there are about seven or eight species of sea turtle, depending on which scientist you speak to. There are really two main groups of sea turtles, there is one species called the Leatherback, which is very different from the rest of them and then the others are your regular hard-shelled turtles. The Leatherback is very unique, it is the largest turtle in the world and they have been recorded as large as 1,000 kilograms, so that is about one ton of turtle, although that's rare that they are that big. They get that big by feeding on nothing but a diet of jellyfish, they are champion divers and have been recorded diving down to 1,500 meters or one and a half kilometers deep. As you can imagine they are able to tolerate very cold water, although they nest in tropical areas.
These tracks were made by a female Green sea turtle. She
makes her way up the beach to dig her nest.
Once she lays her eggs, she returns to the sea
and the hatchlings will be on their own.
Photo: JA Moritmer
The other six or seven species are found mostly in the tropics and temperate seas. In the Seychelles the main turtles we have are the Hawksbill turtle and the Green turtle, which nest here. We also get another type of turtle called the Loggerhead, although that one nests in South Africa and it feeds here on occasion. We also get Leatherbacks feeding here and they nest in South Africa also.
We know that turtles can continue to breed for twenty to thirty years, though not all of them do. We base this estimate on turtles that have been tagged on the nesting beach, so the time that the turtle was first seen on the beach until the very last time that she's been recorded, the maximum time being twenty-five years. If someone actually asked you to design a turtle, you might think it should be an animal that grows up very quickly and then spends a lot of time breeding and that this would make the population grow faster. Turtles do it the other way, they take a very long time to grow up and then they don't spend all that much time breeding. In the past this worked really well for them, its only now that they share the planet with humans that they are having problems with this strategy.
Once a turtle becomes an adult, they do come back to generally the same beach that they hatched from and we know that because of genetic studies that show the genetic characteristics of turtles at particular nesting sites are different from those at other sites.
Mating occurs offshore early in the season, the males stay in the water, they don't come onshore and mating occurs in the water. When ready to lay, the female crawls up onto the beach until she gets into dry sand and then she will start digging her nest. She first uses her front flippers, this lowers her into the sand where the sand is moist and then she will start to dig the actual nest with her back flippers. She'll take a scoop full of sand with one flipper, drop it, kick with that flipper then put the other one in, scoop the sand, drop it and kick it and continue to alternate flippers. When she is finally happy with the nest hole and it may take several tries, she lays about 100 - 200 eggs, depending on the species and on the individual. She then reverses the process and covers the nest with her back flipper and throws sand with her front flippers and then goes back to sea.
The eggs are on their own, they are buried in the sand at a depth of about 80 centimetres and are warmed by the sun. After about 50 - 60 days, they will hatch out and come up through the sand working as a team. The hatchlings work together. After coming up through the surface of the sand, they run straight out to sea. Meanwhile the female will stay offshore for about a two-week period while she makes new eggs. So the female will lay an average of 3 - 5 clutches throughout the season and then she goes away for several years. She goes back to her foraging ground which can be located hundreds or thousands of miles away and gets fat, she needs fat to be able to make the trip back again. Then she will do this on a cycle depending on the species and the location every 2 - 8 years maybe.
Dr. Jeanne Mortimer.
Photo: R. Bour
Back to the hatchlings, they certainly have more predators than the adults, as is certainly the
case with all animals. Little animals get eaten more than big animals. I think sometimes
filmmakers tend to exaggerate the predation on hatchlings. You may have seen films where they show
birds swooping down, lizards eating them and crabs sucking them into their holes, well that does
happen but it doesn't happen to that extent. Usually what happens is the hatchlings come out of the
nest at night, when all of the birds are asleep and the lizards are asleep, the crabs are awake but
there aren't that many crabs usually. As soon as a hatchling comes out of its nest it runs towards
the brightest point on the horizon. On a natural beach this will take them directly to the sea.
They run down the beach and once they reach the water they just swim, swim, swim until they get into
deeper water. The most dangerous period for them is in the shallow waters as there will be many
fishes that will come up and grab them. But once the turtle reaches water about ten meters deep,
the amount of predation really drops off because a lot of your predatory fishes are living at the
bottom. I have followed hatchlings out to sea looking at predation and sure enough in shallow water
the fishes are coming up and grabbing them, but as you get into deeper and deeper water you will see
the same species of fish and they will actually look up at the hatchlings going overhead and
it's kind of like - no, not worth it!
What you have to consider is, in an animal that takes twenty, thirty, forty years to become adult, there are an awful lot of bad things that can happen while they are growing up. So although the predation is heaviest when they are hatchlings and much of it occurs then, mortality occurs throughout their lives, and the hatchling period probably isn't quite as dangerous for them under natural circumstances as we like to imagine.
Stay tuned for upcoming Odyssey logs when Jeanne delves further into the mysterious life cycle of sea turtles and the challenges we face if we are to save the world's remaining wild populations.
Log by Genevieve & Chris Johnson with Dr. Jeanne Mortimer