Life cycle of the marine turtle
All species of marine turtles have the same general life cycle – they grow slowly and take decades to reach sexual maturity. They occupy differing habitats at various stages of their lives and are capable of long migrations of up to thousands kilometres between foraging grounds and nesting beaches.
There are seven turtle species recognised worldwide, five of which occur in the Southern African and Indian Ocean island waters.
• Chelonia mydas (green)
• Lepidochelys olivacea (Olive Ridley)
• Caretta caretta (loggerhead)
• Eretmochelys imbricate (hawksbill)
• Dermochelys coriacea (leatherback)
Life cycle schematic
Hatching and emergence
Almost all the eggs that have developed properly will hatch at the same time and the hatchlings combine their efforts to scrape at the walls and roof of their egg chamber. This brings down sand which filters through the struggling hatchlings and forms a new floor. The floor gets thicker as more sand falls and the hatchlings rise towards the surface rather like a lift. On nearing the surface those hatchlings at the top can tell if the surface of the beach is too hot and they will stop climbing until the beach cools down. Most hatchlings emerge onto the surface of the beach at night.
The rush to the sea
When ready, the hatchlings emerge onto the surface of the beach as rapidly as possible and sprint towards the sea guided by the light over the sea horizon. Speed is of the essence as many predators try to catch and eat them. Frigate birds, kites, ghost and hermit crabs, feral dogs, mongoose and jackal all kill and eat hatchlings. It is estimated that only about two out of every 1 000 hatchlings survives to adulthood.
To find the sea, hatchlings orientate towards the brightest direction and use the topography of the surrounding horizon line. Once in the sea, they use a combination of cues (wave direction, current, and magnetic fields) to orientate themselves to deeper offshore areas. Crossing the beach and swimming away is believed to imprint the hatchlings with the cues necessary to find their way back to the same beach when they are ready to breed.
The pelagic years
Once well away from the beach, most hatchlings are carried by ocean currents into the open ocean where they will spend varying periods of time feeding on floating organisms, returning to the coastline only when they are large enough to avoid most of their old predators.
As immature turtles, they may drift on ocean currents for many years or live for years in the one place before maturing and making a long breeding migration of up to 3 000km from their feeding grounds to a nesting beach.
Growth to maturity
Depending on various factors such as temperature, availability of food, competition and individual species’ characteristics, marine turtles can take between 15 and 30 years to reach sexual maturity. Green turtles, almost totally herbivorous, will graze for hours building up their fat reserves to a point where they will feel the urge to reproduce.
The hatchlings are rarely seen again until their shell length is between 20 and 40cm, which may be five to 10 years after hatching. At this time, the young, free-swimming turtles migrate back to inshore foraging areas. They remain in these areas until they are ready to breed and the cycle begins again.
When the urge to reproduce has developed, males and females return to the beaches on which they hatched so many years before. This is a remarkable feat as it may require a swim of hundreds or even thousands of kilometres over open ocean and facing many dangers. Such migrations are amongst the longest in the world and are a source of wonder and much scientific research. It is thought that turtles are able to navigate using the Earth’s lines of magnetic force and also use their amazing sense of smell to recognise the water close to their original beaches.
Courtship and mating
Green turtle males and females converge on their nesting coast to mate offshore. This is an exciting affair with males becoming very aggressive and active in pursuit of a receptive female. It is not uncommon to see six or seven males surrounding a single female.
Mating, with the male locked by his special claws onto the back of the female, can last up to several days since he has thousands of ova to fertilise. Both male and female turtles mate with a number of partners. The females store sperm in their bodies to fertilise the three to seven clutches of eggs laid during the season. Mating generally takes place offshore a month or two prior to the turtle's first nesting attempt for the season, usually in summer.
The adult male turtles return to their feeding sites. Female turtles cycle between mating and nesting, making between eight and 10 nests per season.
The female, now ready to lay, normally emerges onto the beach after dark and makes her way up the beach to above the high-water mark. She is capable of walking hundreds of metres before finding a suitable site where she settles, digs a body pit in the sand with her fore-flippers, and then an egg hole using her hind flippers.
Into this hole she will deposit between 100 and 150 soft-shelled eggs about the size of a ping-pong ball. When she has completed laying, she gently covers the eggs with sand using first her hind flippers and then disguises the entire nest with great sweeps of her fore flippers.
This entire operation can take up to two hours. Thereafter the exhausted female returns to the sea leaving two clear tracks that record her visit. These tracks are valuable to scientists use them to estimate how many females are nesting on a breeding beach.
In this offshore area she begins to make the next clutch of eggs, fertilising them from her sperm store. After the nesting season, females return to their distant foraging areas and may not nest again for two to eight years.
A clutch of eggs normally lies some 60cm under the surface of the beach and will take between 45 to 75 days to hatch, depending on the temperature and humidity of the surrounding sand.
The sex of the hatchlings is determined by the temperature of the nest on the 19th day of incubation. This “pivotal” temperature varies from beach to beach but is normally about 28° Celsius. Lower than that and the clutch will produce mainly males, higher than that and it will produce mainly females.
The Cycle Begins Again
After a nesting season adult turtles will return to their foraging grounds and if they survive they may return again to lay more eggs after periods of 3 or more years. Many will never return but having laid at least one full season’s worth of eggs they will have done enough to provide the population with some survivors and guarantee that the existence of these wonderful and harmless animals will continue into the future.