Marine Biology

Leatherback Sea Turtle Nesting Habits

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The endangered leatherback sea turtle, the world's largest living reptile, reaches sexual maturity at between six and ten years of age, after which it will nest every two or three years. While its range is fully global, nesting occurs only in subtropical climates such as those in the southern Pacific and surrounding the Gulf of Mexico. In northern, subtropical climates, nesting begins in March and continues for the next four months, during which time the female can nest as often as eleven times, though the average is closer to five. Although a single female could theoretically keep nesting until the age of forty, the vast majority of females only manage a single nesting season before they are killed.

Since the leatherback sea turtle is the most pelagic of the sea turtles, the female seeks out nesting sites where warm, dry sand above the high tide mark is as close to deep sea water as possible. Generally the female emerges from the sea at night during high tide, so as to make the dry-land distance to be traversed as short and perhaps as dark as possible. Often the area is prone to the kind of high waves that help sweep the female inland, and which will later help the new hatchlings quickly work their way into the sea. However, this proximity to rough water can sweep away an unhatched nest as well. The area where the nest is to be built must also be mostly free of rocks, wood, coral, and other protruding debris, and is generally backed by vegetation which gives the sand a degree of stability.

Having found an appropriate site, the female will begin building her nest: first sweeping out a body pit with her front flippers; and then using her hind flippers, one at a time, to dig out a deeper pit where the eggs will go. Once the back pit is as deep as her flippers, usually about 70 centimetres, the female goes into torpor while eggs are being laid. Around 70 yolked eggs are typically laid in a single clutch, two and three at a time. The fertile eggs have white, cartilaginous shells and are roughly five centimetres across. About three dozen smaller, infertile eggs will also typically be laid on top of the fertile eggs as part of the same clutch, a practice unique among turtles to the leatherback. Once all the eggs have been laid, the female rouses and begins covering the eggs with sand, after which she spends some time camouflaging the area before she returns to the sea until the next time she is ready to nest. The entire process will have taken less than two hours.

From 55 to 75 days later the hatchlings will emerge, also at night. Only about one in three eggs will hatch at all: and of these, the vast majority of hatchlings won't survive to maturity. During one part of the eggs' development, the temperature of the sand will determine the gender of the hatchling: with females being born only when the sand temperature remains at 30 degrees Celsius or higher.

Over the past three decades the number of nesting females worldwide has plummeted to a quarter of the 115,000 estimated in 1980. The single largest decreases have been seen along the Pacific coasts of Mexico and Costa Rica, which previously held the world's largest leatherback nesting population (over 65% of the total population); but which now draw fewer than a thousand nesting females per year. The remaining largest nesting populations are currently in French Guiana and Columbia, drawing about five thousand nesting females each; as well as roughly six hundred nesting females in West Papua and Indonesia. No nesting beach outside these areas draws more than a hundred nesting females per year.

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