Turtle - Reproduction | Britannica

Nesting and egg laying

Leatherbacks and other sea turtles are migratory in that they traverse hundreds of kilometres from their main feeding areas to nest on the beaches where they hatched. Annual migration also occurs in some river-dwelling turtles, including the South American arrau (Podocnemis expansa) and the Asian river turtle. These turtles move tens of kilometres along rivers in order to find large sandbars on which to nest. The females of all aquatic species must leave the water to find nesting sites. Some merely move to the banks adjacent to the streams they live in; others travel hundreds of metres across land to find appropriate nesting conditions. Nesting is an arduous affair that exposes females to increased predation.

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The number of eggs in a single “clutch” is variable both within and between species. Small species typically lay few eggs—only one or two in the Asian black marsh turtle or the pancake tortoise. The number of eggs increases with body size among species and occasionally within a species. However, the largest turtle, the leatherback sea turtle, produces fewer eggs (average 50–90 eggs per clutch) than do smaller sea turtles such as the hawksbill (140–160 eggs) and olive ridley (105–110 eggs). Similarly, the large Aldabran tortoise (60–80 cm [24–32 inches]) lays 12–14 eggs, yet the common snapping turtle (20–35 cm [8–14 inches]) lays 20–30 eggs, and the Suwanee cooter (14–28 cm [5.5–11 inches]) lays 15–20 eggs.

In most species, eggs are laid annually; a few species lay every other year, and some lay twice in one nesting season. The sea turtles generally nest in three- to four-year cycles, the female usually laying multiple clutches of eggs during each nesting season. Within the season, cycles of egg laying occur about two weeks apart, allowing the female time to rest from the energy-demanding excursion ashore and for ovulation and shelling of the eggs. Turtle eggshells can be leathery, as in sea turtles, or brittle, as in many tortoises. Calcium carbonate is a constituent of both types of shells; the leathery ones simply have less.

Nest digging is a fixed behavioral pattern in all but a few species. Most turtles dig chambers in which the eggs are laid. Once the female finds a desired nesting site, she begins to dig the chamber with alternate scooping movements of the hind limbs. As one hind limb supports the rear half of the body, the other one moves inward under the tail and, with a semicircular twist of the foot, spades into the soil and makes a quick sideward flip, dropping the soil to the outside as the hind foot locks into its supportive position. The opposite foot repeats the pattern, and in a slow, steady alternation the nest is dug. Digging stops when the female has reached a depth equal to the length of her outstretched hind limbs. Then, bracing herself on both hind limbs and with the tail centred over the nest, she drops her eggs into the hole. After she has expelled all of her eggs, the hind limbs resume their alternate movement, but now they drag the loose soil back into the nest. The female departs when the nest is filled. A few species, such as the stinkpot, or common musk turtle (Sternotherus odoratus), dig a shallow nest with both the fore- and hind feet.