Sea turtles, including species like the Green, Loggerhead, and Leatherback, have a life cycle spanning millions of years. While each species has unique characteristics, their fundamental life cycle involves vast oceanic migrations and brief terrestrial appearances. This cycle begins with reproduction and culminates decades later with a return to the same sandy shores. The stages—from incubation to growth and eventual sexual maturity—connect the ocean and land.
Mating and Nesting: The Start of the Cycle
The life cycle begins when adult turtles congregate in coastal waters near nesting grounds. Mating typically occurs offshore, with females often mating with multiple partners before coming ashore. The female then migrates to a nesting beach, often returning to the general area where she hatched years before.
The female hauls her body onto the dry sand, making her vulnerable. She selects a spot above the high-tide line, digging a large body pit with her front flippers. She then excavates a deep, flask-shaped egg chamber using her hind flippers. The female deposits an average of 100 to 120 soft, leathery eggs into this chamber, though the number varies by species.
After laying the clutch, the female covers the eggs with sand, packing the chamber and scattering sand over a wide area to camouflage the nest. This process can take several hours before she returns to the ocean. A single female may repeat this, laying multiple clutches (typically three to five) at intervals of 10 to 15 days throughout one nesting season.
Hatching and the Dash to the Sea
The eggs incubate within the warm sand for 45 to 70 days, depending on the species and temperature. Temperature-Dependent Sex Determination (TSD) occurs during the middle third of incubation, dictating the embryo’s sex. Cooler sand temperatures, below approximately 27.7 degrees Celsius, produce male hatchlings.
Warmer temperatures above 31 degrees Celsius result in female offspring, with a temperature around 29 degrees Celsius yielding a mixed ratio. Developing embryos use a temporary projection on their beak, called a caruncle or “egg tooth,” to break out of their shells. The collective effort of the hatchlings moving within the nest pushes the sand above them, allowing the group to rise to the surface.
Hatchlings emerge together at night, waiting for the sand temperature to drop. Once free, they immediately begin their instinctive “dash” toward the ocean, a journey fraught with danger from predators like crabs and birds. Their primary navigational cue is the brightest, lowest horizon light, which is the reflection of the moon and stars over the open water.
Artificial light sources can disorient the hatchlings, causing them to crawl inland, often resulting in their demise. Successfully reaching the water, the turtles are met by the powerful waves, which they must swim past to reach deeper waters. This initial high-energy swimming effort takes them far offshore, concluding the terrestrial phase of their lives.
The Oceanic Juvenile Phase
Once in the open ocean, the young turtles enter the “Lost Years,” a phase lasting several years to over a decade. During this time, their small size and pelagic nature make them difficult to track. They utilize major ocean currents and convergence zones as passive transport mechanisms and foraging areas.
These oceanic zones often accumulate floating debris and mats of Sargassum seaweed, which serve as nursery habitat. The Sargassum provides shelter from predators and a concentrated source of food, including small invertebrates. During this time, the turtles are surface-pelagic, living in the upper water column of the open sea.
As juveniles grow, their habitat and dietary needs evolve. When they reach the approximate size of a dinner plate, they transition from the open ocean to nearshore coastal waters and shallow foraging grounds. This movement often coincides with a change in diet, such as the Green turtle shifting from an omnivorous diet to a primarily herbivorous one focused on sea grasses and algae.
Adulthood and Return to the Nesting Grounds
The final stage is marked by the onset of sexual maturity, which varies significantly by species. For the smaller Kemp’s Ridley, maturity may be reached around ten years. For others, such as the Loggerhead and Green turtle, it can take 20 to 50 years. Once mature, migratory cycles between foraging grounds and breeding areas begin.
Adult turtles spend most of their lives in foraging areas, building energy reserves required for reproduction. Males migrate to breeding grounds every year to seek mates. Females only undertake nesting migration every two to four years to restore their energy. The journey back to the natal nesting beach can span thousands of kilometers, likely involving the Earth’s magnetic field for navigation.
Upon completing the nesting season, females return to their foraging grounds. The cycle of migration, feeding, and breeding continues over a long lifespan that can extend for up to a century.