Sea turtles reproduce exclusively through sexual reproduction, involving the combination of genetic material from two parents. This process uses internal fertilization, where the male deposits sperm inside the female to fertilize the eggs. Reproduction is a multi-stage process that begins in the water and culminates with the emergence of young on land.
Aquatic Courtship and Fertilization
The reproductive cycle begins in the ocean, where adult sea turtles gather in coastal waters near the female’s nesting beaches for mating season. Courtship often involves one female being pursued by multiple males, leading to aggressive competition. A male initiates mating by approaching the female and gently biting her neck and flippers to gauge her receptivity.
If the female accepts, copulation takes place on or just below the water’s surface. The male climbs onto the female’s back, securing his grip onto her carapace using specialized claws on his front flippers. He then extends his reproductive organ to inseminate the female.
Mating can last for several hours, requiring the female to periodically surface to breathe while carrying the male’s weight. Females often mate with multiple males and can store sperm for several months in their oviducts. This storage allows them to fertilize all egg clutches laid during the nesting season without needing to mate again.
The Terrestrial Nesting Phase
After mating, the female crawls onto the beach, typically at night during high tide, to begin nesting. She uses her powerful front flippers to excavate a shallow body pit above the high-tide line. She then uses her rear flippers to precisely dig a deeper, vase-shaped egg chamber, which can be up to a meter deep in some species.
Into this chamber, the female deposits a clutch of eggs, which have a soft, leathery shell. Clutch sizes generally range from 50 to over 150 eggs per nest, varying by species. Once laid, she covers the chamber with sand to conceal it from predators and protect the contents.
Temperature-Dependent Sex Determination
The unique aspect of the terrestrial nesting phase is Temperature-Dependent Sex Determination (TSD), where the surrounding nest temperature determines the hatchlings’ sex. The temperature during the middle third of the incubation period is the determining factor.
Warmer sand temperatures produce a greater proportion of female hatchlings, while cooler temperatures result in more males. The pivotal temperature that yields a roughly 50:50 sex ratio is often around 29 degrees Celsius (84.2 degrees Fahrenheit). Temperatures above this point increase the likelihood of female offspring, while temperatures below it favor male offspring.
Hatchling Emergence and Orientation
After an incubation period of 45 to 70 days, hatchlings break free and begin cooperatively digging to the surface. They synchronize their movements, using the collective effort of their siblings to move the sand above them. This synchronized emergence is often triggered by a drop in sand temperature, causing them to erupt en masse at night.
Once on the beach, the hatchlings immediately begin a rapid crawl toward the ocean in a process called “sea-finding.” They orient themselves toward the brightest, lowest horizon—the open ocean—and away from dunes and vegetation. Artificial lights from coastal development can confuse this process, causing the hatchlings to crawl inland.
Upon reaching the water, the hatchlings enter a period of frantic swimming, known as the “swimming frenzy,” to quickly move away from predator-dense nearshore waters. They use a sequence of cues, initially orienting into the waves and then potentially switching to an internal magnetic compass, to maintain an offshore heading.