The leatherback sea turtle is the largest sea turtle species, distinguished by its rubbery, shell-less carapace and an unparalleled capacity for long-distance migration. These reptiles undertake massive journeys essential for their survival and reproduction. Migration is driven by two fundamental biological needs: the necessity of warm, tropical beaches for successful nesting and the pursuit of energy-rich, cold-water foraging grounds. Leatherbacks constantly move between these two geographically distant environments.
The Necessity of Warm Nesting Beaches
The initial migration is a reproductive journey that compels adult females to return to the tropical and subtropical beaches of their birth. Female leatherbacks typically nest every two to four years, an interval necessary for them to rebuild the massive energy reserves spent on migration and egg production. During a single nesting season, a female will return to the beach multiple times, laying an average of six to nine clutches at intervals of about 9 to 12 days.
Each nest contains approximately 100 eggs, including an average of 73 fertilized eggs and around 25 smaller, yolkless eggs. The temperature of the sand where the eggs incubate is the sole determinant of the hatchling’s sex, a phenomenon known as temperature-dependent sex determination. Sand temperatures around 29.5 °C are considered the pivotal temperature, producing an equal ratio of males and females.
Temperatures below this pivotal point result in a greater proportion of male hatchlings, while warmer sands produce more females. The narrow temperature range required for successful hatching means that beaches must be warm enough to promote development but not so hot that they become lethal to the embryos, which can happen when sand temperatures approach 35 °C. The need for specific thermal conditions on land anchors the leatherback’s migratory cycle to tropical latitudes.
Following the Jellyfish: High-Latitude Feeding
Once reproduction is complete, leatherbacks begin the second, longer phase of migration, traveling thousands of kilometers to high-latitude foraging areas. These destinations often include the nutrient-rich, colder waters off the coasts of Canada, Alaska, or South America. This journey is driven by hyperphagia, or massive feeding, to replenish the energy stores depleted during the breeding season.
The leatherback’s diet is highly specialized, consisting almost exclusively of soft-bodied prey like jellyfish and sea squirts. Although jellyfish are relatively energy-poor, the turtles compensate by consuming enormous quantities where these gelatinous organisms aggregate in dense patches. They can consume up to 73% of their own body mass in jellyfish per day.
This high-volume consumption allows them to achieve an energy intake that is three to seven times their daily metabolic requirements. The rapid accumulation of blubber at these feeding grounds is essential, as leatherbacks are “capital breeders” who must build up all necessary energy before returning to the tropics for the next reproductive cycle. This specialized feeding strategy requires migration to cold, productive waters.
Unique Biology for Extreme Temperature Shifts
The leatherback’s ability to migrate into frigid waters, which would cold-stun other sea turtles, results from unique physiological adaptations. Unlike most other reptiles, the leatherback maintains a core body temperature significantly warmer than the surrounding water, a process known as regional endothermy. This capability stems from features including their immense size, which minimizes surface area relative to body volume, a concept known as gigantothermy.
The turtle’s constant, vigorous swimming generates muscle-derived heat, which is retained by a thick layer of insulating fat. Furthermore, they possess a sophisticated system called the countercurrent heat exchange (CCHE), particularly in their large flippers.
In this system, the arteries carrying warm blood away from the core are positioned close to the veins carrying cooler blood back to the core. Heat is transferred from the outgoing warm blood to the incoming cool blood, preventing excessive heat loss from the extremities and conserving the warmth generated by the muscles.
Recent research suggests the leatherback’s CCHE is specifically adapted to keep the flipper muscles warm and functional in cold water, rather than solely insulating the core. This specialized heat retention allows the leatherback to forage in waters as cold as 0.4 °C and to dive to depths of up to 1,280 meters.