Nesting, Incubation and Emergence
Very little is known about why sea turtles nest on some beaches and not on others. In Florida, loggerheads nest by the thousands on the central east coast, while identical looking beaches to the north see far fewer loggerheads. I live on Singer Island, Florida just north of Palm Beach, and we see a lot of turtles on our beach during the summer months.
This nesting distribution may reflect conditions that existed centuries ago, when temperature, beach profiles or the lack of predation made some areas preferable to sea turtles. Today, humans are affecting the places where sea turtles nest. Beach erosion caused by coastal armoring and navigational inlets, artificial lighting and beach renourishment are all impacting once pristine beaches. These changes will likely have lasting effects on future nesting patterns. The more we understand about how, where and when sea turtles nest, the better we will be able to protect their nesting habitat.
Most females return faithfully to the same beach each time they are ready to nest. Not only do they appear on the same beach, they often emerge within a few hundred yards of where they last nested.
Only the females nest, and it occurs most often at night. The female crawls out of the ocean, pausing frequently as if carefully scoping out her spot. Sometimes she will crawl out of the ocean, but for unknown reasons decide not to nest. This is a “false crawl,” and it can happen naturally or be caused by artificial lighting or the presence of people on the beach. Most females nest at least twice during the nesting season, although individuals of some species may nest only once and others more than ten times. Sea turtles are generally slow and awkward on land, and nesting is exhausting work.
Constructing the Nest
The female turtle crawls to a dry part of the beach and begins to fling away loose sand with her flippers. She then constructs a “body pit” by digging with her flippers and rotating her body. After the body pit is complete, she digs an egg cavity using her cupped rear flippers as shovels. The egg cavity is shaped roughly like a tear drop and is usually tilted slightly.
Laying and Burying the Eggs
When the turtle has finished digging the egg chamber, she begins to lay eggs. Two or three eggs drop out at a time, with mucus being secreted throughout egg-laying. The average size of a clutch ranges from about 80 to 120 eggs, depending on the species. Because the eggs are flexible, they do not break as they fall into the chamber. This flexibility also allows both the female and the nest to hold more eggs. Nesting sea turtles appear to shed tears, but the turtle is just secreting salt that accumulates in her body. Many people believe that while laying her eggs a sea turtles goes into a trance from which she can not be disturbed.
This is not entirely true. A sea turtle is least likely to abandon nesting when she is laying her eggs, but some turtles will abort the process if they are harassed or feel they are in danger. For this reason, it is important that sea turtles are never disturbed during nesting. Once all the eggs are in the chamber, the mother turtle uses her rear flippers to push sand over the top of the egg cavity. Gradually, she packs the sand down over the top and then begins using her front flippers to refill the body pit and disguise the nest. By throwing sand in all directions, it is much harder for predators to find the eggs. After the nest is thoroughly concealed, the female crawls back to the sea to rest before nesting again later that season or before beginning her migration back to her feeding ground. Once a female has left her nest, she never returns to tend it.
Incubation takes about 60 days, but since the temperature of the sand governs the speed at which the embryos develop, the hatching period can cover a broad range. Essentially, the hotter the sand surrounding the nest, the faster the embryos will develop. Cooler sand has a tendency to produce more males, with warmer sand producing a higher ratio of females.
Emerging from the Nest
Unlike baby alligators, which are liberated from their nest by their mother, sea turtle hatchlings must do it all themselves. To break open their shells, hatchlings use a temporary, sharp egg-tooth, called a “caruncle.” The caruncle is an extension of the upper jaw that falls off soon after birth. Digging out of the nest is a group effort that can take several days. Hatchlings usually emerge from their nest at night or during a rainstorm when temperatures are cooler. Once they decide to burst out, they erupt from the nest cavity as a group. The little turtles orient themselves to the brightest horizon, and then dash toward the sea.
If they don’t make it to the ocean quickly, many hatchlings will die of dehydration in the sun or be caught by predators like birds and crabs. Once in the water, they typically swim several miles off shore, where they are caught in currents and seaweed that may carry them for years before returning to nearshore waters. There are many obstacles for hatchlings in the open ocean. Sharks, big fish and circling birds all eat baby turtles, and they die after accidentally eating tar balls and plastic garbage. The obstacles are so numerous for baby turtles that only about one in 1,000 survives to adulthood.
The ability of a sea turtle to migrate hundreds (and occasionally thousands) of miles from its feeding ground to its nesting beach is one of the most remarkable acts in the animal kingdom. That adult females return faithfully to nest on the very beach where they were born makes the feat even more amazing. Research into where and how sea turtles migrate has been a focus of scientists for decades. The information collected is vital to the development of conservation strategies for the species. We now know that sea turtles undergo migration throughout their lives, beginning with the first frenzied swim as a hatchling.
During its first critical 48 hours, a hatchling must travel from the beach to a place in the ocean where it is relatively safe from predators and where it can find food. Many hatchlings in the Atlantic and Caribbean make their way into Gulf stream currents, which are filled with floating sargassum weed. There the young turtles find an ample food supply and few predators. After several years of floating around the Atlantic, these young turtles are big enough to venture back into nearshore waters.