Animal Behavior/Nesting Behavior
Over the course of a turtle’s nesting season, a female sea turtle will leave her homing ground and migrate to nesting grounds to build a nest and lay eggs. One species of turtle, Chelonia mydas, the Green sea turtle, lives off the coasts of Brazil but nests each year on the beaches of Ascension Island, a tiny island in the mid-Atlantic Ocean (Papi et al.). These turtles typically make their way to the island between the months of December and March (Sims).
After migrating to a nesting ground, it is time for the female to choose a nesting site along the beach. Many factors play into this decision and a female will scope out a site for a few days before she actually decides to build a nest. However, once the decision of where to nest has been made, females will most likely nest at the same beach and site for the rest of their reproductive years (CCC&STSL).
All species of sea turtles exhibit similar nesting behavior, possibly differing only in inter-nesting period length (the period of time between two nestings during the same nesting season). On the night of nesting, a sea turtle will emerge from the sea and scope out her surroundings one more time before digging a nest. If there seems to be any unnatural distractions (i.e. lighting or human presence), a female may return to the sea without building a nest (“false crawl”). Most often, however, she will proceed to dig a nest on the beach. To do this, the female turtle first digs a relatively shallow depression, known as a body pit, in the sand using her front flippers and body to push sand out of the way. Then she will excavate a single egg chamber, a deeper cavity opening to the body pit, into which she will oviposit an entire clutch of eggs during nesting. Finally, after constructing this egg chamber, she will begin ovipositing her eggs. It is extremely rare that a sea turtle will stop nesting before laying is complete but will happen if the female senses extreme danger (CCC&STSL).
After laying eggs, the female will camouflage her nest by filling in the body pit with sand and return to the sea only to nest at the same site at a later date in the nesting season. After nesting a second time, the female sea turtle will return to the sea and migrate back to her homing ground (CCC&STSL).
Many nesting grounds, such as that of the Green sea turtle, are extremely far from the species’ homing ground. This migration of sea turtles during the nesting season has led to the development of many different hypotheses of how these adult female turtles continually find the common nesting ground and then how offspring navigate to the homing grounds. First, it has been suggested that sea turtles may use the earth’s magnetic field to aid in navigation between homing grounds and nesting grounds. There have, however, been experiments that have weakened this hypothesis in adult sea turtles. One such experiment involved placing magnets on the back of experimental Green sea turtles in order to disrupt the magnetic field possibly detected by the sea turtles and the courses these turtles followed on the return journey from Ascension Island were tracked. The results show that, although hatchlings may be able to navigate via earth’s magnetic field, it is very possible that this ability is absent in adults (Papi et al.). However, another hypothesis has been suggested and states that turtles may use olfactory cues from the ocean currents and winds to navigate through the ocean to nesting grounds (Sims).
"General Behavior Patterns of Sea Turtles." Caribbean Conservation Corporation and Sea Turtle Survival League (CCC&STSL). Accessed 3 December 2004. http://www.cccturtle.org/behav.htm
Papi, F., P. Luschi, S. Akesson, S. Capogrossi, and G.C. Hays. "Open-sea Migration of Magnetically Disturbed Sea Turtles." The Journal of Experimental Biology 203:27 (2000): 3435-3443.
Sims, David W. "Homing is a Breeze for Sea Turtles." Nature (London) 423: 6936 (2003): 128.