Members of the Nesting Patrol
quickly realize that turtle hatchlings must be born with well-developed
systems for finding their way about in the world, without guidance from
parents. Hatchlings that have emerged from the dark nest onto the open beach
immediately begin to crawl toward an ocean that very often they cannot
perceive from the nest site. The young turtles enter the sea and immediately
establish offshore headings, which are maintained long after the hatchlings
swim beyond sight of land. Thus, even as hatchlings, marine turtles are
finely tuned migratory animals equipped with an array of mechanisms that
guide them across vast expanses of trackless ocean.
In recent years, considerable progress has been made in characterizing the orientation cues that guide the hatchlings during their offshore migration.
Initially, they use visual cues to guide them to the ocean.. Among these are the brightness and color of the ambient light, and the presence or absence of dark shapes and silhouettes that they perceive at the low elevation of their field of vision. Under natural conditions, hatchlings are attracted toward the bright and low oceanic horizon and instinctively are repelled by the dark, elevated silhouettes of vegetation and dunes that usually border the land-ward edge of the beach. An artificial light source may become a supernormal stimulus that falsely indicates the seaward direction. At such high levels of light stimulation, hatchlings may ignore shape cues and other features of the beach, or perhaps not even perceive them. As a consequence, they crawl land-ward to their doom.
Moonlight on the beach does not seem to affect the behavior of hatchlings,
contrary to that of the human species. The common misconception that
hatchlings emerge only during the full moon probably originated because
hatchlings are most easily observed on bright, moonlit nights. But there is
no evidence that departure from the nest is in any way correlated with the
lunar phase. (The date of emergence is determined by the date eggs were
deposited in the nest and by the length of the incubation period, which in
turn depends in part upon nest temperature.)
As hatchlings complete their crawl across the beach and enter the surf, they are lifted off the sand by incoming waves. As soon as their flippers lose contact with the sand, hatchlings begin to swim vigorously; and simultaneously they dive. These responses position turtles in the wave undertow, which quickly transports them a short distance off the beach. They may miss that first wave and get tumbled about a bit, or even end up beached again with all four flippers waving in the air. But eventually they get the hang of it, and off they go.
Almost immediately after passing the first barrier of breaking waves, hatchlings clearly set a course toward the open ocean and maintain it as they swim away from shore. The little turtles apparently rely on wave cues to establish their headings. Experiments have demonstrated that hatchlings confined in floating barriers placed at various distances offshore all swam straight into manufactured waves regardless of the wave direction relative to the beach. At times when no waves were present, the confined hatchlings either adopted courses in seemingly random directions or swam in circles. Because waves entering shallow water normally refract until they approach the beach directly, orienting into the waves leads the turtles seaward, and there is no evidence that other directional cues are needed at this early stage of the offshore migration.
However, as they move farther from land, waves no longer provide a reliable indicator of offshore direction, for waves can come from almost any direction. Hatchlings on a Florida beach were fitted with tiny radio transmitters attached to the carapace. The radio signals revealed that they swam off the beach directly into the oncoming waves , but they continued on the same seaward bearing after entering offshore areas where wave direction no longer coincided with their established course. The ability to maintain course seemingly independent of wave direction implies that, after they have distanced themselves from land, hatchlings use one or more alternative sources of directional information to guide their movements.
Technically sophisticated laboratory experiments have demonstrated that the hatchlings can orient to the magnetic field of the earth. Thus, one possibility is that magnetic compass orientation supplants wave orientation as hatchlings distance themselves from shore. Recent experiments also indicate that hatchlings may set their magnetic compasses by wave cues, as they initially set out from shore. It remains to be determined whether the preferred magnetic direction may be established even earlier during their crawl down the beach. What is becoming quite evident is that the hatchlings, by the time they arrive in the open ocean, can navigate by detection of magnetic impulses of which we humans are totally oblivious.