One of the most remarkable and mysterious elements of sea turtle biology is the ability of turtles to return to nest in the same geographic area from which they originated. This behavior, often referred to as natal homing, is particularly astonishing because many sea turtles migrate long distances away from their home areas before returning. Explaining how turtles leave a beach as hatchlings and then, years later, locate the same area of coastline after traveling immense distances through the open sea has posed a daunting challenge for biologists who have long struggled to explain the phenomenon without invoking magic.
The first indication that turtles might return to nest on or near their natal beaches came from results of early tagging programs in the 1950s and 1960s (Carr, 1967; Mrosovsky, 1983). These studies revealed that some female green turtles nest in the same areas year after year, which in turn fueled speculation that the nesting locations chosen by adult turtles might be the same beaches where they themselves began life as hatchlings. Initially, there was no way to test the idea because no suitable method for marking turtles existed; the only tags small enough to be placed on hatchlings detached long before turtles grew to maturity. The development of molecular techniques in the early 1990s, however, triggered an explosion of genetic evidence consistent with natal homing (e.g., Meylan et al., 1990; Bowen et al., 1993, 1994; Bowen and Avise, 1995). It is now known that most, if not all, sea turtles, display some degree of natal homing, although the precision of the homing may vary considerably among different populations and species (Bowen and Karl, 2007; Lohmann et al., 2008c).
Although it is now clear that natal homing occurs in sea turtles, little is known about how it is accomplished. For purposes of discussion, it is helpful to consider the process as being composed of two distinct elements. First, a turtle needs to be able to distinguish its natal beach or region from others, a process that might, in principle, involve information about the target area that the turtle has either learned or inherited. Second, a turtle must be able to navigate to the target area from a considerable distance away. The guidance mechanisms used might be unique to natal homing, or they might instead be the same ones that turtles use whenever they travel over long distances, regardless of the destination.
The question of how a turtle identifies its natal beach or region has prompted considerable speculation. It has been widely assumed, but never demonstrated, that the process of natal homing is linked to a special form of learning known as imprinting. Although precise definitions of imprinting differ (e.g., Hasler and Scholz, 1983; Alcock, 2009; Goodenough et al., 2010; Zupanc, 2010), the hallmarks of imprinting are that the learning occurs during a specific, critical period (usually early in the life of the animal), the effects are long-lasting, and the learning cannot be easily modified. For natal homing, the concept is that sea turtles imprint on some characteristic of their natal beach as hatchlings and then use this information to locate the beach years later as adults.
From a scientific perspective, the best way to study whether imprinting occurs in sea turtles would be to raise turtles under conditions in which various potential imprinting cues are manipulated, release the turtles into the ocean to mature, and allow them to return to nest as adults. This approach provides a powerful way to investigate which elements of early experience, if any, affect the behavior of adults. A similar methodology was used to demonstrate that young salmon imprint on the "chemical signature" of the water in their home rivers and use this information, as adults, to relocate their natal tributary when it is time for them to spawn (reviewed by Hasler and Scholz, 1983; Dittman and Quinn, 1996; Lohmann et al., 2008a; Zupanc, 2010).
For sea turtles, experimentation of this type is challenging for several reasons. All sea turtle species are threatened or endangered, so that limited numbers are available for experimental manipulations. In addition, sea turtles have an extremely long maturation period, with most populations and species requiring one or more decades to reach sexual maturity. Given these constraints, it is not surprising that little is known about imprinting in sea turtles, or even whether it truly occurs.
In this chapter, we begin by briefly summarizing the evidence for natal homing and the likely reasons that natal homing evolved in sea turtles. We then discuss the hypothesis of natal-beach imprinting in sea turtles, with an emphasis on chemical and geomagnetic cues, the two types of sensory information upon which turtles have been proposed to imprint. In addition, we summarize the limited experimental and correlational evidence consistent with each idea. Finally, given that clear evidence for imprinting does not yet exist in sea turtles, we discuss whether it is necessary to invoke imprinting to explain natal homing.