Spend World Sea Turtle Day With the Loggerheads

June 16th is World Sea Turtle Day, which celebrates the creatures and acknowledges the threats they face. There are seven living sea turtle species, and the most abundant species that nests in the United States is the loggerhead sea turtle.

Loggerhead turtles migrate intermittently throughout their lives. As hatchlings, the turtles swim from their natal beaches into the open ocean, often taking refuge in circular current systems (gyres) that serve as moving, open-sea nursery grounds. As juveniles, many take up residence in coastal areas but migrate seasonally between summer and winter habitats. Finally, as adults, turtles periodically leave their feeding grounds and migrate to mating and nesting areas, after which most return to individual feeding sites. This itinerant lifestyle depends on a sophisticated suite of orientation cues and guidance mechanisms that enable the turtles to maintain consistent headings and navigate accurately across vast expanses of ocean.

At no point in the life cycle are orientation abilities more vividly displayed, or more crucial to survival, than during the hatchling stage. For hatchlings, minimizing time on the beach reduces exposure to environmental extremes and terrestrial predators. Similarly, rapid and direct movement through shallow, near-shore waters reduces exposure to fish and avian predators that are concentrated in coastal areas. It is therefore not surprising that natural selection has favored orientation mechanisms that guide young turtles quickly and reliably to the relative safety of the open ocean along paths that approximate straight lines.

The migration of hatchling loggerheads differs from the first migration of birds and other terrestrial animals in two important ways. First, turtles migrate through an underwater environment where visual landmarks are absent and celestial cues often cannot be perceived. Second, whereas most young birds have the opportunity to practice or refine at least some orientation skills during short forays prior to migration, turtles remain in an underground nest chamber until the moment they emerge to migrate. Thus, young turtles must somehow guide themselves through a completely unfamiliar environment even as they experience it for the first time.

Hatchlings usually emerge from their nests at night and begin to crawl seaward almost immediately. Carr and Ogren demonstrated that green turtle hatchlings (Chelonia mydas) use cues specific to their location to guide themselves to the sea rather than following a specific compass heading that is independent of local conditions. When hatchling green turtles were translocated from the east coast of Costa Rica to the west coast, they crawled westward toward the sea in the new location, even though such headings would have led inland at the original emergence site. Although such a translocation experiment has not yet been done with loggerheads, sea-finding cues used by hatchling loggerheads and green turtles appear to be similar in most regards.

Stranded debris and irregularities in the beach surface frequently obstruct the view of hatchlings at the nest site. Thus, turtles must often ascertain the seaward direction without having a direct view of the ocean. Visual cues are nevertheless of primary importance in sea finding (Lohmann et al. 1997). Hatchlings released on a beach with one or both eyes covered either crawled in circles or moved along circuitous, seemingly random paths. Some hatchlings responded to beach slope when tested in darkness under laboratory conditions, but such nonvisual cues appear to exert little or no influence on directional movement if visual cues are present.

Although visual cues are apparently crucial in enabling hatchlings to reach the sea, it has proven challenging to identify the precise visual stimulus or stimuli to which turtles respond. During the past century, a number of slightly different explanations for sea finding have been proposed.

One of the first hypotheses was based on the fact that water reflects more light than does land. As a consequence, the oceanic horizon is usually brighter than the landward horizon. This consideration, along with evidence that hatchlings prefer brighter lights to dimmer ones, led to the hypothesis that hatchlings locate the ocean by crawling toward the brightest horizon.

In a study involving hundreds of natural loggerhead emergences in Australia, however, Limpus (1971) investigated conditions under which hatchlings failed to locate the sea. He concluded that hatchlings moved towards that part of the horizon line that was at the lowest angle of elevation from the turtles’ position, even if this area was not the most brightly illuminated. The lowest-horizon hypothesis also accurately predicted both the movement of hatchlings released at various locations along the beach and the orientation of turtles tested in an arena in which horizon elevation could be manipulated.

Additional evidence that horizon elevation is important in sea-finding was obtained by Salmon et al. (1992), who tested turtles inside a circular arena where black paper could be used to obscure various regions of an otherwise illuminated background. Loggerhead hatchlings consistently moved away from elevated silhouettes and toward the lowest illuminated horizon, even when doing so meant moving toward an area of dimmer, lower light instead of toward an area where the light was brighter but higher. The authors concluded that hatchlings find the sea by moving away from elevated silhouettes, a response that also takes them toward the lowest horizon.

Experiments on the beach have confirmed that loggerhead hatchlings do not always move in the direction that human optical measurements indicate is brightest. Does this mean that the brightest-direction hypothesis is incorrect? The question is not easily answered because turtles may assess brightest direction using different rules than human instruments employ. Hatchling turtles appear to integrate brightness measurements over only a limited part of the visual field that falls within a specific “cone of acceptance”. For both photometers and hatchlings, the height and breadth of the acceptance cone critically influence assessments of what direction is brightest. Experiments suggest that the acceptance cone of loggerhead hatchlings spans about 180° in horizontal breadth but extends no more than 30° upward or downward from the horizon. This restricted vertical dimension suggests that light close to the horizon influences hatchlings more strongly than does light higher up in the visual field.

Viewed in the context of acceptance cones, the tendency of hatchlings to orient away from silhouettes (or toward the lowest horizon) might plausibly be interpreted as an epiphenomenon of brightest-direction orientation. Dune silhouettes darken areas near the horizon that fall within a hatchling’s cone of acceptance; thus, a turtle orienting away from an elevated silhouette might do so because it perceives little light in that direction and brighter illumination elsewhere.

On the other hand, the same results can potentially be explained without invoking brightest-direction orientation at all. As discussed previously, hatchlings might simply assess the angle of horizon elevation and crawl towards that part of the horizon that is lowest, a process that would not require any assessment of brightness. Another possibility is that hatchlings rely on form vision to directly perceive the trees and dunes that line the beach on the landward side and then move away from these objects. Although form vision in sea turtles has not yet been studied in detail, laboratory experiments suggest that hatchling loggerheads can perceive darkened vertical stripes and other shapes, an ability that might assist them in distinguishing the landward and seaward horizons under some conditions. Thus, several different visual stimuli, as well as several different modes of neural processing, might potentially be involved in sea finding.

In summary, it seems reasonable to conclude that visual cues are of primary importance in sea-finding. From a functional perspective, the elevation of the horizon and the brightness of the light also appear to be important factors under at least some conditions. Less clear is precisely which feature or features of the visual environment turtles discern (horizon elevation, brightness within the cone of acceptance, the shape of silhouettes, other as yet unidentified visual cues, or some combination). Disentangling the various potential cues will be challenging because altering any one can change others.

Immediately after entering the ocean, loggerhead hatchlings establish an offshore course by swimming into waves. In the laboratory, turtles tethered in a wave tank oriented randomly in still water but swam into waves when waves were present. In the ocean, hatchlings swam into approaching waves when tethered in floating arenas placed offshore. At times when no waves were present, hatchlings either swam in circles or established courses in apparently random directions.

Similar results have been obtained with hatchling green turtles and leatherbacks (Dermochelys coriacea). Thus, hatchlings of at least three sea turtle species appear to maintain seaward orientation early in the offshore migration by using wave propagation direction as an orientation cue. Because waves and swells entering shallow, coastal areas are refracted until they approach a beach directly, swimming into waves reliably guides turtles away from land and toward the open sea.

Considerable progress has been made in unraveling the directional cues that guide hatchling loggerheads from the eastern coast of Florida to the Gulf Stream. An important caveat, however, is that detailed studies have been carried out only with hatchlings from southern Florida. Thus, whether the mechanisms outlined here are generally applicable to loggerhead populations in other parts of the world is not known. Different mechanisms, or perhaps different uses of the same cues, appear likely to exist among sea turtles that nest in different ecological settings. For example, whereas Florida hatchlings need to maintain only a single, consistent heading from the nest to the Gulf Stream, a different pattern might exist among populations that nest on island beaches. In such settings, following a straight-line course away from the beach would lead hatchlings emerging on opposite sides of an island toward very different directions and might result in at least some turtles moving away from appropriate developmental habitats. How hatchlings that emerge on islands guide themselves has not yet been investigated.

Loggerhead Sea Turtles is available from Smithsonian Books. Visit Smithsonian Books’ website to learn more about its publications and a full list of titles. 

Excerpt from Loggerhead Sea Turtles by Alan B. Bolten and Blair E. Witherington © 2003 by the Smithsonian Institution