Functional Autonomy of Land and Sea Orientation Systems in Sea Turtle Hatchlings

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Abstract
Sea turtle hatchlings emerge from underground nests on oceanic beaches and immediately confront two separate problems in orientation. First they must locate the ocean and crawl to it; then they must orient offshore while they swim out to sea in a migration lasting several days. Visual cues guide hatchlings from the nest to the sea (1, 2), but little is known about the cues used by turtles in the ocean. Nevertheless, the crawl across the beach has long been considered essential to swimming orientation because hatchlings released offshore without a crawl reportedly fail to orient seaward (3, 4). Here we report that hatchling leatherback (Dermochelys coriacea) and green (Chelonia mydas) sea turtles released offshore consistently swam toward approaching waves and oceanic swells. Wave tank experiments confirmed that swimming hatchlings oriented into waves. A crawl across the beach was not a prerequisite for wave orientation in either the field or lab, indicating that hatchling sea turtles possess two separate orientation systems, each based on different sensory cues and capable of functioning autonomously. The first guides hatchlings on land to the sea; the second, based on wave detection, functions during the ocean migration. In five field experiments with green turtles and five others with leatherbacks, we monitored the swimming orientation of hatchlings released at various distances offshore near Fort Pierce, Florida. A total of 45 green turtle and 48 leatherback hatchlings were tested. All experiments were conducted between July and September in 1988 and 1989.