Animal orientation

This dissertation examined the natal origins, home-range, and in-situ foraging behavior of an aggregation of sub-adult hawksbill turtles (Eretmochelys imbricata) found off the coast of Palm Beach County, Florida. Surveys were conducted on approximately 30 linear km of reef between 15 and 30 m in depth. Tissue samples were retrieved from 112 turtles for mtDNA haplotype determination. GPS-linked satellite transmitters were deployed on six resident sub-adults, resulting in both minimum convex polygon (MCP) and 95%, 50%, and 25% kernel density estimates (KDE) of home-range size. A foraging ethogram was developed, and sequential analysis performed on thirty videos (141 total minutes) of in-situ foraging behavior. Seventeen total haplotypes were identified in this aggregation, the majority (75%) of which represented rookeries on Mexico’s Yucatan Peninsula. Other sources, from most to least important, include Barbados, Costa Rica, Puerto Rico, Antigua, and the U.S. Virgin Islands.

The cues used by marine turtles to locate foraging areas in the open ocean are largely
unknown though some species (especially the green turtle [Chelonia mydas], the
loggerhead [Caretta caretta], and the leatherback [Dermochelys coriacea]) somehow
locate areas of high productivity. Loggerheads can detect airborne odors, but a capacity
to orient has not yet been investigated. In this comparative study, tethered loggerheads
and leatherbacks were exposed to dimethyl sulfide (DMS) or food odors in a laminar
flow of air. Turtles did not orient into the air current. Free-swimming loggerheads and
green turtles were also exposed to air- or waterborne food (squid) odor plus a neutral
visual stimulus. Both species showed increases in swimming activity and biting behavior
to both stimuli. These results suggest that airborne odors are likely not used to locate
distant areas, but that they are used in localized food searching efforts.

Recent studies have shown that hatchling loggerhead sea turtles possess the ability to orient to the earth's magnetic field. These experiments did not identify the specific component of the field used by turtles to determine direction. One of the field's most important characteristics, inclination, has been implicated as the specific property used by birds to orient. This study investigated the possibility that sea turtles use the inclination of the earth's field in a similar manner. Results show that turtles determine direction with the use of an inclination compass similar to the one used by birds to orient. This study has important implications regarding the mechanisms used by animals to orient and navigate.

Artificial lighting disrupts sea turtle hatchling orientation from the nest to the sea. I studied how a light-induced landward crawl affects the ability of hatchlings to later crawl to the sea, and swim offshore from a dark beach. A brief (2 min) landward crawl had no effect on orientation, as long as waves (used as an orientation cue while swimming) were present. In the absence of waves (a flat calm sea), landward-crawling hatchlings failed to swim offshore while those crawling seaward were well oriented. A longer (2 h) landward crawl impaired the ability of hatchlings to crawl to the sea. These results demonstrate that previous exposure to artificial lighting compromises subsequent orientation, both on land and in the sea. On the basis of my results, I suggest several changes to current management practices, currently used when releasing misoriented turtles in the wild.

Pole-mounted street lighting on coastal roadways is often visible in adjacent areas. At roadways near sea turtle nesting beaches, these lights can disrupt the nocturnal orientation of hatchlings as they crawl from the nest to the sea. Our objective was to determine if an alternative lighting system (light-emitting diodes, embedded in the roadway pavement) prevented orientation disruption of loggerhead hatchlings. Hatchlings at the beach oriented normally when the embedded lights were on, or when all lighting was switched off. However, turtles showed poor orientation when exposed to pole-mounted street lighting. Light measurements revealed that street lighting was present at the beach, whereas embedded lighting was absent. I conclude that embedded lighting systems restrict light scatter, leaving adjacent habitats dark, and therefore protect the turtles from artificial lighting allowing for normal seafinding.

This study's objective was to determine if the transfer of a crawling direction to a magnetic compass in loggerhead hatchling sea turtles ( Caretta caretta L.) was facilitated by how long the turtle crawled (an "endogenous timing" component). I first determined how long it took hatchlings to crawl from their nest to the ocean. Two types of experiments were then carried out. In the first, crawling time varied. In the second, both crawling time and direction varied. I found that at most beaches hatchlings crawled to the ocean in less than 5 min. My experiments showed that if crawls are too short (1 min), or too long (5 min), vector transfer is weakened compared to a 2 min crawl. I also found that a period of non-directional crawling interfered with the ability of a 2 min crawl to promote calibration. These results confirm that efficient transfer of a crawling vector, maintained by visual compass, to a swimming vector, maintained by a magnetic compass, depends upon an endogenous timing program in hatchlings. The temporal properties of that program are, in turn, apparently shaped by where their mothers place nests on the beach.

Recent studies show that sea turtles use both magnetic and visual cues to successfully orient. Juvenile green sea turtles from the near shore reefs of Palm Beach County, Florida were brought to the lab to determine whether the sun could serve as a visual orientation cue. When tethered during the day in a large outdoor tank west of the ocean, the turtles oriented east to northeast. To determine whether the sun's position was used to maintain their heading, I altered the turtles' perception of time by entraining them to a light cycle advanced by 7 h relative to the natural cycle. When tested afterward in the same outdoor tank the turtles oriented northwest, the predicted direction after compensating for the sun's movement over 7 h across the sky. Orientation was unchanged when the turtles bore magnets that negated the use of magnetic cues. These results are consistent with the hypothesis that the turtles used the sun for orientation.

Loggerhead sea turtles nest on either the Atlantic or Gulf coast of Florida. The hatchlings from these nests migrate offshore in opposite directions. The purpose of my study was to determine if Gulf coast hatchlings use magnetic maps, as Atlantic coast hatchlings do, both to locate areas favorable for survival in the Gulf of Mexico and to orient appropriately within surface currents that could transport them into the Atlantic Ocean. To find out, I presented Gulf coast hatchlings with magnetic fields corresponding to different locations inside the Gulf, and within currents leading into (Florida Straits) and within (Gulf Stream) the western portion of the Atlantic Ocean. I conclude that Gulf coast hatchlings (i) use a high resolution magnetic map for navigation within the Gulf of Mexico, (ii) initially remain within the eastern Gulf, but later may (iii) gain entry into currents that transport them into Atlantic waters.

There are four distinct subpopulations of loggerhead sea turtles (Caretta caretta) in Florida as determined behaviorally by geographic fidelity, and genetically by mitochondrial haplotypes. The South Florida subpopulation consists of females nesting on the southeastern and southwestern coasts of Florida and their offspring. Previous research shows that west coast hatchlings exhibit higher levels of nocturnal swimming during the postfrenzy period than east coast hatchlings. This study attempted to determine how these differences in migratory behavior develop. A reciprocal translocation experiment was conducted to distinguish between environmental and genetic factors. No consistent differences in hatchling swimming behavior were seen based on geography. Movement of nests resulted in lower levels of nocturnal swimming behavior in hatchlings compared to hatchlings that emerged from natural nests, suggesting that the relocation of nests may not provide a natural incubation environment for developing hatchlings.