Animal navigation

Species that are ontogenetic migrators have early life stages (juveniles) that live
shallower in the water column than the adults and therefore experience a brighter
environment than the adults. This work provides evidence that juveniles and adults of the
ontogenetically migrating crustacean species Gnathophausia ingens, Oplophorus
gracilirostris, and Systellaspis debilis have evolved visual adaptations to their respective
environments. The juveniles use apposition optics that provide greater resolution,
whereas the adults use superposition optics that maximize sensitivity. These animals also
have regional specializations to aid in viewing a light field that is brighter above than
below, such as accessory screening pigments located dorsally and superposition type
optics ventrally. The non-ontogenetic migrators Notostomus elegans and Notostomus
gibbosus possess superposition optics as both juveniles and adults, implying that the
changes seen in ontogenetic migrators are indeed visual adaptations.

Hatchling loggerhead sea turtles emerge from their nests on oceanic beaches, crawl to the surf zone, and swim out to sea. How do turtles maintain oriented headings once they lose contact with land? I tested the hypothesis that by swimming into surface waves hatchlings establish an offshore heading (directional preference), and that once out to sea this heading is transferred to, and maintained by, a magnetic compass. This hypothesis was supported by laboratory and field experiments, described herein. A directional preference can also be established by oriented crawling (from the nest to the surf zone). Thus hatchlings possess two mechanisms (crawling and swimming) for the establishment of an offshore heading. The use of these alternative mechanisms probably assures that turtles escape from shore under the broad range of conditions which they naturally encounter after emerging from their nests.

Hatchling sea turtles emerge at night from underground nests, crawl to the ocean, and swim out to sea. In this study, I determined how offshore orientation and shallow-water predation rates varied under natural (sand bottom and patch reef) and modified (submerged breakwater and open-beach hatchery) ecological circumstances. Hatchling offshore orientation in the sea was normal under all conditions; there were no significant differences in either scatter or direction among groups. However, predators (tarpon, snapper, barracuda, jacks, and grouper) took more hatchlings as they swam over submerged reefs, and after they entered the water in front of hatcheries. Predators were concentrated at both of these sites probably because prey (small fishes and invertebrates at patch reefs and turtles entering the water where nests were concentrated in hatcheries) occur in greater abundance.

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.

Little is known about the visual capabilities of marine turtles. The ability to discriminate between colors has not been adequately demonstrated on the basis of behavioral criteria. I used a three-part methodology to determine if color discrimination occurred. FIrst, I exposed naèive, light-adapted hatchlings to either a blue, green or yellow light. I manipulated light intensity to obtain a behavioral phototaxis threshold to each color, which provided a range of intensities we knew turtles could detect. Second, I used food to train older turtles to swim toward one light color, and then to discriminate between the rewarded light and another light color ; lights were presented at intensities equally above the phototaxis threshold. Lastly, I varied light intensity so that brightness could not be used as a discrimination cue. Six turtles completed this task and showed a clear ability to select a rewarded over a non-rewarded color, regardless of stimulus intensity. Turtles most rapidly learned to associate shorter wavelengths (blue) with food. My results clearly show loggerheads have color vision. Further investigation is required to determine how marine turtles exploit this capability.

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.

The mechanisms that rodents employ to navigate through their environment have been greatly studied. Cognitive mapping theory suggests that animals use distal cues in the environment to navigate to a goal location (place navigation). However, others have found that animals navigate in a particular direction to find a goal (directional navigation). The rodent brain contains head direction cells (HD cells) that discharge according to the head direction of the animal. Navigation by heading direction is disrupted by lesions of the anterodorsal thalamic nuclei (ADN), many of which are HD cells. Aim 1 tested whether male C57BL/6J mice exhibit direction or place navigation in the Morris water maze. Aim 2 tested the effects of temporary inactivation of the ADN on directional navigation. Together, these data indicate that C57BL/6J mice also exhibit preference for directional navigation and suggest that the ADN may be crucial for this form of spatial navigation.