How sea turtle hatchlings know where to crawl

From Acapulco, Mexico, at a meeting of the American Geophysical Union

Field tests suggest that newly hatched sea turtles need a variety of senses, not just sight, to find their way to the ocean.

The female black sea turtle (Chelonia agassizi) buries her eggs in sand 100 meters or so from the shoreline. As soon as the hatchlings emerge, they head for the water. Most previous studies of how these young creatures find their way into the surf have focused on the role of visual cues such as moonlight, says Gabriel Gutiérrez-Ospina, a neurobiologist at the National Autonomous University of Mexico in Mexico City. However, the results of his group’s experiments now suggest that other senses help the turtles set off in the right direction.

The 15 turtles in the experiment’s control group, which were allowed to crawl across the beach unhindered, made the 120-m trip in an average of just over 15 minutes, says Gutiérrez-Ospina. During the nighttime experiments, researchers set bright lights along the paths of another 15 turtles. Apparently attracted to the lights, the hatchlings veered off course, and none reached the ocean within 20 minutes. Likewise, hatchlings whose eyes were covered with patches or whose nostrils were temporarily plugged with dental wax didn’t make it to the sea in that time, says Gutiérrez-Ospina.

Twelve of 15 hatchlings with weak magnets glued atop their heads made it to the sea within 20 minutes. However, no turtle adorned with a magnet strong enough to mask Earth’s magnetic field reached the surf within the same period.

The team’s results suggest that hatchling turtles rely primarily on sight to reach the sea but also use their sense of smell and their ability to discern Earth’s magnetic field, says Gutiérrez-Ospina. The findings also suggest that those supplementary senses don’t ensure successful navigation when visual clues are absent or distracting, the researchers note.