Scientists Answer Unresolved Questions About Loggerhead Turtle Migrations
May 16, 2012

Scientists Answer Unresolved Questions About Loggerhead Turtle Migrations

A new study led by Kenneth J. Lohmann, a marine biologist at the University of North Carolina at Chapel Hill, provides insight into exactly how young loggerhead turtles complete a transoceanic migration that involves moving through the entirety of the North Atlantic basin.

These turtles promptly leave the safety of their nests in Florida, after emerging from their shells, and once they have completed the migration alone and intact, the loggerhead turtles will arrive in North America's coastal waters. The research from the study was partially funded by the National Science Foundation (NSF) and will be published in The Journal of Experimental Biology in the June 2012 issue.

The team asserts in their most recently published paper that the two-inch long hatchlings migrate through the vast, open ocean by using beneficial currents to their advantage, inert drifting when possible, and tactical swimming when needed. These traveling techniques allow the baby turtles to store precious energy while migrating across such extensive distances.

"Young turtles probably rely on a strategy of 'smart swimming' to optimize their energy use during migrations," stated Lohmann.  He also said, "The new results tell us that a surprisingly small amount of directional swimming in just the right places has a profound effect on the migratory paths that turtles follow and on whether they reach habitats favorable for survival."

Researchers used a simulation created by using real ocean current information and “virtual turtles” that would swim for random amounts of time. This boldly challenged the popular belief that loggerhead turtles merely drift during their impressive migration and that they have no part in where they end up. "Most researchers have assumed that, because ocean currents in some places move faster than young turtles can swim, the turtles cannot control their migratory paths," Lohmann stated. "This study shows otherwise." The research proposes that even the minimum effort given from the young loggerheads may indeed affect where they arrive.

David Stephens, a program director at the NSF, stated, "The research team's results have important implications for 'weakly moving animals,' including larval fish, butterflies and ballooning spiderlings.” He also said, "All those things that we've thought of as 'just drift along with the current' might, after all, have a lot of control over where they're going, with minimal effort!"

“This discovery may be particularly useful in understanding commercially important creatures, such as fish and crab, that have weakly swimming larvae that, like turtles, have often been assumed to drift passively.” Lohmann stated. This could improve supervision at fisheries.

Lohmann's team published a correlating paper in the April 2012 issue of Current Opinion in Neurobiology.  Complied over a period of ten years, the research, funded by the NSF, reveals that loggerhead young are able to maneuver confidently and know exactly where they are by using a somewhat inherited “magnetic map” by using the changes in the earth´s magnetic field to navigate properly across the North Atlantic basin. The subtle changes that occur throughout the world can be compared to “road signs” that turtles use as markers. Every time a change occurs in the magnetic field, the young loggerhead will make an adjustment in its swimming course.  These results were found by utilizing laboratory experiments in which young loggerheads were presented with magnetic changes like those in their natural migratory settings. Remarkably, the turtles reacted the same way that turtles swimming through the ocean would and these results show that the brains of the loggerhead turtle are “hardwired” from birth to allow them to navigate through their natural and dangerous waters.

"The results also indicate that turtles obtain both latitude and longitude-like information from the oceanic magnetic field," Stephens stated. "They may thereby obtain much richer spatial representations from magnetic fields than do humans with their compasses."

According to Lohmann, loggerhead turtles, born helpless and unable to dive, make easy prey for birds and predatory fish that swim beneath them. They are only able to move about on the surface of the water, and are abundant in low coastal waters. The migrating habits of these turtles can be compared to a “hail Mary pass”, a play in football games. The young turtles seek protection in the open ocean, where they can mature in relative safety. Adult turtles are less likely to be attacked by predators, so young loggerheads wait until they are large before going back to the North American waters. Even with these precautions, it is estimated that only one in four thousand baby loggerheads from Florida will survive their struggles and become adults.

“All species of sea turtles are listed as threatened or endangered. The new research may provide insights that are helpful in conservation.” Lohmann stated. He added, “For example, different populations of loggerheads around the world are likely to have different magnetic maps with each map specific to a particular migratory pathway in one part of the world. If loggerheads in one geographic area go extinct, it will probably be impossible to replace them with turtles from another area, because the new arrivals will lack the inherited instructions needed to navigate within and from their transplanted homes.”

Another reason why turtle conservation proves to be difficult is conditions that may alter their perceptions of the magnetic fields. According to Lohmann, one popular method of aiding baby turtles consists of surrounding the underground nests with metal wire, which prevents predators such as raccoons from pilfering the eggs. Although the intentions of humans are good, a method such as this can hinder the baby turtle´s navigation once they surface from the nests.