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“Smart swimming,” magnetism said to help baby turtles through epic journey

May 16, 2012
Courtesy of the National Science Foundation
and World Science staff

It has been one of the more en­dur­ing mys­ter­ies of bi­ol­o­gy: how ba­by sea tur­tles, barely long­er than a thumb, to un­der­take one of the most spec­tac­u­lar migra­t­ions in the an­i­mal king­dom—alone.

Now, sci­en­tists say they may have a han­dle on how the log­ger­head tur­tle hatch­lings do it: an in­ter­nal com­pass that re­sponds to chang­ing mag­net­ic fields, and “s­mart swim­ming,” all hard­wired in­to their young brains.

A baby logger­head. (Credit: K. Loh­mann, Dept. of Biol­ogy, UNC Cha­pel Hill)

“A sur­pris­ingly small amount of di­rec­tion­al swim­ming in just the right places has a pro­found ef­fect on the mi­gra­to­ry paths that tur­tles fol­low,” said Ken­neth J. Loh­mann, a ma­rine bi­ol­o­gist at the Uni­vers­ity of North Car­o­li­na at Chap­el Hill who has stud­ied log­ger­heads.

The “smart swim­ming” al­lows the tur­tles, born on the Flor­i­da coast, to get where they need to go with­out put­ting in­or­di­nate strain on their ti­ny bod­ies, he ex­plained. There are oth­er log­ger­head popula­t­ions born else­where with dif­fer­ent migra­t­ion pat­terns, but Loh­mann has fo­cused his work on the Flor­i­da group.

Part of the an­swer to how they man­age their jour­ney is that many don’t. An es­ti­mat­ed three in four tur­tles fall prey to hun­gry birds, fish­es or oth­er per­ils be­fore adult­hood. 

But those that sur­vive nav­i­gate around the whole North At­lantic ocean, in­to wa­ters that pro­vide fa­vor­a­ble con­di­tions for their growth, only years lat­er re­turn­ing to their birth ar­eas to spawn. All this they ac­com­plish with­out ev­er hav­ing met their par­ents. The moth­ers lay eggs on the beach and de­part, leav­ing them to hatch around eight weeks lat­er. Upon hatch­ing, the defense­less young­sters head for the wa­ter and swim franti­c­ally away to­ward deep wa­ters, where their preda­tors are less abun­dant. In the open ocean, rafts of plants provide young logger­heads with food, rest­ing places and camou­flage.

Approximate migratory route of Florida logger­heads. The path­way coin­cides with a warm-water curr­ent sys­tem known as the North Atlan­tic Sub­tropi­cal Gyre, marked in a simpli­fied form with red arrows. (Credit: K. Loh­mann, Dept. of Biol­ogy, UNC Cha­pel Hill)

In a study pub­lished in the June is­sue of The Jour­nal of Ex­pe­ri­men­tal Bi­ol­o­gy, Loh­mann and col­leagues ar­gue that young log­ger­heads swim only in places where cur­rents threat­en to push them off course. Where the cur­rents are fa­vor­a­ble, they just drift, con­serv­ing en­er­gy. These find­ings—based on com­put­er sim­ula­t­ions com­bin­ing ocean cur­rents and ‘vir­tual tur­tles’ swim­ming for var­i­ous pe­ri­od of time—chal­lenge a long­stand­ing be­lief that young sea tur­tles drift pas­sively and that their dis­tri­bu­tion is de­ter­mined en­tirely by ocean cur­rents, Loh­mann said.

“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,” he ex­plained. “This study shows oth­erwise.”

A re­lat­ed pa­per by Loh­mann’s team ex­plains how young log­ger­heads know where they are and in what di­rec­tion to steer as they mi­grate. The pa­per, which ap­pears in the April is­sue of Cur­rent Opin­ion in Neuro­bi­ol­o­gy, re­ports that the tur­tles are guid­ed at least partly by an in­her­it­ed “mag­net­ic map.” In other words, the Earth’s mag­net­ic field dif­fers slightly in dif­fer­ent ge­o­graph­ic ar­eas, and the tur­tles’ inter­nal map en­ables them to in­stinc­tively use dif­fer­ences in these fields as naviga­t­ional mark­ers. Each change in the field elic­its a change in the tur­tle’s di­rec­tion, which in turn steers the tur­tle along its route.

The new pa­per sum­ma­rizes a dec­ade of re­search in which sci­en­tists in­ves­t­i­gated the tur­tles’ mag­net­ic map, us­ing lab­o­r­a­to­ry ex­pe­ri­ments in which young log­ger­heads were ex­posed to mag­net­ic fields that ex­ist along the nat­u­ral mi­gra­to­ry route. Amaz­ing­ly, Loh­mann said, the di­rec­tion that tur­tles swam in the lab in re­sponse to var­i­ous mag­net­ic fields matched ob­serva­t­ions of the steer­ing de­ci­sions made by tur­tles when swim­ming through com­pa­ra­ble mag­net­ic fields in the ocean. The re­sults in­di­cate the tur­tles’ brains are hard-wired to nav­i­gate their mi­gra­to­ry routes from birth, he added.

“The re­sults al­so in­di­cate that tur­tles ob­tain both lat­i­tude and longitude-like in­forma­t­ion from the oceanic mag­net­ic field,” said Da­vid Ste­phens, a pro­gram di­rec­tor at the U.S. Na­t­ional Sci­ence Founda­t­ion, which funded the re­search. “They may there­by ob­tain much richer spa­tial rep­re­senta­t­ions from mag­net­ic fields than do hu­mans with their com­passes.”

All spe­cies of sea tur­tles are list­ed as threat­ened or en­dan­gered. The new re­search may pro­vide in­sights that are help­ful in con­serva­t­ion, Loh­mann said.

For ex­am­ple, dif­fer­ent popula­t­ions of log­ger­heads around the world are likely to have dif­fer­ent mag­net­ic maps, Loh­mann ex­plained, with each map spe­cif­ic to a par­tic­u­lar mi­gra­to­ry path­way in one part of the world. If log­ger­heads in one ar­ea die out, 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.

In ad­di­tion, con­di­tions that im­pair the func­tion­ing of tur­tles’ mag­net­ic sense may jeop­ard­ize sur­viv­al. Loh­mann said that in Flor­i­da and else­where, a com­mon con­serva­t­ion prac­tice is to sur­round tur­tle nests on the beach with wire cages to pro­tect the tur­tle eggs from rac­coons. But such cages al­so dis­tort the lo­cal mag­net­ic field, he said, and may thus impair the hatch­lings’ abil­ity to nav­i­gate.

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It has been one of the more enduring mysteries of biology: how baby sea turtles, barely longer than a thumb, to undertake one of the most spectacular migrations in the animal kingdom—alone. Now, scientists say they may have a handle on just how the loggerhead turtle hatchlings do it: an internal compass that responds to changing magnetic fields, and “smart swimming,” all hardwired into their young brains. “A surprisingly small amount of directional swimming in just the right places has a profound effect on the migratory paths that turtles follow,” said Kenneth J. Lohmann, a marine biologist at the University of North Carolina at Chapel Hill who has studied loggerhead turtles. The “smart swimming” allows the turtles, born on the Florida coast, to get where they need to go without putting inordinate strain on their tiny bodies, he explained. There are other loggerhead populations born elsewhere with different migration patterns, but Lohmann has focused his work on the Florida group. Part of the answer to how they manage their journey is that many don’t. An estimated three in four turtles fall prey to hungry birds, fishes or other perils of the sea before adulthood. But those that survive navigate around the whole Atlantic ocean, into waters that provide favorable conditions for their growth, only years later returning to their birth areas to spawn. All this they accomplish without ever having met their parents. The mothers lay eggs on the beach and depart, leaving them to hatch around eight weeks later. As soon as they are born, they head for the water and swim frantically away toward deep waters, where their predators are less abundant. In a study published in the June 2012 issue of The Journal of Experimental Biology, Lohmann and colleagues argue that young loggerheads swim only in places where currents threaten to push them off course. Where the currents are favorable, they just drift, conserving energy. These findings—based on computer simulations combining ocean currents and ‘virtual turtles’ swimming for various period of time—challenge a longstanding belief that young sea turtles drift passively and that their distribution is determined entirely by ocean currents, Lohmann said. “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,” he explained. “This study shows otherwise.” A related paper published last month by Lohmann’s team explains how young Florida-hatched loggerheads know where they are and in what direction to steer as they migrate around the North Atlantic basin. The paper, which appears in the April issue of Current Opinion in Neurobiology, reports that the turtles are guided at least partly by an inherited “magnetic map.” The Earth’s magnetic field differs slightly in different geographic areas. The turtles’ magnetic map enables them to instinctively use differences in these fields as navigational markers. Each change in the magnetic field elicits a change in the turtle’s swimming direction, which in turn steers the turtle along its migratory route at each location. The new paper summarizes a decade of research in which scientists investigated the turtles’ magnetic map, using laboratory experiments in which young loggerheads were exposed to magnetic fields that exist along the natural migratory route. Amazingly, Lohmann said, the direction that turtles swam in the lab in response to various magnetic fields matched observations of the steering decisions made by turtles when swimming through comparable magnetic fields in the ocean. The results indicate the turtles’ brains are hard-wired to navigate their migratory routes from birth, he added. “The results also indicate that turtles obtain both latitude and longitude-like information from the oceanic magnetic field,” said David Stephens, a program director at the U.S. National Science Foundation, which funded the research. “They may thereby obtain much richer spatial representations from magnetic fields than do humans with their compasses.” All species of sea turtles are listed as threatened or endangered. The new research may provide insights that are helpful in conservation, Lohmann said. For example, different populations of loggerheads around the world are likely to have different magnetic maps, Lohmann explained, 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. In addition, conditions that impair the functioning of turtles’ magnetic sense may jeopardize survival. Lohmann said that in Florida and elsewhere, a common conservation practice is to surround turtle nests on the beach with wire cages to protect the turtle eggs from raccoons. But such cages also distort the local magnetic field, he said, and may thus compromise the hatchlings’ ability to navigate.