Baby sea turtles can feel Earth’s magnetic field on their first journey

Hatchling loggerhead sea turtles begin life with a remarkable challenge. They crawl from the beach into the surf and then navigate thousands of miles across open ocean – alone, with no experience, and no adults to guide them.

A new study from the University of North Carolina at Chapel Hill (UNC) reveals how they pull off this navigational feat.

The researchers show that baby sea turtles rely on a tactile magnetic sense – likely driven by tiny magnetite crystals in their bodies – to determine where they are on Earth’s magnetic map.

At the same time, a separate, light-dependent magnetic sense acts more like a compass, helping the turtles keep a steady heading.

Together, these systems give the hatchlings the ability to both know where they are and which way to go before they ever leave sight of land.

How turtles detect magnetism

Animals are thought to detect magnetism in two main ways. One relies on light-sensitive molecules whose chemistry is subtly altered by Earth’s magnetic field (a “see the field” mechanism).

The other depends on microscopic particles of magnetite that physically tug in response to the field (a “feel the field” mechanism).

Loggerheads hatch with both a magnetic compass and a magnetic map. But scientists had not yet identified which sensor feeds the map – the ability to pinpoint location.

Seeing vs. feeling magnetism

UNC researchers devised a clever conditioning assay: hatchlings can learn to associate a specific magnetic field – the kind naturally found at a real ocean location – with food.

Instead of salivating like Pavlov’s dogs, the turtles perform a conspicuous “dance,” lifting their heads and front flippers above the water when they expect to be fed.

As study co-author Alayna Mackiewicz put it, “they are very food motivated and eager to dance when they think there is a possibility of being fed.”

The team fed hatchlings while they were immersed in a magnetic field that mimics the Turks and Caicos. Another group experienced a field patterned after waters near Haiti.

Later, both sets of turtles “danced” when exposed to the same magnetic signatures. This proves the youngsters had learned that field as a place where food appears.

Pulse reveals true sensor

To work out whether the turtles were seeing or feeling the magnetic map, the team used a classic magnetoreception test: a brief, strong magnetic pulse.

Such pulses temporarily scramble magnetite-based sensors but leave light-dependent mechanisms unaffected.

The researchers moved each trained hatchling into a coil that delivered the pulse. Then they placed it back into the learned magnetic field and watched what happened.

If the map relied on a magnetite “feel,” the dance should fade after the pulse; if it relied on a light-dependent “see,” the dance should persist.

Testing what turtles sense

After pulsing, the hatchlings danced significantly less when re-exposed to their trained fields.

That drop in response indicates that their ability to locate themselves on the magnetic map depends on a magnetite-based, tactile sense, one that the pulse temporarily disrupted.

In other words, loggerhead babies appear to feel where they are. Crucially, this doesn’t mean they ignore the other magnetic pathway.

Decades of research suggests that turtles use a light-dependent magnetoreception system as a compass to maintain headings over long stretches of open ocean.

The new study shows these two systems likely divide the labor. One system tells the turtles which direction they’re going, and the other tells them where they are on the globe.

Turtle dance in magnetic fields

The behavioral conditioning that made this test possible took patience. Mackiewicz and co-author Dana Lim spent two months working with newly hatched turtles.

The team repeatedly paired the Turks and Caicos and Haiti-like magnetic fields with feeding until the youngsters reliably performed their food-anticipation “dance” in those fields.

Only then could the team introduce the magnetic pulse and compare pre- and post-zap behavior.

Why turtles need magnetism

Mapping the sensory biology behind sea turtle navigation helps explain how animals with tiny brains and no mentors traverse an ocean they’ve never seen. It also sharpens conservation efforts.

Loggerheads imprint on natal beaches and rely on magnetic cues throughout their migrations.

Policymakers can design better rules on coastal development and electromagnetic noise when they understand which cues matter and how turtles detect them.

Moreover, they could even clarify how we interpret shifts in turtle routes as Earth’s magnetic field slowly changes over time.

The authors note that hatchlings may integrate multiple cues. These can include magnetic, visual, wave- and current-direction, and even chemical signals, depending on the context.

But this study pins down an essential piece: when loggerhead babies consult their inborn magnetic map, they do it by feeling Earth’s field.

The study is published in the Journal of Experimental Biology.

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