There's been a long-held theory that female Loggerhead sea turtles return to the place where they were born to lay their own eggs, once they reach maturity. Yes, loggerheads like monarch butterflies do come equipped with a built-in GPS that’s been passed down and perfected through evolution. But there's now also some new evidence that in addition to internal compasses, there's another trigger at play. Of recent date a new study points to turtles relying also on an external factor.
Finding their way in the world . . .
Immediately upon leaving its nest, each hatchling will scramble to the ocean to begin an epic 8,000-mile solo journey around the North Atlantic basin. Young loggerheads that survive this incredible migration will return to the coastal waters of North America in about 6 to 12 years — only after they have grown large enough to fend off predators. (Adult loggerheads are distinguished by their huge heads and large crushing jaws.)
Unfortunately, only about one in 4,000 hatchlings reach adulthood. The ones that do are solitary ocean creatures — males live their entire lives in the depths, while the females only come ashore to nest and lay eggs. This is where that 'external factor' comes into play.
Coastlines are magnetic . . .
New research indicates that Loggerheads are attuned to Earth’s magnetic field which may may get them closer to their original birthplace, but not always.
That’s because each part of the coastline has its own magnetic signature, which Mamma Loggerheads remember and later use in tandem with their internal compass.
What can change the exact spot from the exact birthplace to a new nesting ground is that these magnetic fields change slowly over time — prompting these Mamas to shift their landfalls, according to a study published by Current Biology in 2015.
"It's pretty fascinating how these creatures can find their way through this vast expanse of nothing," said study co-author J. Roger Brothers, a biology graduate student at the University of North Carolina, Chapel Hill.
Researchers have been tracking subtle shifts in Earth's magnetic field along the eastern coastline of the United States, using man-made compasses to measure how their strength and other properties transition over time.
These researchers use their data combined with that supplied by volunteers provided over the last few years. Their combined results supported their hypothesis: The loggerhead sea turtle nests moved in tandem with the shifts in the magnetic field.
As a result of the shifts, these Mama Loggerheads get confused. For instance, on some islands such as Hunting Island in South Carolina, different parts of other eastern coastal beaches may have ‘similar’ magnetic fields.
In these cases, the turtles can get disoriented and their internal compasses may be telling them to head toward a familiar magnetic field, not necessarily the one where they were born, but one further north into North Carolina, or one further south into Georgia or Florida. Sometimes its very slight from one zone to another, on the same beach.
"It's through these navigational errors that we might see a population structure where, regardless of geographic distance or environmental similarities between the two nesting beaches," he says. "The difference in the Earth's magnetic field is a really strong predictor of how genetically similar or different the two nesting populations are,“ said J. Roger Brothers, the lead author of the study.
So, while there’s no place like home, and works of fiction more often than naught like to tidy up their denouements with a happy ending, in reality — at least with the loggerheads — “close enough” can be just as good!