A female baby sea turtle hatches out of its egg in the sand on a Florida beach. It races for the ocean to catch the waves and swim out to the open sea. After years at sea traveling thousands of miles across the open ocean, the turtle, now a pregnant mother, finds its way back to the exact same beach where it was born. This amazing feat is described in a new paper in Current Biology by Roger Brothers and Ken Lohman at the University of North Carolina:
Ever since John James Audubon tied silver threads to the legs of young songbirds and observed their return the following year, evidence has accumulated that many animals return to their natal areas after migrating to distant locations. An extreme example exists in loggerhead sea turtles, which leave their natal beaches as hatchlings and traverse entire ocean basins before returning to nest, at regular intervals, on the same stretch of coastline where they hatched. How sea turtles accomplish natal homing has remained an enduring mystery of animal behavior. [Emphasis added.]
The biologists analyzed nesting sites over a 19-year period. They found that nest sites of returning sea turtles followed changing magnetic field lines. As the earth’s magnetic field moves over the years, certain field lines of equal inclination, called isolines, move accordingly. In some years at a given location on the coastline, the isolines spread apart. In other years, the lines come closer together. The nest sites followed this same pattern, showing that the sea turtles were homing in on their natal beach by following the magnetic field inclination that matched their birth site. The hatchlings must have "imprinted" on that field line when they raced for the sea years earlier.
The precision of this sensory ability is incredible. Turtles are apparently able to gather information not just on the inclination angle of the magnetic field, but its intensity as well:
These results provide strong evidence that nesting sea turtles use Earth’s magnetic field to locate their natal beaches. Although the exact geomagnetic component (or components) exploited by turtles cannot be determined from the analyses, the findings are consistent with the hypothesis that nest site selection depends at least partly on magnetic signatures consisting of inclination angle, field intensity, or a combination of the two.
Science Magazine gives a brief review of the findings:
Much like shifting sand, magnetic fields slide slightly over time, and their strength also increases as one moves away from the equator, akin to latitude. This property gives each stretch of coast a unique geographic marker, known as an isoline. The team found that in years when these magnetic isolines moved apart, the turtle nests spread out over a larger area — by 1 or 2 kilometers. Conversely, when isolines converged, the nests squeezed into a smaller patch of beach, suggesting the turtles follow shifting magnetic tracks to their favorite nests. The findings also argue that a magnetic address is imprinted on loggerhead turtles at birth to point the way home.
The apparatus for sensing the earth’s magnetic field appears to be in the turtle’s brain, although this is still poorly understood. A news release about the paper from Cell Press says:
Brothers says that little is known about how turtles detect the geomagnetic field. Most likely, tiny magnetic particles in the turtles’ brains respond to the Earth’s field and provide the basis for the magnetic sense, but no one knows for sure.
Another clue indicates that turtles use more than a magnetic sense to find home. Some turtles nest on islands, where the isolines would spread out to the open sea at times. "In these cases," Brothers and Lohman suggest, "turtles might use magnetic cues to navigate to the vicinity of the island and then use odorants or other supplemental local cues to locate the nesting beach."
Remarkably, salmon show this same ability. Brothers and Lohman write:
In a previous study, the migratory route of salmon approaching their natal river was shown to vary with subtle changes in the Earth’s field. Whereas the endpoint of the salmon spawning migration was presumably the same regardless of route, our findings demonstrate for the first time a relationship between changes in Earth’s magnetic field and the locations where long-distance migrants return to reproduce.
Joining the contenders for this skill set are more unrelated animal types:
… our results provide the strongest evidence to date that sea turtles find their nesting areas at least in part by navigating to unique magnetic signatures along the coast. In addition, our results are consistent with the hypothesis that turtles accomplish natal homing largely on the basis of magnetic navigation and geomagnetic imprinting. These findings, in combination with recent studies on Pacific salmon, suggest that similar mechanisms might underlie natal homing in diverse long-distance migrants such as fishes, birds, and mammals.
So here we have a highly-precise navigational ability, able to cue on very faint properties in the earth’s magnetic field, then on olfaction, and possibly on "other supplemental local cues" to find home across thousands of miles. The sensory "instruments" involved are integrated so that they are able to coordinate their functions for the same goal. Furthermore, the baby turtles, with their tiny brains, must have the ability to memorize the natal signatures of odors and magnetic field properties at birth, then recall those memories years later as large adults. (Sea turtles return about every two years to lay eggs.)
That would be a conundrum enough to explain by unguided processes like natural selection. But then, adding to the difficulty for Darwinism, similar abilities are found in distantly related animals like fish, birds, and mammals. Even if a Darwinian could show a possible line of descent from fish to mammal, the abilities involved would have been lost and regained multiple times, because not all fish, birds, and mammals use magnetic navigation. Given the complexities of the sensory systems involved, this would represent a case of "convergent evolution" on steroids. If the origin of this capability in one type of animal is highly implausible by mutation and selection, how about four times or more?
A design perspective, by contrast, would expect that unrelated animals on a common planet would share similar capabilities for their needs. The earth’s magnetic field is global. It isn’t surprising that very different animals would be designed to use that feature of the earth.
Image: I, Thierry Caro [GFDL (http://www.gnu.org/copyleft/fdl.html), CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/) or CC BY-SA 2.5-2.0-1.0 (http://creativecommons.org/licenses/by-sa/2.5-2.0-1.0)], via Wikimedia Commons.