Mother turtles find their way back to nesting beaches by looking for unique magnetic signatures along the coast, according to a new study published in Current Biology.
Loggerhead turtles, for example, leave the beach where they were born as hatchlings and traverse entire ocean basins before returning to nest, at regular intervals, on the same stretch of coastline as where they started. How the turtles accomplish this natal homing has remained an enduring mystery until now.
Several years ago, Kenneth Lohmann, the co-author of the new study, proposed that animals including sea turtles and salmon might imprint on magnetic fields early in life, but that idea has proven difficult to test in the open ocean. In the new study, Brothers and Lohmann took a different approach by studying changes in the behavior of nesting turtles over time.
“We reasoned that if turtles use the magnetic field to find their natal beaches, then naturally occurring changes in the Earth’s field might influence where turtles nest,” Brothers says.
The researchers analysed a 19-year (1993–2011) database of loggerhead nesting sites on the Atlantic coast of Florida, an area encompassing the largest sea turtle rookery in North America. Their analyses confirmed the predictions of the geomagnetic imprinting hypothesis.
In some times and places, the Earth’s field shifted so that the magnetic signatures of adjacent locations along the beach moved closer together. When that happened, nesting turtles packed themselves in along a shorter stretch of coastline, just as the researchers had predicted. In places where magnetic signatures diverged, sea turtles spread out and laid their eggs in nests that were fewer and farther between.
Turtles are long lived, and females undertake reproductive migrations periodically throughout their adult lives. Thus, the population of turtles that migrate to a given beach to nest each year consists of two subsets: a group of first-time nesters, and another, typically larger group of older “re-migrants” that have nested in the area during previous years.
Loggerhead turtles are thought to reach adulthood when they are between 23 and 29 years old. Much younger than this they return to coastal areas from the open sea and continue to mature there.
Sea turtles likely go to great lengths to find the places where they began life because successful nesting requires a combination of environmental features that are rare: soft sand, the right temperature, few predators, and an easily accessible beach.
“The only way a female turtle can be sure that she is nesting in a place favorable for egg development is to nest on the same beach where she hatched,” Brothers says. “The logic of sea turtles seems to be that ‘if it worked for me, it should work for my offspring.'”
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.
Evidence for Geomagnetic Imprinting and Magnetic Navigation in the Natal Homing of Sea Turtles, Brothers, J. Roger et al., Current Biology DOI: http://dx.doi.org/10.1016/j.cub.2014.12.035
Casale P, Mazaris A, Freggi D (2011). Estimation of age at maturity of loggerhead sea turtles Caretta caretta in the Mediterranean using length-frequency data. doi: 10.3354/esr00319