Several marine animals, including salmon, elephant seals, and sea turtles, disperse across vast expanses of ocean before returning as adults to their natal area to reproduce. How animals accomplish such feats of natal homing has remained an enduring mystery. Salmon are known to use chemical cues to identify their home rivers at the end of spawning migrations. Such cues, however, do not extend far enough into the ocean to guide migratory movements that begin in open-sea locations hundreds or thousands of kilometers away. Similarly, how sea turtles reach their nesting areas from distant sites is unknown.
The geomagnetic imprinting hypothesis for sea turtles and salmon, described in a 2008 paper published in the Proceedings of the National Academy of Sciences, proposes that these animals imprint on the magnetic field of their home areas when young and then use this information to return as adults years later. During the past decade, strong evidence has accumulated to support this idea.
What is imprinting?
In behavioral biology, imprinting refers to a special form of learning. Although precise definitions of imprinting vary, the hallmarks of imprinting are that the learning occurs during a specific, critical period (usually early in the life of the animal), the effects are long-lasting, and the learning cannot be modified easily. For natal homing, the concept is that migratory marine animals learn to recognize the unique magnetic field of their home region before leaving and can then identify it when it is time for them to return.
How can the Earth’s magnetic field be used to identify specific locations?
To a first approximation, the Earth’s magnetic field resembles the dipole field of a giant bar magnet (see diagram below). Field lines leave the southern hemisphere and curve around the globe before reentering the planet in the northern hemisphere. Several geomagnetic elements vary predictably across the surface of the globe. For example, at each location on the globe, the magnetic field lines intersect the Earth’s suface at a specific angle of inclination. At the magnetic equator, the field lines are parallel to the ground and the inclination angle is said to be zero. The field lines become progressively steeper as one moves toward the magnetic poles; at the poles themselves, the field lines are perpendicular to the Earth’s surface. Thus, inclination angle varies predictably with latitude, and an animal able to detect this field element (as sea turtles are known to do) may be able to detect whether it is north or south of a particular area. Similarly, the strength or intensity of the Earth’s field also varies across the surface. Turtles are also known to detect this magnetic parameter.
Right: Diagram of the Earth’s magnetic field. (A) Diagram illustrating how field lines (represented by arrows) intersect the Earth’s surface and how inclination angle (the angle formed between the Earth’s field and the Earth) varies with latitude. At the magnetic equator (the curving line across the Earth), field lines are parallel to the Earth’s surface. The field lines become progressively steeper as one travels north toward the magnetic pole, where the field lines are directed straight down into the Earth and the inclination angle is 90 degrees. (B) The field present at each location on Earth can be described in terms of total intensity and inclination angle. (The total intensity of the field can also be resolved into two vector components — the horizontal and vertical field components — but whether any animal can detect these is not known.)
In principle, geomagnetic parameters can be used to identify particular coastal areas. To understand why this is possible, it is helpful to look at how magnetic inclination and intensity vary across a large part of the Earth. The map on the left (Fig. A) shows the isolines of magnetic inclination along the North American coast. (An isoline is a line along which a parameter is constant, so in this case, every location along a particular isoline has the same inclination.) A similar map of isolines is shown for magnetic intensity in Fig. B.
The basic concept behind the geomagnetic imprinting hypothesis can be illustrated easily using the west coast of North America as an example. The coastline is aligned approximately north-south, whereas the isolines of inclination and intensity in this region trend east-west. As a consequence, every area of coastline is marked by a unique magnetic inclination angle (Fig. A) and also by a unique intensity (Fig. B). Thus, in effect, every coastal area has a unique “magnetic signature” that can potentially be used to identify a natal region and distinguish it from all other locations along the same coast. The same is true along the east coast of North America and, indeed, along most continental continents worldwide.
In principle, if salmon in the Pacific northwest — or sea turtles in the Atlantic southeast — were to imprint on the inclination angle and/or intensity of their home area, then returning there years later might be relatively simple. The animal might need only to locate the coast, and then swim north or south along it until the correct isoline is encountered. Moreover, such a migrant could potentially determine if it is north or south of the target by comparing the field at its present location with the remembered value at the natal area. Locations north of the natal area will have fields with steeper inclinations and stronger intensities than the field at the natal location; locations south of the natal area will have fields with inclinations less steep and intensities less strong than exist at the natal area.
What is the evidence for geomagnetic imprinting?
Several studies providing strong evidence for geomagnetic imprinting in sea turtles and salmon have now been published. To learn more about the evidence for geomagnetic imprinting, follow the links below:
Evidence for geomagnetic imprinting in sea turtles
Evidence for geomagnetic imprinting in Pacific salmon
Brothers, J. R. and K. J. Lohmann. 2018. Evidence that magnetic navigation and geomagnetic imprinting shape spatial genetic variation in sea turtles. Current Biology https://doi.org/10.1016/j.cub.2018.03.022 [New York Times] [Science Alert]
Lohmann, K. J., Putman, N. F., and C. M. F. Lohmann. 2008. Geomagnetic imprinting: a unifying hypothesis of long-distance natal homing in salmon and sea turtles. Proceedings of the National Academy of Sciences. 105: 19096-19101. [Download PDF]
Lohmann, K. J., Lohmann, C. M. F., Brothers, J. R., and N. F. Putman. 2013. Natal homing and imprinting in sea turtles. In: Biology of Sea Turtles (Editors: J. Wyneken, K. J. Lohmann, and J. Musick). Vol. 3, pp. 59-77. CRC Press: Boca Raton.
Brothers, J. R. and K. J. Lohmann. 2015. Evidence for geomagnetic imprinting and magnetic navigation in the natal homing of sea turtles. Current Biology. 25: 392-396. [Download PDF] [BBC] [Nature] [National Geographic] [Science] [Los Angeles Times] [The Scientist]
Putman, N. F. and K. J. Lohmann. 2008. Compatibility of magnetic imprinting and secular variation. Current Biology. 18: R596-597. [Download PDF]
Putman, N. F., Lohmann, K. J., Putman, E. M., Quinn, T. P., Klimley, A. P., and D. L. G. Noakes. 2013. Evidence for geomagnetic imprinting as a homing mechanism for Pacific salmon. Current Biology. 23: 312-316. [Download pdf] [BBC] [National Geographic] [New York Times] [Smithsonian]