Geomagnetic Imprinting Hypothesis of Natal Homing in Salmon and Sea Turtles
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.
geomagnetic imprinting hypothesis for sea turtles and salmon,
described in a 2008 paper
by the Lohmann Lab 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
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.)
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:
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: