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Review
. 2008 Dec 9;105(49):19096-101.
doi: 10.1073/pnas.0801859105. Epub 2008 Dec 5.

Geomagnetic imprinting: A unifying hypothesis of long-distance natal homing in salmon and sea turtles

Kenneth J Lohmann  1 , Nathan F Putman, Catherine M F Lohmann
Affiliations
Review

Geomagnetic imprinting: A unifying hypothesis of long-distance natal homing in salmon and sea turtles

Kenneth J Lohmann et al. Proc Natl Acad Sci U S A. .

Abstract

Several marine animals, including salmon and sea turtles, disperse across vast expanses of ocean before returning as adults to their natal areas 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. However, both salmon and sea turtles detect the magnetic field of the Earth and use it as a directional cue. In addition, sea turtles derive positional information from two magnetic elements (inclination angle and intensity) that vary predictably across the globe and endow different geographic areas with unique magnetic signatures. Here we propose that salmon and sea turtles imprint on the magnetic field of their natal areas and later use this information to direct natal homing. This novel hypothesis provides the first plausible explanation for how marine animals can navigate to natal areas from distant oceanic locations. The hypothesis appears to be compatible with present and recent rates of field change (secular variation); one implication, however, is that unusually rapid changes in the Earth's field, as occasionally occur during geomagnetic polarity reversals, may affect ecological processes by disrupting natal homing, resulting in widespread colonization events and changes in population structure.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.
Fig. 1.
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 field lines 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°. (B) Diagram illustrating four elements of geomagnetic field vectors that might, in principle, provide turtles with positional information. The field present at each location on Earth can be described in terms of total field intensity and inclination angle. The total intensity of the field can be resolved into two vector components, the horizontal field intensity and the vertical field intensity. (Whether animals can resolve the total field into vector components is not known.)
Fig. 2.
Fig. 2.
Isolines of magnetic field elements along the coasts of North America derived from the World Magnetic Model for the year 2005. (A) Isoclinics (isolines of magnetic field inclination). Each region of the west coast is marked by a different inclination angle; a similar situation exists for the east coast. Adjacent isoclinics represent differences in inclination of 2°. (B) Isodynamics (isolines of total field intensity). Adjacent isolines represent differences in intensity of 1,000 nT.
Fig. 3.
Fig. 3.
Evidence for a magnetic map in juvenile green turtles. Juvenile turtles were captured in feeding grounds near the test site in Melbourne Beach, FL. Each turtle was exposed to a magnetic field that exists at one of two distant locations (represented by stars along the coastline). Turtles exposed to the field from the northern site swam approximately southward, whereas those exposed to the field from the southern site swam approximately north. In the orientation diagrams, each dot represents the mean angle of a single turtle. The arrow in the center of each circle represents the mean angle of the group. Dashed lines represent the 95% confidence interval for the mean angle. Figure is modified from ref. . See text for details.
Fig. 4.
Fig. 4.
Navigational errors that would hypothetically accrue due to secular variation, assuming various absences, geographic locations, and strategies of geomagnetic imprinting (see text). Animals were assumed to be unable to detect or compensate for secular variation; they were also assumed to be absent from the natal region until reproductive maturity. Analyses were done for four different situations: (i) salmon leaving the Columbia River in Washington state and returning after 3 years; (ii) Kemp's ridley turtles leaving their nesting beach at Rancho Nuevo, Mexico, and returning after 10 years; (iii) loggerhead turtles leaving Melbourne Beach, FL, and returning after 20 years; (iv) loggerhead turtles leaving Mon Repos, Queensland, Australia, and returning after 30 years. In each case, the measurements reported indicate the distance by which an adult animal would miss its natal site if it were to return to the coastal location which has the same magnetic inclination or intensity that marked the natal site at the time the animal departed. Measurements represent means of repeated simulations at 5-year intervals from 1900 to the present. In other words, the salmon simulation involved a fish leaving the coast in 1900 and returning in 1903, a second fish leaving the coast in 1905 and returning in 1908, a third fish leaving in 1910 and returning in 1913, and so on. Error bars represent 95% confidence intervals of the mean.
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