Diverse animals detect the Earth's magnetic field and use it as a cue in orientation and navigation. Most research on magnetoreception has focused on the directional or `compass' information that can be extracted from the Earth's field. Because the field varies predictably across the surface of the globe, however, it also provides a potential source of positional or `map' information, which some animals use to steer themselves along migratory pathways or to navigate toward specific target areas. The use of magnetic positional information has been demonstrated in several diverse animals including sea turtles, spiny lobsters, newts and birds, suggesting that such systems are phylogenetically widespread and can function over a wide range of spatial scales. These `magnetic maps' have not yet been fully characterized. They may be organized in several fundamentally different ways, some of which bear little resemblance to human maps, and they may also be used in conjunction with unconventional navigational strategies.
Salmons, for example, use Earth’s magnetic field to create a large-scale mental map which they follow to find suitable feeding grounds, a study published today in Current Biology has found. The salmons are born in rivers and live out the early part of their lives in freshwater before travelling hundreds or thousands of kilometres out into the open ocean, where they spend most of their adulthood.
It has long been suspected that salmon use Earth’s magnetic field to navigate during this long migration. But until now, the way that young salmon swim from their streambed out into the open ocean with no previous knowledge of the sea, nor any parents or experienced fish to follow, has been a mystery. Previous work has suggested they might be guided, in part, by taking cues from regional magnetic ﬁelds to determine the best course – termed the “inherited magnetic map”. Because this map is inherited the salmon do not require any previous knowledge of their migration path or location.
The idea of an inherited magnetic map has long been speculated, but until now there has been no empirical evidence to suggest that salmon can determine their geographic position using the geomagnetic field. Nathan Putman from Oregon State University has now shown that salmon do navigate using an inherited magnetic map.
Dr. Putman and colleagues tested young Chinook salmon against different magnetic fields, either north or south of their typical ocean range, and found that the fish orientate themselves back towards their home range. If a fish was exposed to a north magnetic field, for instance, it would change its swimming direction back south.
Putman and colleagues also examined magnetic field components (magnetic intensity and the inclination angle) to determine which feature the fish use as a cue and found that neither of the features alone elicited the complete turn-around response, indicating that salmon rely on a combination of the two.
The results of the study also suggest this trait is inherited, as salmon are able to navigate without requiring any previous learning.