Smartphones and GPS-based navigation systems have made it pretty easy for humans to figure out how to get from point A to point B. But migratory animals lack not only the thumbs to key in search terms for a destination on an iPhone, but the technology itself. Migratory birds and sea turtles, however, may have something better: They can sense variations in Earth’s magnetic field as they travel huge distances across the globe to the same mating grounds or beaches year after year.
Now we know more about how salmon do it, too. In a study published February 6th in Current Biology scientists have found that salmon use Earth’s magnetic field like a map, much like hatchling turtles. Birds, by contrast, use magnetic fields more like a compass: They know what direction they’re facing, but they need other information to know where they’re supposed to go.
Salmon appear to inherit this map as genetic hand-me-downs from their parents: Young fish that were raised in captivity still responded as expected to artificial magnetic fields. “If they’re using magnetic fields to figure out where they are, when they’re in a different magnetic field, they should change their swimming direction,” says Nathan Putman, a researcher at Oregon State University (O.S.U.) and lead author of the study. And indeed, the fish “go in different directions in different magnetic fields,” he says.
In the study researchers took young Chinook salmon and put each one in a tank. Using magnetic coils, they were able to emulate the magnetic fields at different parts of the salmons’ oceanic ranges. They found that when the fish were exposed to fields like those at the northernmost parts of their usual range, they wanted to swim south to return to the center of their ranges. Conversely, fish exposed to a southern-range magnetic field tended to orient themselves northward. “Those fish know where they are and they know where they should be and they know how to get there,” says David Noakes, a professor of fisheries and wildlife also at O.S.U. and a co-author of the paper. “That’s pretty impressive,” he says.
Unlike migratory birds, Noakes says, most salmon don’t have the opportunity to learn from their forebears where they should go; they only migrate out to the ocean and back to their natal streams to spawn once in their lives before they die. “Their parents aren’t around to help them do things,” he says. “They have to do everything in their life right…. They don’t get to play the game again.” If they get it wrong, Noakes says, then the fish will either starve to death, get eaten or end up in the wrong currents. This means the migratory knowledge has to be “internal to the animal,” he notes.
The study also found that the salmon appear to use a combination of two characteristics of the magnetic field at a given location: its intensity, or strength, coupled with its inclination, or the angle of the field compared with Earth itself. When fish were exposed to a mismatched field, such as a northern intensity paired with a southern inclination, or vice versa, they became disoriented, indicating that they use both types of information to figure out where they are and where they should go.
Whereas the study showed that salmon inherit some innate maps, it is not clear exactly how the animals perceive them. “We can’t say what a fish or a turtle is picturing,” Putman says. What the scientists are stuck with, he says, is looking at the behavior of the salmon and asking how they behave. “It’s behaving like it has some information about where it is,” he says, meaning that it has some sort of internal map based just on the magnetic field. Although fish may use other environmental characteristics in the wild, such as the sun or olfactory cues, those variables were controlled in this study, meaning that the fish only relied on the magnetic field to decide on which direction they should swim.
James Gould, an evolutionary biologist at Princeton University who was not involved in the study, wrote in a commentary on the paper that the fish may not have a map like the kind we might imagine, but instead something akin to an Excel spreadsheet, with “lists of magnetic coordinates with the seasonally appropriate directional responses filled in.” That is, the fish may simply perceive the magnetic field at a given location and, like a GPS, have essentially a voice command in its head that tells it where to swim. Putman thinks this is likely the case for the salmon. “If I had to bet, I would say that is probably what’s happening,” he says.