By Nathan Putman
How does a young animal find its way to an unfamiliar location hundreds or thousands of kilometres from where it was born?
A reasonable idea might be to find an older, experienced migrant and follow. This might work well for caribou or some songbirds, but what about the many marine animals such as tuna, salmon, eels or sea turtles that never even meet their parents?
Our experiments published in Current Biology indicate that juvenile Chinook salmon (sometimes called king salmon) make their journey as if they have a GPS, based not on satellite links but the Earth’s magnetic field. This is possible because the Earth’s magnetic field varies predictably across the globe: the intensity of the field increases from the equator to the poles, and the angle at which magnetic field lines intersect the surface of the Earth similarly increases towards the poles. This forms a grid of coordinates that animals capable of sensing it can use to approximate their position.
This is different to a compass, in which the magnetic field is used to find or maintain a direction. A compass can help you walk in a straight line, but it won’t tell you where you are. For that a map is needed, and quite conveniently for salmon they seem to come with one pre-installed.
Salmon orienteering We were able to reveal this with surprisingly simple techniques. We created a model of the North Pacific’s magnetic field by wrapping a wooden frame with copper wires running horizontally and vertically around the perimeter, spaced at certain intervals. Passing an electric current of a specific amperage through the wires recreated the intensity and inclination angle found at any location in the North Pacific.
We placed juvenile salmon in several five-gallon buckets within the contraption and changed the magnetic field around them, while a camera overhead photographed how their direction of swimming changed with the magnetic field. Overall, fish that found themselves to the north of their typical oceanic range swam southwards, and those displaced to the south swam northwards. Fish that remained in the local field, that is, the field they expected to be in, did not have a preference, which indicated that those preferences observed in the other simulated fields are attributable to the change in magnetic conditions (a reassuring “control” for our experiments).
To demonstrate that salmon were using both the intensity and inclination of the magnetic field to determine where they were, we tested them in a field that paired the northern intensity with the southern inclination angle and vice versa. These combinations of magnetic parameters do not exist in the North Pacific, so only if the fish used one, rather than both, of the parameters would they be able to orient themselves correctly. We found the fish swam and directed themselves randomly, showing their confusion.
What’s really interesting is that we used entirely naïve salmon in our experiments – they had never been anywhere but the testing facility, had never swum the route to learn the magnetic gradients, nor met another salmon that had. So the magnetic map, and compass, and knowledge of which direction to set off in are probably inherited traits, equipping the salmon to successfully navigate the world’s largest ocean as soon as they hit saltwater.
Field knowledge from birth Admittedly, it does seem fantastic that animals can know where they are on the globe by taking readings of the Earth’s subtle, invisible magnetic field. It’s been argued that the magnetic field is too weak and the gradients to shallow for animals to assess a latitudinal and longitudinal position. Others have likewise suggested that as the magnetic field gradually drifts over time, it doesn’t make a good map.
Work by Ken Lohmann and me on sea turtles seems to have convinced most people that hatchling turtles are using inherited magnetic instructions (at least, in part) to guide their ocean migration. There is considerable scepticism among some as to whether this could work for other animals, while others have taken to it with gusto: some in the creationist/intelligent design camp claim the apparent “uniqueness” of turtles' magnetic navigation system proves turtles are a “special creation” and thus disprove the theory of evolution by natural selection.
But these experiments definitively show that juvenile salmon, like turtles, inherit an ability to detect and orient themselves to magnetic fields to help them cross the oceans. It appears that the unpredictable ocean environment imposes a strong selective pressure for animals to be able to determine where they are, so as to help them find food, favourable temperatures, and avoid predators. Many of these things are very difficult to directly detect; salmon and turtles seem to have evolved hard-wired orientation responses and this leads them to find better than average locations, which in turn leaves them better fed, better protected and more likely to reproduce.
Given the behaviour observed in these two distantly related species of marine migrants, convergent evolution seems to have decided that the cues within the magnetic field are the right tool for the job.