Animal migration is one of the great wonders of the natural world. Monarch butterflies, Arctic terns and humpback whales, among other species, travel thousands of miles to escape harsh seasonal weather and find more hospitable climes, like New Yorkers who high-tail it for Florida when the first snowflake drops.
But, unlike humans, animal species don't have airlines and highways to guide them. How do they make their amazing journeys?
With the help of magnets, according to new research.
An international team of scientists identified cellular machinery in migratory trout that allows them to detect and respond to the Earth's magnetic field. The ability explains how animals are able to maintain their long-distance pilgrimages even after human activity changes the visual layout of the migratory path.
Experts have believed for some time that animals use the planet's magnetic field as a rough road map. But to prove it, they needed to find cells that could act as magnets.
The path to finding those cells has been full of stops and starts. For instance, a high-profile paper purporting to show that the beaks of birds contained magnetic cells was debunked in April. The cells, it turned out, were unrelated to navigation.
One of the reasons for the difficulty is that magnetic cells can't be clustered together, as vision cells are in the retina. Instead, since each cell creates its own little magnetic field, the cells must be spread out to avoid disrupting one another.
In the new report, published online this week by the Proceedings of the National Academy of Sciences, researchers not only said that they had found the magnetic cells, but also that the cells are much more powerful than anyone had imagined.
"These cells are magnetic monsters," said Caltech geobiologist Joseph Kirschvink, who worked on the study.
The team used a simple technique to isolate the key cells from rainbow trout. First they extracted cells from the olfactory epithelium, the part of the fish that senses smell. Then, while observing the cells under a microscope, they induced a magnetic field around them that rotated clockwise. If any of the cells were magnetic, they would spin along with the rotating magnetic field.
That's exactly what they found: A small number of the cells began to spin. Further analysis revealed that the cells were loaded with a magnetic mineral called magnetite.
Though the method may seem obvious in retrospect, the researchers said they were amazed it worked because it required the cells to be extremely magnetic.
"The earlier estimates of the amount of magnetic particles in each cell were really low," said Michael Winklhofer, a geophysicist at Ludwig-Maximilians University in Munich and the study's senior author. But it turned out that each cell contained about 100 magnetite crystals, rather than the five they had expected based on their mathematical models.
That surprising strength solves another long-standing mystery related to magnetic navigation. Once every few hundred thousand years, the Earth's magnetic field reverses, and during that time there is a period when the field is almost nonexistent. Scientists have wondered how migrating animals managed to navigate during these periods. But with so many crystals in each cell, "the challenge of using the magnetic field to navigate when it goes to near-zero is overcome," said Michael Walker, an ecologist at the University of Auckland in New Zealand who wasn't involved in the study.
How do the cells work? A big clue comes from the fact that the cells spun exactly in time with the rotation of the magnetic field. That indicates the magnetite was firmly attached to the cell membrane rather than floating freely.
And that provides a mechanism by which the cells might send signals to the brain: When the field changes, the magnetic pull puts physical stress on the cell membrane, causing channels to open and ions to flow in and out. Such a mechanism has been demonstrated in other systems, and there's preliminary evidence that it happens in these cells too, Kirschvink said.
Walker said the new study makes a very good case that the magnetic field sensors have been found. Definitive proof will come when scientists show that changing the magnetic field causes the cells to fire predictably, just like changes in the visual environment prompt predictable changes in retinal cells.
Still unanswered is whether other migratory species utilize the same mechanism for detecting Earth's magnetic field. Kirschvink said he could use the same method to test cells from other animals, and he expects the answer to be yes.
"This study should be representative of what we find in other organisms," he said. "It's a game changer."