In the early days of sailing, merchants would pilot ships around the Mediterranean carrying goods for trade. Early sailors would follow coastlines from port to port for their livelihoods. Navigation into open water was tricky. When land was not visible, one had to rely on celestial cues for positioning. In open water, the only landmarks available to the intrepid sailor were not landmarks at all. They instead relied upon the Sun and stars. Latitude, or how far north or south you are, is easy to discern based on the position of the stars or how high the Sun peaks above the horizon.
Longitude, or how far East or West you are, was far more difficult to discern. Not until the invention of reliable, seaworthy clocks in the mid 1700s could longitude be calculated at sea. Access to this information, latitude and longitude, enables the sailor (or anyone, really) to discern their position on the planet.
These navigation techniques are meaningless on a bad day. Overcast weather in short term meant no navigational information. Long term, it could be disastrous.
Between 300 and 200 BC, the Chinese made a revolutionary discovery. They discovered that a needle magnetized by iron ore would orient itself to the North or South if suspended by a string. Although the direct application of this technology for navigation may seem obvious in hindsight, it would be another 1000 years before compasses served any practical navigational purpose.
In the event of a storm or overcast conditions, a navigator, equipped with sophisticated tools such as cork, a needle, and a bucket, comes to the ship’s rescue. He fills the bucket with water. He produces a needle that he proceeds to rub against a piece of iron ore before using it to impale a piece of cork. He drops the needle and cork into the bucket. Slowly, the needle begins to turn in the water. It will point to the north, allowing the navigator to discern the orientation of the ship and confirm their heading.
Navigation has come a long way since the invention of the compass. The compass was a significant achievement. The use of compasses in ancient china and medieval Europe represents humans joining a distinguished class of life on Earth. We became the newest species to navigate using the Earth’s magnetic field.
How do animals navigate?
In the boreal forests of northeastern Canada, the blackpoll warbler begins getting restless. It is September and days are beginning to become shorter. This little bird is packing on fat by gorging itself on everything it can find to eat. It’s packing for a long journey. Every fall, the blackpoll warbler migrates from Canada to the forests of northern South America.
Many birds make this migration, but the blackpoll warbler does something unique. While most migrants travel over land through Mexico into Central America, the blackpoll warbler takes a more direct route. Flying out over the Atlantic ocean, this bird takes a due south heading and flies for three days over the open ocean.
How does the warbler achieve this feat? It can sense the Earth’s magnetic field. Sailors using compasses to maintain their heading in fog were using the same tool to maintain their heading.
Birds are not the only species that navigate this way. Monarch butterflies making the trek from North America into Mexico navigate by the position of the sun during the day. On cloudy days they are able to navigate using the magnetic field as a backup. More impressively, researchers recently uncovered evidence that bogong moths, which migrate nocturnally, undergo massive treks to their home caves with the help of the magnetic field. This is the first insect species discovered to migrate at night by the magnetic field.
The ocean is no stranger to magnetic navigation, either.
Pacific salmon, which live their adult lives in the open ocean, likely navigate back to their home rivers through a combination of following chemical cues and magnetic navigation. Although researchers are not sure how exactly Salmon are using the magnetic field to navigate, research on European eels may provide a clue. Recently, the European eel was found to “memorize” the magnetic direction of tidal flows from their recruitment estuary, giving biologists a clue about how these animals handle their 5,000 kilometer transatlantic migration from their birthplace in the Sargasso sea to the estuaries in which they spend the majority of their time.
Another clue as to how animals might navigate with the Earth’s magnetic field comes from research on sea turtles, which suggest that they, at birth, are imprinted with a sense of their home beach’s “magnetic signature“.
While there are many species that navigate using the Earth’s magnetic field, few species seem to solely rely on the field itself as their primary navigational tool. It is apparent that magnetic navigation is an essential tool for long-distance migrations, but not the only one at their disposal. Just like early sailors who navigated by not only compass and map but also Sun and stars, long-distance migrants use multiple strategies to achieve their impressive migrations.
How do animals sense the magnetic field?
Although an ability to sense the Earth’s magnetic field exists in many species, it is still not known what physiological mechanisms allow this. In science, magnetoreception is an “area of ongoing research”, which is scientist-speak for “we don’t know it works yet”. This does not mean that clues haven’t been uncovered.
One prevailing clue, discovered in 2018, is the presence of a magnetically-sensitive protein present in the eyes of birds. This protein is of a class of proteins called cryptochromes. It is possible that magnetically-sensitive cryptochromes could be adapted for extra functions as favored by selection, such as the utility of being able to sense magnetic fields for navigation. The hypothesis that cryptochromes could be used to “see” the magnetic field has been strengthened by the discovery of magnetically-sensitive cryptochromes in other species capable of magnetoreception, such as the monarch butterfly.
Even though the mechanism for magnetoreception is not well understood, experiments involving the manipulation of magnetic fields provide conclusive evidence of magnetoreception in many species. Most of these experiments involve subjecting groups of animals to various magnetic fields which, while we humans would have no idea anything has changed, this entirely changes some aspect of the animal’s behavior. For example, the evidence that bogong moths use magnetoreception to navigate comes from a recent experiment that involved placing the moths in a tube with visual navigational cues and the ability to manipulate the magnetic fields within. When the magnetic field and the visual cues disagreed, the moths flew in random directions as if they were completely disoriented.
Understanding more about how animals sense the Earth’s magnetic field may inform conservation efforts. Migratory species are often in peril due to their reliance on multiple habitats. If our understanding of animal magnetoreception improves, we could potentially use that knowledge to protect species. Many sea turtle populations are at risk due to encroachment on their nesting habitat. Theoretically, conservation biologists could relocate sea turtle nesting habitat by imprinting the magnetic signature of more favorable and easily protected beaches into the memory of baby sea turtles at birth. If this is successful, those turtles would return to beaches where they would be less at risk. Although not an ideal situation, it must be admitted that this is a creative solution to a dire conservation issue.
It is also thought that magnetic field distortion around cell phone towers and windmills disorients birds, leading to collisions with these structures. By better understanding the mechanisms of detection, it should be possible to design workarounds for this issue as well.
Even though nobody is certain of how it works, one thing is certain – magnetoreception is one of the most fascinating and enduring mysteries of biology.