As soon as autumn comes in the west and the leaves start falling, humans start preparing for winter. What do other animals, such as birds, insects and fish, do to prepare for the cold?
The great migrations start. Thousands and thousands of creatures go to the places where their ancestors have always gone to sit out the winter and come back in warmer weather. But how do they know where to go? Migratory birds use a combination of methods to navigate like landmarks and seeing the position and light of the sun and stars. Now there is evidence that magnetic fields play an important role in navigation.
Our senses: sight, sound, taste, smell, and touch, are meant to help us deal with the environment, keep us out of danger and find food and shelter. Other animals need different information about the world to survive. So they have our senses and more : echolocation, infrared vision, magnetic sense. Radar, compasses, and infrared detectors are all man-made contraptions that enable humans to stretch beyond our natural senses. Other animals have these inbuilt in their bodies.
The flow of molten material in the earth’s core and the flow of ions in the atmosphere generate a magnetic field that surrounds the earth. Amazingly, animals can sense this magnetic field. We need compasses to help us navigate by detecting the magnetic north. Chitons, honey bees, sharks, sea turtles, rays, homing pigeons, bats, dolphins, migratory birds, whales, bass, tuna, mosquitoes, butterflies, mollusks, trout, salmon, frogs, salamanders, termites, lobsters, mole rats, even some bacteria have an inbuilt magnetic sense that enables them to go long distances in the correct direction.
Researchers have found that all these animals have deposits of magnetite in their nervous systems. Magnetite is a type of iron oxide with natural magnetic properties. In fact, it is the most magnetic naturally occurring mineral on Earth and was once used in compasses, under the name lodestone, by mariners from the twelfth century. Magnetite crystals, align themselves with magnetic fields and act like microscopic compass needles. What is magnetic information and how do animals use it to navigate?
The liquid core of the Earth gives off a magnetic field. Energy goes from the Earth’s South Pole and reenters the Earth at the North Pole . Evidence suggests that animals can navigate by detecting the strength of the magnetic field and the angle at which the field meets the earth which is distinct for each spot on the globe. Scientists have suggested that as we have specialized cells to see and smell, animals have specialized nerve cells called “magnetoreceptors” , an internal compass that is hooked up to the body such that the “needle” of the compass moves with the changing magnetic field and triggers other cells that help the animal sense where to direct its movements. Studies from rainbow trout showed that brain areas that are sensitive to the magnetic field have nerve cells that contain magnetite.
True navigation is when an animal returns to a place without using landmarks from the destination and the journey. Normal navigation for the human means relying on visual landmarks. If you’ve traveled the route many times, you may even be able to do it blindfolded by using non-visual landmarks, such as smell, sound, remembering distances and knowing when to turn. Studies have shown that in addition to using those types of landmarks, some animals use the earth’s magnetic field to navigate with an internal compass that enables them to sense direction.
Migratory birds use magnetic clues to find their way south in fall and north in spring. This was demonstrated in 1966 by Wolfgang Wiltschko of the University of Frankfurt, Germany for the first time. He showed that even when migratory birds are caged, when the time comes for migration they faced the direction they want to migrate to, e.g., south in autumn. If the magnetic field is changed, so that magnetic south appeared in a different direction, birds assembled in this new direction.
At the age of 6 weeks, pigeons who have a lot of magnetite in their beaks adopt a home loft that remains permanent throughout their lives. Pigeons use different landmarks to return to the home loft, including sight and smell. But magnetic information seems to be the strongest guide. When scientists disrupted the magnetic field by glueing magnets to the back of their necks, pigeons could not find their way back to their lofts.
Loggerhead sea turtles are born on the Florida beaches. After the turtles hatch, they crawl across the beach out into the ocean. Hatchlings swim along the east coast of the United States and then swim northeast with the Gulf Stream. They complete the circle within the Atlantic Ocean, and then they return to the same beach to nest. The return trip takes them across thousands of miles in the open ocean. Olive Ridley turtles are born on the coast of Orissa and Andhra and do the same in the Indian/Pacific oceans, coming back to the exactly the same place 30 years later to lay their eggs. Magnetic information becomes important once they are in the open sea. This has been studied in the laboratory by observing the swim direction of hatchlings in a small pool. When the magnetic field surrounding the swimming hatchlings is reversed, the hatchlings change their swim direction
Spiny lobsters live in caves, come out to forage for food and then return to the same cave. They also have an annual migration. Even when they have been placed miles away in a new place by experimenters, they have been found to return to their cave.
Certain types of bacteria have bodies of magnetite. There is no sensible reason unless the magnetic properties of this element were useful in some way. Biologists think that bacteria(e.g., Magnetospirillum magnetotacticum) may use these internal compasses to sense where ‘down’ is so that they can find food.
The bobolink has the longest migratory path, more than 7000 miles, of any songbird in the West. A study, conducted by Robert Beason of SUNY- Geneseo, validates that magnetite in the bird’s brain functions as part of the bird’s compass. Beason took migrating bobolinks and treated them with a strong magnetic pulse which reversed the polarity of the magnetite in their bodies. Before the magnetic pulse, the birds hopped toward the southeast, which is their normal migratory direction in the fall. After the treatment, the birds changed direction and hopped northward. This is what would be expected if the compass sense relied on magnetite.
In 1983, Baker, Mather and Kennaugh of the University of Manchester discovered “magnetic bones in human sinuses,” a tiny, shiny crystal of magnetite in the ethmoid bone located between the eyes, just behind the nose. But studies show that humans have no capacity for direction finding by magneto-reception. However since magnetite absorbs microwave radiation and reacts to static and low-frequency fields, this means that fields from power lines, cellular telephones, computers and monitors could be affecting us right now. And the people who say that migratory birds are dying off because of the cell phone towers may well be right.