Autumn is here. The leaves on the trees are changing colors. It is one of those seasons where you would like to take a walk outside and forget about raking up those leaves in the front yard. But still, there is the anxiety that we have so much to do, look forward to, or even dread in some cases. We have to take the fans down, take out the woolens from the closets, and start up the heaters in our homes. Some of us even prepare for the next season by getting out our shovels from the storage shed to get ready for the big snowstorm that will occur in winter. While all of us are busy in our day to day "human" schedules, do we ever take a moment to think about what other animals, such as birds, insects and some species of fish, do to prepare for the new season? Where do they go? How do they know when it is time to head south? What is it in their brains that tells them that this is the direction in which to head?
All of these questions are fascinating to many biologists, including neurobiologists. Animals, such as birds, marine mammals, salamanders, fish, and even certain species of insects, rely on numerous environmental cues as well as internal cues in order to be able to know which way to migrate.
Migratory birds are good examples of organisms that need environmental cues in order to "reason" their way around (1). Migratory birds use a combination of methods to navigate. The methods include visual cues, solar navigation, magnetic fields, star navigation, and polarized light (1),(2). There has been much evidence that magnetic fields play an important role in navigation.
Recently, scientists have found that migratory birds not only rely on visual cues that humans can see, such as landmarks and constellations of stars, but also on cues that humans are not able to detect (3). A study done by Wolfgang Wiltschko showed that when European robins were exposed to an altered magnetic field in a laboratory, the birds oriented themselves to the artificial field. Another type of bird that is believed to have a magnetic sense is the homing pigeon (4). In the October 16, 1981 issue of Science, a group of German scientists observed that when homing pigeons had strong magnets glued to their backs, the pigeons were disoriented and flew off randomly (5). The scientists concluded that the earth's magnetic field serves as a primary guidance source for navigation.
The built-in compass of migratory birds, such as the bobolink which has the longest migratory path (more than 7000 miles) of any songbird in the New World, is magnetite (6), (3). Research suggests that cells in the bird's brain contain magnetite, an iron oxide crystal that aligns with magnetic north similar to a compass needle. Scientists suggest that these cells may serve as receptors that send directional information to the brain.
A study, conducted by Robert Beason of SUNY- Geneseo, validates that magnetite functions as part of the bird's compass. Beason took migrating bobolinks and treated them with a strong magnetic pulse. This magnetic pulse reverses 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 (3).
In order to see if there was another way, in which bobolinks sensed magnetic fields, Beason numbed the trigeminal nerve, which is associated with the magnetite cells, with Novocain. The result was the birds went back to their southeast orientation. This suggests that there must be another way for the birds to detect the magnetic fields when input from the magnetite cells are blocked (3),(7).
The other way that birds detect magnetic fields is by their sense of vision (3). Beason and his colleagues have studied how bobolinks respond to different wavelengths of light. When exposed to red light, bobolinks become disoriented. However, when exposed to green, blue or white light, their sense of direction remains the same. With these wavelengths of light, the pigment molecules behave like weak magnets. The visual information is relayed from the bird's eyes to the brain. Therefore, light-sensitive pigments in the birds' eyes serve as magnetic sensors. When there is no green, blue or white light striking the pigment molecules, the bird loses sense of direction (3).
Many animals have the ability to sense the geomagnetic field and utilize it as a source of directional (compass) information. Studies have shown that salamanders and frogs use magnetic fields for orientation when they have to find a way to escape from danger, such as from predators (9). Other animals that have been known to migrate via the detection of the Earth's magnetic field include sparrows, pigeons, bobolinks, yellow fin tuna fish, honeybees, and bacteria (8). Magnetite has been found in the tissues of all these organisms (8).
There are three main ways that animals perceive the geomagnetic field (9). One is by mechanical reception, which is the principle behind a compass needle. In animals, magnetoreceptors such as magnetite are used. A second mechanism found in animals is electric induction, where a movement in a magnetic field will lead to an induced electric field. Studies have shown that Elasmobranch fish have a certain sensory organ that perceives electric fields and magnetic fields (9). The third way animals perceive magnetic fields is through chemical reception.
Other animals do not use magnetic fields to navigate. Sharks are sensitive to electric fields, and this is likely to help them navigate (8). Salmon that migrate use their olfactory system to detect waters in which they had spent their early lives (8). Similarly, insects have a strong sense of smell and an acute chemical sense to help them navigate (8).
While some animals use magnetic fields to help navigate and some do not, all of these animals have reasons to migrate. An environmental cue, such as change in atmospheric temperature or scarcity of food, influences the decision for the animal to migrate. The nervous system is directly linked to migration because migration is part of the life cycle and it depends on internal cues. Metabolic changes, such as increases in deposits of fats and increased food consumption, result from hormones secreted from the pituitary glands of these animals (8).
So, even though migration may seem like an external event, in reality it is one that involves many internal processes. The means by which animals migrate seem less trivial and more complex than the some of the everyday tasks that humans encounter. So, the next time you see a flock of birds flying overhead, just remember the intricacies and complexities behind it all.
2)Physiology of Bird Migration
4) How Do Birds Find Their Way?
5) Bird Navigation Alaska Science Forum ,
6)Peterson's Birds --Bobolinks
9)The Magnetic Sense of Animals