Animals possess certain mechanisms other than their senses that help them navigate through their environment. This is an adaptation taken by many animals whose other senses are limited by the environment they are in. Having another way to move around and survive in such environments is definitely contributory to a species' success and survival. One such sensory mechanism is a system controlled and shaped by magnetism.
Sensory mechanisms governed and guided by the magnetic field of the planet are perhaps the one of the most underrated sensory systems there are. Many animals have been using this system to survive throughout their natural histories. Many of their natural biological tendencies are created, shaped and guided by the earth's magnetism. Such animals have mechanism that allows magnetic force to govern their travelling behaviour across landscapes, feeding mechanisms and other biological signals that trigger the start or the end of biological processes.
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These sensory mechanisms are without any doubt, highly important to these animals, and this importance spawned many studies about it. Many scientists have tried to identify and pinpoint the exact biological mechanism that these animals have, but unfortunately, most of them are met with vagueness. Many hypotheses have been developed for the mechanisms of this amazing sensory system, but to this day, there exists no single explanation to solve this mystery. Hypotheses continue to pile up as investigative data continue to pour in still.
This paper will serve as a compilation of the hypotheses and explanations given by scientific papers, journals and dissertations on the magnetic sensory mechanism of different species of animals. The author will cite, explain these mechanisms in light of their biological mechanisms and their purposes for the organism that possesses them.
II. Magnetic Sensory Systems
There exist several theories on how animals are guided by the magnetic field of the earth. One of the oldest theories that tried to explain the mechanism is the theory of magnetoreception via biological magnetite. Biological magnetite and other inorganic components are found in certain cells and they may even have the ability to store information of previous flights (Bokkon and Salari, 110). This theory states that certain animal cells contain magnetite and these particles manage to align themselves to the earth's magnetic field and use this to navigate. Many theories that have emerged to explain magnetic sensory systems contain principles that are from the old magnetoreception theory and other theories and some new principles that were established from data.
Numerous studies have suggested that magnetic sensory reception is not simply the alignment of particles along a magnetic field (Fleissner, Stahl and Thalau). The animal's response to the earth's magnetic field is coordinated and mediated by complex nervous function (Johnsen and Lohman). Other theories combine other naturally-occuring abiotic factors and principles with the old magnetoreception theory.
Perhaps the most common purpose for magnetic sensory systems for animals is the fact that these systems help them determine their exact migration route. Animals, especially those that migrate need intricate navigation systems to help them get to their destination and return to their place of origin. This system is very essential as their very life cycle depends on it. Their feeding and reproduction pattern depends on this migration path, and without an exact migratory navigation system, the species' survival would be compromised entirely. This purpose is most common in birds, turtles, fish and other migratory animals.
Magnetic sensing occurs in a very intricate system of physicochemical interactions between biological organs, cells and even chemicals.
III. Magnetic Sensory Systems in Certain Animals
Perhaps the species where magnetic sensory system was apparent were those that migrated. Migratory birds and some domesticated ones are among the species that are most studied for magnetic sensory mechanisms because of their amazing ability to come back to their region of origin without difficulty.
One of the species of birds that most frequently studied when it comes to magnetic sensory mechanisms is the homing pigeons. These birds are known to have an excellent navigational mechanism that enables them to leave and come back to their location of origin. There many studies that try to shed some light on the magnetic sensory systems of homing pigeons as their magnetic sensory mechanism remains to have no concrete biological theory that can explain it fully.
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Many scientists propose that there is no magnetic sensory systems as these birds only navigate by detecting light intensity. In a way, they only know their path because they can tell where the sun sets and where it rises. Another hypothesis by intelligence scientists is that they learn where to fly and come back because of conditioning and learning. But these hypotheses are not consistent with the fact that cellular components of many animals, including birds, contain inorganic iron compounds, these components are intricately arranged so that it forms a complex biological navigation network and that even inexperienced pigeons are able to fly and return.
A studies by Fleissner et. al. and Wild et. al revealed much information on the possible physiological basis of the homing pigeons' magnetic sensory system.
Fleissner and his colleagues revealed that there are structures in the upper beak of the pigeons that contain iron through X-ray analysis. Histological sections of their dendrites, specifically those of the trigeminal nerve contain iron particles. This system apparently is arranged in such a delicate network of neurons. Iron particles contained in such cells are arranged in such a way that it follows a specific three-dimensional structure that is bilaterally symmetrical. The study also discovered that maghemite, not magnetite, was the predominant magnetic inorganic compound in the birds. These two compounds which the birds' body are detected in such quantities that are enough for it to create for a bird a three dimensional vector that would allow the bird to pinpoint its location by proper alignment with the earth's field. The study suggests that the birds' ability to locate its position is independent of other factors like light intensity.
Wild et. al supported Fleissner et al.'s finding regarding the location of magnetoreceptors in the pigeons' bodies. Via anatomical experiments, they found that the iron-containing receptors found in the upper beak of homing pigeons are connected with the ophthalmic branch of the trigeminal nerve. But other than this finding, they also discovered that inexperienced pigeons do not find it difficult to navigate through their course. This study also discovered a rather radical finding when it comes to magnetic sensory systems as it may have forged the link between olfaction and magnetic sensing. The experimenters severed the ophthalmic and olfactory branch of the trigeminal nerve of 2 groups of trained adult homing pigeons, respectively. The ones with severed ophthalmic branches did not display any reduction on homing capabilities, but those with severed olfactory branches displayed a large drop in their navigational skills. Furthermore, severed olfactory branches in young pigeons totally upset their navigational ability. Therefore, magnetic sensory capabilities are also hardwired to other sensory systems as well, and in this case, the sense of smell.
Another study that related a physical sense with magnetic sensing is the study by Hogben et al. which related the organ of sight with magnetic navigation in birds. According to the study, the eyes itself may influence magnetic sensing in the sense that there are many chemical reactions in the retina that may influence how the birds receive magnetic signals. Oscillating magnetic fields affect the birds severely as they experience disorientation and deviation from a flight path. Certain chemical products from the chemical reactions in the retina influence the way that these oscillations are received and read by the bird.
Also, birds that are released from a certain timezone can go to another area of a different daylight cycle and still come back to their place of origin. If the light detection hypothesis alone was right, then these birds wouldn't have the capability to return considering that the differing light cycles would have thrown the birds' navigational system off. Gagliardo et. al studied the ability of the birds to correct their path even in the event that light intensity and brightness varies throughout their course. The ability of birds to correct their path despite the changes in time and light intensity, according to the study, can be attributed to the fact that the homing pigeons have an internal compass that allowed them to navigate independent of light cycles. The experiment proved this by comparing the deviation of two groups of clock-shifted pigeons---one carrying magnets and one without---from their path of flight. The group that deviated from the path the most are the birds that carried magnets, and the results are consistent with the hypothesis that homing pigeons do navigate with a magnetic sensory system.
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But there is much debate that exists if whether or not light does affect the capability of birds to navigate using their magnetic sensory systems. Another study that relates light intensity with magnetic navigation is the study by Sandberg and Petterson. They studied the migration patterns and effects of light intensity and seasons to snow buntings (Plectrophenax nivalis), a migratory bird. Tests were conducted to see the effects of changing light intensity that comes with the shift of seasons. Snow buntings were subjected to conditions that are meant to mimic conditions during spring and autumn and their direction of flight followed the artificial magnetic field applied in the area. The birds showed no change in flight pattern although the conditions of the sky were absent of any signs of the season. Only the artificial magnetic field remained, and this is what they followed. But some tests that involved clear skies and an artificial vertical magnetic field still suggests the interaction of light intensity with flight patterns. The birds flew an axial orientation, which was identical to the flight directionality of the birds during both seasons.
The phenomenon widely observed wherein other light sources disorient the path of certain birds that fly and navigate using magnetic sensory systems open doors to more questions regarding the interaction of magnetic systems with light systems.
The study by Poot et. al shows that light may affect the navigational capabilities of birds, especially the migratory ones, especially those that are nocturnal. The study focused on learning the effects of artificial light sources to the flight paths of the birds. It was not uncommon to see birds dying in their course because they encounter artificial means of lighting along the way. The study, which aimed to provide information for the development of bird-friendly lighting equipment, found out that the birds' magnetic navigational system is affected by the wavelength of light. Magnetic navigation can only be possible in the presence of lights with shorter wavelengths and those with longer wavelengths tend to disrupt it.
Perhaps there exists a balance between different factors that govern birds' migration patterns. Celestial clues like light intensity and seasons may work hand in hand with the earth's magnetic field to provide an excellent system to guide the flight and migration of birds (Wiltschko and Wiltschko).
Turtles are known for their habit of swimming over very long distances to feed, mate and reproduce. This habit must be widely supported by an innate navigational system, whose nature is vague. Whether or not these animals contain mechanisms that are governed by light, sea currents or even magnetism is still a matter of much debate.
Sea turtles have been recently the subject of a study by Luschi et al. when they tried to study these organisms' ability to navigate through their environment. The study involved the researchers attaching magnets to the heads of young green turtles (Chelonia mydas) and leaving the rest untreated. The turtles were released to open sea, and those with magnets displayed a significant course change with regards to distance and length as compared to their untreated counterparts. Magnetically-treated turtles also displayed a significant reduction in their ability to sense the direction of ocean currents.
Magnetic navigational systems is one of nature's greatest mysteries and yet one of nature's most important ones. The studies that tackle magnetic sensory systems have discovered lots of new things that may shed some light regarding this mechanism's true nature and biological basis.
While many scientists try to figure out and pinpoint the real mechanism of these systems, many animals are extremely dependent on this system for their continued survival. Seeking to understand this mechanism can help us shape our activities in such a way that our activities do not take away the natural balance of this natural balance.
V. Works Cited
Bokkon, Istvan and Vahid Salari. "Information storing by biomagnetites." Journal of Biological Physics (2009): 109-120.
Fleissner, Gerta, et al. "A novel concept of Fe-mineral-based magnetoreception: histological and physicochemical data from the upper beak of homing pigeons." Naturwissenschaften (2007): 631-642.
Gagliardo, Anna, et al. "Re-orientation in clock-shifted homing pigeons subjected to a magnetic disturbance: a study with GPS data loggers." Behavioral Ecology and Sociobiology (2009): 289-296.
Hogben, Hannah, et al. "Possible involvement of superoxide and dioxygen with cryptochrome in avian magnetoreception: Origin of Zeeman resonances observed by in vivo EPR." Chemical Physics Letters (2009): 118-122.
Johnsen, S. and KJ Lohmann. "The physics and neurobiology of magnetoreception." Natural Review of Neuroscience (2005): 703-712.
Luschi, Paolo, et al. "Marine Turtles Use Geomagnetic Cues during Open-Sea Homing." Current Biology (2007): 126-133.
Poot, Hanneke, et al. "Green Light for Nocturnally Migrating Birds." Ecology and Society (2008).
Sandberg, Roland and Jan Petterson. "Magnetic Orientation of Snow Buntings (Plectrophenax nivalis), a specied breeding in the high Arctic: Passage Migration Through Temperate Zone Areas." The Journal of Experimental Biology (1996): 1899-1905.
Wild, Martin, et al. "Navigational abilities of adult and experienced homing pigeons deprived of olfactory or trigeminally mediated magnetic information." The Journal of Experimental Biology (2009): 3119-3124.
Wiltschko, W. and R. Wiltschko. Magnetic orientation and celestial cues in migratory orientation. Basel, Switzerland: Birkhiuser Verlag, 1990.