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Magnetoception is a type of navigation used by some animals. It is navigation that uses electromagnetic fields, and has been observed in bacteria. It is useful because scientists might be able to come up with different types of navigation.
Magnetoception is a type of navigation used by some animals. It is navigation that uses electromagnetic fields, and has been observed in bacteria. It is useful because scientists might be able to come up with different types of navigation.
Many factors contribute to bird navigation during long distance migrations, including learning, instinct, their solar compass (perception of sun orientation and day length), landmarks, olfactory cues, and a curious ability still being researched called magnetoception (or magnetoreception) which enables them to detect the orientation of the earths magnetic field. The mechanism is not fully understood but some tests indicate that homing pigeons, for example, might exploit magnetite contained in small deposits around the beak. Another theory suggests that a complex protein called cryptochrome might affect bird vision by altering the sensitivity of retinal neurons such that, in effect, birds might be able to 'see' the earth's magnetic field.
Orientation and navigation
Navigation is based on a variety of senses. Many birds have been shown to use a sun compass. Using the sun for direction involves the need for making compensation based on the time. Navigation has also been shown to be based on a combination of other abilities including the ability to detect magnetic fields (magnetoception), use visual landmarks as well as olfactory cues.
Long distance migrants are believed to disperse as young birds and form attachments to potential breeding sites and to favourite wintering sites. Once the site attachment is made they show high site-fidelity, visiting the same wintering sites year after year.
The ability of birds to navigate during migrations cannot be fully explained by endogenous programming, even with the help of responses to environmental cues. The ability to successfully perform long-distance migrations can probably only be fully explained with an accounting for the cognitive ability of the birds to recognize habitats and form mental maps. Satellite tracking of day migrating raptors such as Ospreys and Honey Buzzards has shown that older individuals are better at making corrections for wind drift.
As the circannual patterns indicate, there is a strong genetic component to migration in terms of timing and route, but this may be modified by environmental influences. An interesting example where a change of migration route has occurred because of such a geographical barrier is the trend for some Blackcaps in central Europe to migrate west and winter in Britain rather than cross the Alps.
Migratory birds may use two electromagnetic tools to find their destinations: one that is entirely innate and another that relies on experience. A young bird on its first migration flies in the correct direction according to the Earth's magnetic field, but does not know how far the journey will be. It does this through a radical pair mechanism whereby chemical reactions in special photo pigments sensitive to long wavelengths are affected by the field. Note that although this only works during daylight hours, it does not use the position of the sun in any way. At this stage the bird is similar to a boy scout with a compass but no map, until it grows accustomed to the journey and can put its other facilities to use. With experience they learn various landmarks and this "mapping" is done by magnetites in the trigeminal system, which tell the bird how strong the field is. Because birds migrate between northern and southern regions, the magnetic field strengths at different latitudes let it interpret the radical pair mechanism more accurately and let it know when it has reached its destination. More recent research has found a neural connection between the eye and "Cluster N", the part of the forebrain that is active during migrational orientation, suggesting that birds may actually be able to see the magnetic field of the earth.
Vagrancy
Migrating birds can lose their way and occur outside their normal ranges. These can be due to flying past their destinations as in the "spring overshoot" in which birds returning to their breeding areas overshoot and end up further north than intended. A mechanism which can lead to great rarities turning up as vagrants thousands of kilometers out of range is reverse migration, where the genetic programming of young birds fails to work properly. Certain areas, because of their location, have become famous as watch points for migrating birds. Examples are the Point Pelee National Park in Canada, and Spurn in England. Drift migration of birds blown off course by the wind can result in "falls" of large numbers of migrants at coastal sites.
This list begins with those five senses defined by Aristotle and hence probably most familiar
to the original poster.
1. Seeing or vision describes the ability to detect light and interpret it as
"sight". There is disagreement as to whether or not this constitutes one,
two or even three distinct senses. Neuroanatomists generally regard it as
two senses, given that different receptors are responsible for the
perception of colour (the frequency of light) and brightness (the energy of
light). Some argue that the perception of depth also constitutes a sense,
but it is generally regarded that this is really a cognitive (that is,
post-sensory) function derived from having stereoscopic vision (two eyes)
and is not a sensory perception as such.
2. Hearing or audition is the sense of sound perception and results from tiny
hair fibres in the inner ear detecting the motion of atmospheric particles
within (at best) a range of 20 to 20000 Hz. Sound can also be detected as
vibration by tactition. Lower and higher frequencies than can be heard are
detected this way only.
3. Taste or gustation is one of the two "chemical" senses. It is well-known
that there are at least four types of taste "bud" (receptor) and hence, as
should now be expected, there are anatomists who argue that these in fact
constitute four or more different senses, given that each receptor conveys
information to a slightly different region of the brain. The four well-known
receptors detect sweet, salt, sour, and bitter, although
the receptors for sweet and bitter have not been conclusively identified. A
fifth receptor, for a sensation called "umami", was first theorised in 1908
and its existence confirmed in 2000. The umami receptor detects
the amino acid glutamate, a flavor commonly found in meat, and in artificial
flavourings such as monosodium glutamate.
4. Smell or olfaction is the other "chemical" sense. Olfactory neurons differ
from most other neurons in that they die and regenerate on a regular basis.
The remaining senses can be considered types of physical feeling.
5. Tactition is the sense of pressure perception.
6. Thermoception is the sense of heat and the absence of heat (cold). It is
also the first of the group of senses not identified explicitly by
Aristotle. Again there is some disagreement about how many senses this
actually represents--the thermoceptors in the skin are quite different from
the homeostatic thermoceptors which provide feedback on internal body
temperature. How warm or cold something feels does not only depend on
temperature, but also on specific heat capacity and heat conductance; e.g.,
warm metal feels warmer than warm wood, and cold metal feels colder than
cold wood, because metal has a higher thermal conductivity than wood. Wind
feels cold because of the heat withdrawn for evaporation of sweat or other
moisture, and because an isolating layer of warm air around the body blows
away; however, in the case of hot air, wind makes it feel hotter, for a
similar reason as the latter.
7. Nociception is the perception of pain. It can be classified as from one to
three senses, depending on the classification method. The three types of
pain receptors are cutaneous (skin), somatic (joints and bones) and visceral
(body organs).
8. Equilibrioception is the perception of balance and is related to cavities
containing fluid in the inner ear. There is some disagreement as to whether
or not this also includes the sense of "direction" or orientation. However,
as with depth perception earlier, it is generally regarded that "direction"
is a post-sensory cognitive awareness.
9. Proprioception is the perception of body awareness and is a sense that
people rely on enormously, yet are frequently not aware of. More easily
demonstrated than explained, proprioception is the "unconscious" awareness
of where the various regions of the body are located at any one time. (This
can be demonstrated by anyone closing their eyes and waving their hand
around. Assuming proper proprioceptive function, at no time will the person
lose awareness of where the hand actually is, even though it is not being
detected by any of the other senses).
Based on this outline and depending on the chosen method of classification,
somewhere between 9 and 21 human senses have been identified. Additionally
there are some other candidate physiological experiences which may or may
not fall within the above classification, for example the sensory awareness
of hunger and thirst.
This list concludes with known non-human senses.
10. Electroception (or "electroreception"), the most significant of the
non-human senses, is the ability to detect electric fields. Several species
of fish, sharks and rays have evolved the capacity to sense changes in
electric fields in their immediate vicinity. Some fish passively sense
changing nearby electric fields, some generate their own weak, electric
fields and sense the pattern of field potentials over their body surface,
and some use these generating and sensing capacities for social
communication. The mechanisms by which electroceptive fishes construct a
spatial representation from very small differences in field potentials
involve comparisons of spike latencies from different parts of the fish's body.
The only mammal which is known to demonstrate electroception is the platypus.
11. Magnetoception (or "magnetoreception") is the ability to detect fluctuations
in magnetic fields and is most commonly observed in birds. Although there is
no dispute that this sense exists in many avians (it is essential to the
navigational abilities of migratory birds) it is not a well understood
phenomenon.
12. Echolocation is the ability to determine orientation to other objects
through interpretation of reflected sound (like sonar). Bats and dolphins
are noted for this ability, though some other mammals and birds do as well.
It is most often used to navigate through poor lighting conditions or to
identify and track prey. There is presently an uncertainty as to whether
this is simply an extremely developed post-sensory interpretation of
auditory perceptions, or actually constitutes a separate sense. Resolution
of the issue will require brain scans of animals while they actually perform
echolocation, a task which has proved difficult in practice.
