Non-Visual Cues – Magnetic Field
The Earth’s geomagnetic
varies in 3 ways that
can convey information
on position and
orientation:
1) Polarity
2) Intensity
3) Inclination
(dip) Angle
Non-Visual Cues – Magnetic Field
Non-Visual Cues – Magnetic Field
Inclination angle varies regularly with latitude.
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Non-Visual Cues – Magnetic Field
Field intensity varies with latitude.
Non-Visual Cues – Magnetic Field
Declination is the difference (in degrees)
between a geographic and magnetic north.
Non-Visual Cues – Magnetic Field
Magnetic senses and/or magnetic orientation
have been demonstrated in:
Magnetotactic bacteria – Magnetospirillum sp.
Magnetite crystals
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Non-Visual Cues – Magnetic Field
Magnetic senses and/or magnetic orientation
have been demonstrated in:
Magnetotactic bacteria – Magnetospirillum sp.
Non-Visual Cues – Magnetic Field
Magnetic senses and/or magnetic orientation
have been demonstrated in:
Invertebrates – flatworms, annelids,
molluscs, arthropods.
Vertebrates – fish, amphibians, reptiles
birds, mammals (including
humans).
Non-Visual Cues – Magnetic Field
The inclination compass.
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Non-Visual Cues – Magnetic Field
Exposure to the horizontal field at the equator
causes a reversal of preferred inclination
angle of migrant birds.
Non-Visual Cues – Magnetic Field
Exposure to the horizontal field at the equator
causes a reversal of preferred inclination
angle of migrant birds.
Non-Visual Cues – Magnetic Field
Polarity compasses are uncommon, but have
been documented in some species.
e.g. spiny lobsters.
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Non-Visual Cues – Magnetic Field
In a polarity compass reversal of the
vertical field component has no effect.
Lohmann et al. 1995. JEB 198:2041-2048.
Non-Visual Cues – Magnetic Field
Field intensity can indicate latitude and
alter behaviour (e.g. fattening in a transSaharan migrant bird)
Non-Visual Cues – Magnetic Field
Local hills or valleys of field intensity
are associated with whale strandings.
Pilot
whale
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Magnetic Senses
There are three proposed mechanisms for
magnetic sensing.
1) Electroinduction
2) Magnetite based
3) Photoreceptor based
Electroinduction
Elasmobranchs that can sense electric fields
may be able to use the same ampullae of
Lorenzini to sense magnetic fields.
Magnetite Based Magnetoreception
Magnetite (Fe3O4) has been detected in a
wide variety of organisms.
Single domain (SD) magnetite crystals are
50 – 70 nm in size, and can hold a permanent magnetic moment.
Super paramagnetic (SP) magnetite
particles are < 50 nm and can only have
a magnetic moment in the presence of an
external magnetic field.
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Magnetite Based Magnetoreception
Magnetotactic bacteria have chains of
SD magnetite.
Magnetite Based Magnetoreception
Studies of rainbow trout (Onchorhyrhus
mykiss) provide the first evidence of a
magnetoreceptor.
Walker et al. 1997. Nature 390:371-376.
Magnetite Based Magnetoreception
Receptor cells are localized in the lamina
propria, deep to the epithelium.
Olfactory lamellae
Walker et al. 1997. Nature 390:371-376.
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Magnetite Based Magnetoreception
The receptor cell is multilobed and has a
1 um chain of SD magnetite particles. It
communicates to the brain via the trigeminal
nerve.
Diebel et al. 2000. Nature 406:299-302.
Magnetite Based Magnetoreception
Magnetic force microscopy demonstrates
the response of the magnetic sensor to
magnetic field alterations.
Diebel et al. 2000. Nature 406:299-302.
Magnetite Based Magnetoreception
SP magnetite receptors have recently
been described under the skin of the
upper margins of the pigeon beak.
Fleissner et al. 2003. J. Comp. Neur. 458:350-360.
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Magnetite Based Magnetoreception
Fleissner et al. 2003. J. Comp. Neur. 458:350-360.
Magnetite Based Magnetoreception
The receptor has 1 nm SP magnetite spherules
located close to the membrane, and associated
iron phosphate platelets that may help
amplify the magnetic field variations.
Fleissner et al. 2003. J. Comp. Neur. 458:350-360.
Magnetite Based Magnetoreception
The SP
magnetite is
localized in a fibrous
cup that is associated
with the membrane
and may trigger
depolarization
by a mechanical
mechanism.
Fleissner et al. 2003. J. Comp. Neur. 458:350-360.
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Photoreceptor Based Magnetoreception
There is ample evidence from a variety of
species (newts, flies, birds) that magnetic
orientation can be
impaired in the dark
or by long
wavelength light.
Deutschlander et al. 1999. JEB 202:891-908.
Photoreceptor Based Magnetoreception
The proposed mechanism involves excitation
of a photopigment that forms radical pairs
of electrons that respond to magnetic
fields.
Inclination compass only.
Localized to eyes and/or pineal (amphibians).
The leading candidate is cryptochome
protein which contains FADH.
Photoreceptor Based Magnetoreception
It is hypothesized that two photoreceptor
systems that are
sensitive to either
short or long
wavelength light
are responsible.
One is excitatory
and the other is
inhibitory.
Deutschlander et al. 1999. JEB 202:891-908.
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Photoreceptor Based Magnetoreception
Under intermediate wavelength they cancel
out. Under full
spectrum light
one system
dominates and the
animal orients to
the axis of the
field that gives
the strongest
response.
Deutschlander et al. 1999. JEB 202:891-908.
Measuring the Magnetic Field
Birds use head scans to measure the strength
and direction of the magnetic field.
Mouritsen et al. 2004. Curr. Biol. 14:1946-1949.
Coordination of Orientation
Marine turtle hatchlings use sky brightness,
beach slope, wave direction, and magnetic
field inclination and intensity.
Lohmann and Lohmann 1998. JAB 29:585-596.
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Coordination of Orientation
Magnetic field inclination angle is used to
keep turtles in the Atlantic gyre.
Lohmann and Lohmann 1998. JAB 29:585-596.
Coordination of Orientation
Magnetic field intensity is also an important
Cue that is used to keep turtles in the
Atlantic gyre.
Lohmann and Lohmann 1998. JAB 29:585-596.
Coordination of Orientation
Turtles and other pelagic species could
Potentially use a bicoordinate system of
inclinations and intensities to navigate.
Equal Inclination
Angle
Equal Intensity
Lohmann and Lohmann 1998. JAB 29:585-596.
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Coordination of Orientation
Prior to migration birds use celestial cues to
calibrate the magnetic compass.
In juveniles the stellar rotation sense is
paramount and without magnetic field
birds orient directly away from the
rotation axis.
Magnetic information is corrected for
declination to produce correct population
specific orientation.
Coordination of Orientation
During the migration season it’s controversial!
Some evidence suggests that the magnetic
compass is paramount and is used to recalibrate celestial compasses (e.g. Wiltschko
et al 1998. JAB 29:606-617).
Recent reanalysis of data indicates that if
birds can see the entire sunset to the
horizon, that the magnetic compass is
recalibrated using skylight polarization
(e.g. Muheim et al 2006. JEB 209:2-17).
Coordination of Orientation
Nightly recalibration of the magnetic compass
using twilight cues
is supported by
recent empirical
evidence.
Swainson’s thrush
Cochran et al. 2004. Science 304:405-408.
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Navigation
How animals convert orientation information
into navigation is poorly understood.
Modifying triggers
Vector navigation – combining orientation
preferences with a time program.
Navigation
Natural selection should favour the shortest
path. Orthodromes or great circle routes
are shortest but
require constant
course adjustment
if there is any
east-west component.
Loxodromes or
rhumblines have a
constant compass
bearing but are
longer.
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