Behavioral Evidence for Magnetic Sensing

Many animal ethologists studying migration have found strong direct evidence that the Earth's magnetic field is used in conjunction with other sensory modalities for guidance. Unfortunately, the location of the neural receptors responsible for static magnetic field sensing are known in few cases (see Section 2.4.2). Some magne-toreceptors are thought to use cells containing microscopic magnetic iron oxide (magnetite) crystals that interact with the Earth's magnetic field to create forces that are somehow transduced to nerve spikes. Other workers have implicated elements of the visual system in magnetoreception. The bottom line is that at this time there is no hard evidence for a specific mechanism for magnetoreception, even in Tritonia Pd5 neurons. Much basic neurophysiology and neuroanatomy has yet to be done.

The list of animals thought to use the Earth's magnetic field for guidance is impressive. These include but are not limited to

Vertebrates, including homing pigeons, bobolinks, sea turtles, rainbow trout, sockeye salmon, yellowfin tuna

Arthropods, including spiny lobsters, honeybees, the mealworm beetle, Tene-brio molitor

Mollusks including the sea slug, Tritonia diomedea, and the snail, Lymnaea

Bacteria exhibiting magnetotaxis have also been found.

Homing pigeons have been widely studied because of their ability to find their home loft under amazing conditions of distance and weather. Some of the earliest evidence that these birds use the Earth's magnetic field for navigation was obtained by Keeton (1971; 1974), who glued small, permanent magnets onto pigeon heads to change the magnetic field vectors experienced by the birds. Homing behavioral studies showed that birds with magnets had random "vanishing bearings," while birds carrying brass control weights had vanishing bearings clustered around the direction of their loft. (The vanishing bearing is the direction the pigeon flies out of sight on the horizon after release.) Keeton found that the birds' magnetic compass sense interacted with their "sun compass." In another important set of pigeon experiments, Walcott and Green (1974) equipped their birds with solenoidal electromagnets on their heads to reverse the N-S polarity of the vertical component of Be. Birds with north up often flew 180° away from home under overcast conditions, but could navigate normally when the sun was out. Birds having south up were able to navigate normally under both overcast and sunny conditions. In another series of experiments, Walcott (1977) applied graded vertical components of 0, 0.1, 0.3, and 0.6 Gauss, N or S up, with the coils worn by the pigeons. He found that even on sunny days, the artificial field caused an increase in the scatter of vanishing bearings. These results suggested that the outputs of both the sun and magnetic compass senses were functionally integrated by the bird, and did not act autonomously. The sensitivity of pigeons to the magnitude of the vertical component of Be suggests that they use the complete Be vector for navigation, not just the horizontal component.

Another interesting experiment on bird magnetic sensing was described by Beason et al. (1995), working with the European bobolink, a migratory bird. These workers sought to verify that the birds' transduction of Be depended on sensors using magnetite crystals (magnetic iron oxide, Fe3O4). A bird's head was placed inside a solenoid, and a 5-ms, 0.5-T pulse was applied to strongly and permanently magnetize magnetite crystals assumed to be in the head and associated with the bird's migratory compass. The pulse was brief to prevent the magnetite crystal(s) from physically rotating to align themselves with the strong external field. Magnetization was done in one of three bird orientations: north anterior, north posterior, and north up. North anterior means that if the beak were iron, it would attract the south end of a compass, etc. Space does not permit a detailed description of Beason et al.'s results; however, in summary, the head magnetization did alter the birds' ability to orient themselves correctly for migration. Birds that were magnetized N-anterior had a significantly different mean heading from that of birds magnetized S-anterior, and each group differed significantly from its control. N-up magnetized birds had two mean headings, 180° apart.

The saga of the spiny lobsters is worthy of consideration here. Lohmann et al. (1995) tested spiny lobsters (Panulirus argus) for their ability to guide their migration or homing movements by using the Earth's magnetic field. An underwater magnetic coil system was constructed with which the experimenters could independently and exactly reverse the vertical or horizontal components of Be by passing appropriate dc currents through the coils. The lobsters were tethered so they would remain in the effective field of the coils; their direction of movement under the various field conditions was noted. Control lobsters (coils unenergized) marched in diverse directions, but were consistent in those directions. When the vertical component of Be was reversed by coil, there was little change in individual march directions. When the horizontal component of Be was reversed by coil, after about 5 min, nearly all of the lobsters were marching in directions 180° from their control directions. This experiment demonstrated that spiny lobsters use the horizontal component of Be for guidance.

In 1984, Lohmann using a sensitive SQUID magnetometer reported that the spiny lobster had four sites showing natural remnant magnetization (NRM), indicating the presence of ferromagnetic material in its body. Three of the four sites were located in the cephalothorax; the NRM of the one in the center was directed to the right, the NRM of one in the left posterior cephalothorax was directed posteriorly, and the NRM of the site in the right posterior cephalothorax was directed anteriorly. A fourth site in the telson-uropods region had NRM directed to the animal's left. The most likely material providing NRM is magnetite crystals, which may lie in a yet-to-be-discovered magnetosensor cell or cell complex.

Many other interesting examples of animal directional responses to the Earth's magnetic field can be found in the literature. Of the animals examined, their responses can be put in two broad categories: those that only use the horizontal component of Be (e.g., the spiny lobster), and others that use both the vertical and horizontal components (entire) of Be (e.g., pigeons, sea turtles).

2.4.2 The Putative Magnetoreceptor Neurons of Tritonia

Tritonia diomedea is a large, North Pacific nudibranch mollusk (sea slug) that has been shown not only to respond behaviorally to induced magnetic fields, but to have two individually identifiable neurons believed to be magnetoreceptors (Cain et al., 1999). The neurons are located in the symmetrical left and right pedal ganglia, and have been designated LPd5 and RPd5, respectively. They have enormous cell bodies, ~500 |im in diameter, and when stimulated by action potentials, their terminal branches secrete neuropeptides that increase the beat frequency of pedal cilia on the left and right sides of the animal, respectively, producing turning.

If all central nerves are intact, Pd5 neurons responded with an increased rate of firing when the horizontal component of the Earth's magnetic field was rotated 60° clockwise The increase in firing rate was delayed for 6 to 16 min following the directional change of B (Lohmann et al., 1991). Curiously, Pd5 neurons did not respond if recorded from an isolated Tritonia brain. However, when all nerves were cut except Pedal 2 and 3, response was obtained. These perplexing results suggests three possibilities: (1) nerves P2 and/or P3 transmit spikes from magnetosensors located in the animal's peripheral tissues; or (2) P2 and/or P3 are the magnetosensors, or (3) cutting the axon of Pd5 keeps it from spiking in response to magnetic stimuli in the isolated brain.

No one as yet has examined the ultrastructure of Pd5 giant neurons, or determined whether or not they contain magnetite. As will be seen below, magnetic fields can theoretically be sensed in several ways other than by the forces acting on magnetized magnetite particles.

2.4.3 Models for Magnetoreceptors

The author can think of four reasonable hypothetical physical models for animal magnetosensor systems. Three of the models make use of the law of physics governing the force acting on a charged particle moving in a magnetic field.

2.4.3.1 The Magnetic Compass Analog

This type is based on a hypothetical, mechanosensory neuron containing microscopic, domain-sized particles of magnetic iron oxide (magnetite). To be maximally effective, the magnetite particles must have a permanent magnetic moment, i.e., be magnetized. Similar to the operation of a magnetic compass, the south pole of the particle aggregate will experience a magnetic force or torque trying to align it with the (north) magnetic vector of the Earth; conversely, the north pole of the magnetite will be attracted to the south magnetic vector. (Early magnetic compasses used a piece of magnetized lodestone, magnetite pivoted on a pin or attached to a wooden float and free to turn in a water-filled bowl.) In the case of cells with magnetized magnetite particles inside them, the tiny internal forces or torques created by non-alignment with the Earth's magnetic field would have to be coupled to a spikegenerating mechanism that can signal by its frequency the degree of nonalignment with Be. If the magnetite crystals are attached to myosin filaments that, in turn, are attached to certain ion-gating proteins in the magnetoreceptor cell's membrane, then the microscopic forces or torques produced by misalignment of the crystals with Be could provide the necessary coupling mechanism.

How many sensors would be required to give unambiguous information on heading in a 360° circle about magnetic north? (See Section 2.6 for how the outputs of four sensors might be resolved to give absolute heading.)

Three key questions must be answered to accept the hypothetical magnetite-containing magnetosensor: (a) How do the magnetite crystals (insoluble in seawater) get inside the hypothetical magnetoreceptor neurons? Is it biogenic, i.e., made by the cell? (b) How do they get permanently magnetized? (c) How do forces and torques on the magnetized magnetite crystals (single domains, or small aggregates) produce changes in receptor resting potential leading to spikes? That is, what is the transducer coupling mechanism?

2.4.3.2 A Hall Effect Analog

Another possible mechanism for a neural magnetoreceptor might be based on the Hall effect. In this scenario, an animal has a cell with a special membrane Figure 2.4-1). Inside the membrane, or attached to its surface, a thin layer of densely packed mac-romolecules actively transports electrons parallel to the membrane surface at an average drift velocity, vn, in the -x direction. The By component of Be that lies perpendicular to vn causes a perpendicular Lorentz force, FL, to act on each moving electron according to the vector equation:

FIGURE 2.4-1 Diagram of a hypothetical Hall-effect-based biological magnetosensor membrane. Electrons are transported at velocity vn in the -x direction, giving a net current density Jx in the +x direction. vn is ± to the Earth's magnetic field, Be. A Lorentz force, FL, acts at right angles to vn and Be and pushes the moving electrons to the outside of the membrane, depolarizing it in that region. This model requires a yet-to-be-discovered array of electron transfer molecules embedded in the membrane.

FIGURE 2.4-1 Diagram of a hypothetical Hall-effect-based biological magnetosensor membrane. Electrons are transported at velocity vn in the -x direction, giving a net current density Jx in the +x direction. vn is ± to the Earth's magnetic field, Be. A Lorentz force, FL, acts at right angles to vn and Be and pushes the moving electrons to the outside of the membrane, depolarizing it in that region. This model requires a yet-to-be-discovered array of electron transfer molecules embedded in the membrane.

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