Biomagnetism

Honey bees that have discovered a suitable flower patch perform a dance pattern upon returning to their hive to describe the new flower source location to their companions. Dissections show that honey bees have naturally formed

FIGURE 2.17 ► Magnetite crystals grown by aquatic bacteria were discovered in Massachusetts by R. Blakemore. Inside these pond bacteria, the crystals form long magnets with the north pole toward their front to orient their swimming direction.

magnetic material in their stomachs. Biological researchers, using a large loop antenna, artificially modified the magnetic field direction in the region of a hive and found that the returning bees make a corresponding flight change in their dance pattern. The scientists concluded that the bees use the local magnetic field as one of their sources of orientation.

In other studies, pond bacteria have been discovered that naturally grow magnetic crystals aligned with the long axis of their body (Figure 2.17). These bacteria orient their swimming with respect to the Earth's local field direction. The well-known sea travelers whales, dolphins, green turtles, blue marlins, and tunas have brains containing magnetic crystals suspended by fine fibers that may be part of their navigation sensing mechanism. Salmon have been shown to alter their swimming direction when scientists artificially modified the local magnetic field directions. Because of evidence that homing pigeons use the Earth's magnetic field as part of their navigation sense, organized rallies for homing pigeon races are cancelled when there is a forecast of natural geomagnetic field disturbances (such as Figure 1.13).

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