MAGNETORECEPTION IN VERTEBRATE ANIMALS

Magnetoreception in Vertebrate Animals

Magnetoreception in Vertebrate Animals

Introduction

Diverse vertebrate animals can sense the earth's magnetic field, but little is known about the physiological mechanisms that underlie this sensory ability. Three major hypotheses of magneticfield detection have been proposed (Purcell, 2005). Electrosensitive marine fish might sense the geomagnetic field through electromagnetic induction, although definitive evidence that such fish actually do so has not yet been obtained. Studies with other vertebrates have provided evidence consistent with two different mechanisms: biogenic magnetite and chemical reactions that are modulated by magnetic fields. Despite recent progress, however, primary magnetoreceptors have not yet been identified unambiguously in any animal (Lohmann & Lohmann, 1993).

Discussion

Behavioral Experiments have demonstrated that diverse animals, including representatives of all five vertebrate classes, can sense the earth's magnetic field and use it as an orientation cue while migrating, homing or moving around their habitat. Relatively little is known, however, about the physiological mechanisms that underlie this sensory ability (Beason & Semm, 2004). Theoretical work on mechanisms of magnetoreception has progressed much more rapidly than have empirical physiological studies. Numerous hypotheses have been proposed, including transduction processes mediated by electromagnetic induction, magnetite, melanin, optical pumping and biradical reactions (Lohmann & Lohmann, 1993). Yet despite these theoretical analyses, little direct neurobiological or anatomical evidence exists to support any of the proposed mechanisms. In no case yet have primary magnetoreceptors been identified with certainty. Several factors have made locating magnetoreceptors difficult (Beason & Semm, 2004). One is that magnetic fields pass freely through biological tissue. Thus, magnetoreceptors need not contact the external environment and might plausibly be located nearly anywhere within the body of an animal. Magnetoreceptors might also be tiny and dispersed throughout a large volume of tissue, or the transduction process might occur as a set of chemical reactions, so that no obvious organ or structure devoted to magnetoreception necessarily exists (Purcell, 2005). Moreover, accessory structures such as lenses, which focus sensory stimuli on receptors and are often conspicuous, are unlikely to have evolved for magnetic sensing because few biomaterials affect magnetic field lines. For now, most of what is known about magnetoreception in vertebrate animals has been inferred from behavioral experiments, from theoretical considerations, and from a limited number of electrophysiological and anatomical studies. This article describes the difference between a magnetic directional (or compass) sense and a magnetic positional (or map) sense, reviews the three main hypotheses of vertebrate magnetoreception, and summarizes the evidence for each (Lohmann & Lohmann, 1993).

In principle, all three mechanisms we have described can provide an animal with directional information that might be used in a magnetic compass sense. The information derived from the field, however, is not identical in all cases. Some magnetite models and the induction model are capable of detecting field polarity (that is, they can potentially differentiate between magnetic north and south) (Purcell, 2005). By contrast, no current model of chemical magnetoreception allows for this (Beason & Semm, 2004). Thus, a chemically based magnetoreceptor should detect only the axis of the ...