Biological Effects of Electric and Magnetic Fields: Sources and Mechanisms by Carpenter David O. Ayrapetyan Sinerik

Author:Carpenter, David O.,Ayrapetyan, Sinerik [Carpenter, David O.; Ayrapetyan, Sinerik]
Language: eng
Format: epub
ISBN: 978-0-08-088689-3
Publisher: Elsevier Ltd.
Published: 1994-03-14T16:00:00+00:00

9

Effects of Magnetic and Electric Fields in Invertebrates and Lower Vertebrates

Martin Kavaliers and Klaus-Peter Ossenkopp

I. INTRODUCTION

There is mounting evidence that animals are able to detect and respond to magnetic fields (Gould, 1984). Virtually all organisms are exposed to the earth’s magnetic field (geomagnetic field) and likely have become adapted to their geomagnetic environment through natural selection. The earth’s magnetic field contains information about direction, location, and time that can be used by potentially all animals.

A review of the various relevant features of the geomagnetic field is provided by Skiles (1985). Briefly, directional information can be obtained from the horizontal and vertical components of the earth’s magnetic field. The magnetic lines of force have polarity (i.e., north – south) and are only horizontal at the earth’s magnetic equator. Elsewhere on the earth’s surface, the lines have an angle of dip or inclination (angle between the magnetic vector and horizon), becoming steeper at higher latitudes (90° at the poles) and being more or less vertical at the geomagnetic poles (0°). Thus, at any location on earth (except at the poles) magnetic polarity (polarity compass) could be used to obtain the direction of north. It is also possible to use magnetic inclination (inclination compass) to provide direction (except near the equator). An animal that faces the direction in which the lines of force of the earth’s magnetic field are descending into the ground will be orienting to the nearest geomagnetic pole. An inclination compass will, therefore, give the direction of either the north or south pole and the equator, while a polarity compass will provide north–south information. Generally, the deviation (declination or variation) between magnetic and geographic north is less than 20°, although extreme values are present at the poles.

Location, in terms of relative magnetic latitude, could be determined from total intensity [0.3–0.6 G (30,000–60,000 nT) at the magnetic equator and poles, respectively], vertical and horizontal intensity, and dip angle of the geomagnetic field. Additional recognition cues are available from perturbations in the geomagnetic field arising from local variations in lithography and topography.

Temporal information is available from the low-amplitude variations in the geomagnetic field that occur over the solar and lunar days, synodic month, and tropical year. At north temperate latitudes, the intensity decreases until local noon by 30 to 100 nT, then increases again. Superimposed on this are irregular fluctuations associated with factors such as sun spot activity and solar flares. There are also slow secular variations in intensity, inclination, and declination related to the westward drift of the field. These slow changes are negligible through the life span of an organism, but may be important over evolutionary time.

The associated electric fields also need to be considered. A magnetic field whose direction or intensity is changing with time is always accompanied by an electric component (Faraday’s law). Therefore, an organism moving in a magnetic field will also experience an electric field. Electric and magnetic fields are neither independent nor absolute entities. Rather, they are components of the same electromagnetic field whose spatial and temporal characteristics will vary according to the movement of an organism.

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