EXPERIMENTAL DESIGN/ SENSORY BIOLOGY

Experimental Design/ sensory biology

Experimental Design/ Sensory Biology

Introduction

The magnetic field of the Earth provides a pervasive and reliable source of directional information that certain animals can use as an orientation cue while migrating, homing, or moving around their habitat. Behavioral experiments have demonstrated that a surprising number of diverse organisms can orient magnetically, including certain species of bacteria, molluscs, insects, fishes, amphibians, reptiles, birds, and mammals. Relatively little is known, however, about the neural mechanism or mechanisms that underlie magnetic field detection and geomagnetic orientation in animals (Beason & Semm, 1991). This article briefly describes the difference between a magnetic compass and a magnetic map sense, reviews the three major hypothesized transduction mechanisms for magnetic field detection, and concludes with an overview of recent neurophysiologic advances.

Discussion

In principle, the Earth's magnetic field provides a potential source of positional information that might be used in such a map sense. Several geomagnetic parameters, such as field intensity and the inclination of field lines relative to the Earth's surface (dip angle), vary across the Earth's surface (Lohmann, Cain, Dodge & Lohmann, 2001). During the past decade, evidence has accumulated steadily that some animals can, indeed, derive positional information from features of the Earth's field. Sea turtles, newts, and honeybees have been shown to distinguish between different magnetic inclination angles or field strengths (Beason & Semm, 1991). Electrophysiologic recordings from birds and trout have also provided evidence for the presence of cells that respond to changes in field intensity. An interesting possibility is that some species possess two separate magnetosensory systems: one for a magnetic compass, the other for a magnetic map. Each system may detect a different feature of the Earth's field, and each may have a different underlying physiologic mechanism (Lohmann, Cain, Dodge & Lohmann, 2001).

Indirect support for a link between magnetoreception and photoreceptors has come from behavioral experiments indicating that the magnetic orientation behavior of birds, newts, and flies changed when the animals were tested under different wavelengths of light. No consistent pattern has yet emerged among species, but wavelength-dependent effects reported so far include random orientation and shifts of about 90 degrees in orientation direction (Walker, Diebel, Haugh & Pankhurst, 1997). Although certain results, such as random orientation, might conceivably be explained as an effect of wavelength on an animal's motivation, 90-degree shifts in direction elicited by specific wavelengths are more challenging to explain as anything other than an effect on a magnetoreceptor system. Several studies have also suggested a link between magnetoreception and the pineal gland. A recent study with newts has revealed that a 90-degree shift in magnetic orientation direction that occurs when newts are tested under a specific wavelength of light can be elicited if the pineal complex, but not the eyes, are illuminated with light of the same wavelength (Lohmann, Cain, Dodge & Lohmann, 2001).

Neurobiological Research

Electrophysiologic and neuroanatomic research into the mechanisms underlying magnetic field detection has lagged behind behavioral work on magnetic orientation and theoretic considerations of possible transduction modes (Walker, Diebel, Haugh & ...