Evidence for the use of magnetic map information by two classes of vertebrates.

摘要

Whether animals navigate using “map” information derived from spatial gradients in the Earth's magnetic field has been debated for half a century. Although there is evidence that certain animals possess the sensory abilities necessary to perceive at least two magnetic components that vary spatially, the use of magnetic map information by experienced adult migrants had not previously been directly tested. In separate sets of experiments, I investigated the possibility of a magnetic component of a navigational map by exposing migrants from two classes of vertebrates (a migratory bird, Zosterops l. lateralis, and the eastern red-spotted newt, Notophthalmus viridescens) to altered magnetic fields, and subsequently testing for orientation. The animals were displaced from their respective home territories, and then housed and tested within electromagnetic coils. The birds exposed to a small increase in magnetic inclination and intensity maintained appropriate migratory orientation, whereas those exposed to an equivalent decrease ceased to show significant orientation. Studies examining the effect of 2° changes of inclination on the orientation of adult newts found that equivalent increases and decreases in inclination alone produced different directional responses. The same 2° inclination manipulation did not affect the newts' compass response. Although silvereyes responded to simultaneous changes in intensity and inclination, it was unclear which of the two magnetic parameters caused the response. However, a change in inclination alone was sufficient to affect the newts' homing orientation. These results suggest that magnetic inclination (or one of its components) is a possible parameter for map-based homing in the short-distance migrant, the eastern red-spotted newt. There followed an attempt to map geomagnetic parameters around the newts' home ponds, to determine whether the magnetic cues used in the laboratory experiments were biologically relevant to newts. If newts are using inclination as a map cue its gradient should increase northwards. Due to equipment limitations, I was unable to show the directions of the magnetic gradients. Although I could not construct a magnetic map of the newts' home pond area, I examined the relative merits of different magnetic parameters for use in a magnetic map by a short-distance migrant.