Magnetic compass mediates nocturnal homing by the alpine newt, Triturus alpestris

Abstract

Experiments were carried out to investigate the use of magnetic compass cues in the nocturnal homing ori- entation of the alpine newt Triturus alpestris. Tests were carried out at a site 9 km to the east-northeast of the breed- ing pond. Newts were tested at night in an outdoor circular arena that provided an unimpeded view of celestial cues, in one of four symmetrical alignments of an earth-strength magnetic field. In tests carried out under partly cloudy skies newts exhibited homeward magnetic compass orientation. Because the moon was visible in some trials, but obscured by clouds in others, we investigated whether the presence of the moon contributed to the scatter in the distribution of magnetic bearings. When the moon was visible, the distri- bution of magnetic bearings was more scattered than when the moon was obscured by clouds, although in neither case was the distribution significant due, in part, to the small sample sizes. Moreover, when the moon was visible, newts oriented along a bimodal axis perpendicular to the moon azimuth, suggesting that the presence of the moon may have affected the newts behavior. To provide a more rigorous test of the role of magnetic compass cues when celestial cues were unavailable, nocturnal tests were carried out during the following migratory season under total overcast. In the absence of celestial compass cues, the distribution of mag- netic bearings exhibited highly significant orientation in the homeward direction. These findings indicate that newts are able to orient in the homeward direction at night using the magnetic compass as the sole source of directional infor- mation. Moon light altered the newts' behavior. However, this apparently resulted from the asymmetrical distribution of moon light in the testing arena, rather than the use of an alternative compass.