Magnetic Compass Orientation in the Eastern Red-Spotted Newt, Notophthalmus viridescens: Rapid Acquisition of the Shoreward Axis
Orientation toward and away from shore (y-axis orientation) plays an important role in the daily and seasonal movements of amphibians. A variety of studies have shown that amphibians use both celestial cues and the geomagnetic field for y-axis orientation. However, few studies have addressed how quickly amphibians are able to learn the direction of a new shore. In the present laboratory-based experiments, we show that the Eastern Red-Spotted Newt, Notophthalmus viridescens, is able to learn the direction of the y-axis with respect to the geomagnetic field within 12–16 h. In conjunction with previous findings, our results suggest that the geomagnetic field is an important and readily used cue for orientation in amphibians.Abstract
Schematic of the newt training tank with the training chute (1) attached. At the shore end of the tank a Plexiglas “fence” (2) allowed water through but prevented the newts from escaping from the tank. Rubber dividers (3) at the shore end of the tank provided newts with shade and hiding places. A water-filled heat exchange system (4) situated beneath the shore (5) controlled the water temperature (see text). Water was drawn past the heat exchanger and up into the section of the tank containing the newts by an air-driven circulation system (6). Dashed lines indicate holes for water circulation in the tank; arrows indicate the direction of water circulation
Bimodal magnetic orientation along the y-axis after 12–16 h of training. (A–B) Bearings of the newts plotted with respect to the direction of magnetic North during testing for newts trained to a west shore and a south shore, respectively. (C) The combined distribution of magnetic bearings from A and B plotted with respect to the direction of shore in training. (D–F) The distributions of the doubled bearings (used to calculate the bimodal statistics) for A, B, and C, respectively. Each dot represents the bearing of one newt. Single-headed and double-headed arrows at the center of each plot indicate the mean vector, or mean bimodal axis, respectively, for each distribution. The length of each arrow is proportional to the mean vector length (“r”), with the diameter of the circle corresponding to r = 1 (in A–C) or with the radius of the circle corresponding to r = 1 (in D–F). Dashed lines indicate the 95% confidence intervals for the mean vectors