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Magnetic Orientation of the Common Toad: Establishing an Arena Approach for Adult Anurans

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Magnetic Orientation of the Common Toad: Establishing an Arena Approach for Adult Anurans

Lukas Landler et al. Front Zool.

Abstract

Background: Magnetic orientation is a taxonomically widespread phenomenon in the animal kingdom, but has been little studied in anuran amphibians. We collected Common Toads (Bufo bufo) during their migration towards their spawning pond and tested them shortly after displacement for possible magnetic orientation in arena experiments. Animals were tested in two different set-ups, in the geomagnetic field and in a reversed magnetic field. To the best of our knowledge, this is the first study testing orientation of adult anurans with a controlled magnetic field of a known strength and alignment.

Results: After displacement, toads oriented themselves unimodally under the geomagnetic field, following their former migration direction (d-axis). When the magnetic field was reversed, the distribution of bearings changed from a unimodal to a bimodal pattern, but still along the d-axis. The clustering of bearings was only significant after the toads reached the outer circle, 60.5 cm from their starting point. At a virtual inner circle (diameter 39 cm) and at the start of the experiment, orientation of toads did not show any significant pattern.

Conclusions: The experimental set-up used in our study is suitable to test orientation behaviour of the Common Toad. We speculate that toads had not enough time to relocate their position on an internal map. Hence, they followed their former migration direction. Bimodality in orientation when exposed to the reversed magnetic field could be the result of a cue conflict, between magnetic and possibly celestial cues. For maintaining their migration direction toads use, at least partly, the geomagnetic field as a reference system.

Figures

Figure 1
Figure 1
Testing site. Location of the pond in Vienna with the surrounding park and the direction towards the testing site. Toads are diverted by the drift fence until they can cross the street through the tunnels.
Figure 2
Figure 2
Experimental set-up. Experimental set-up with Helmholtz-coils. At the start of each trial the toad was placed in the centre of the arena and covered with a pot (release device).
Figure 3
Figure 3
Results. Bearings and mean directions of the initial orientation (a), orientation at the inner circle (b) and orientation at the wall of the arena (c); in the figure the radius of the circle corresponds to a mean vector length (r) equal to one. Graphs in the upper row show the results obtained under natural conditions, graphs in the lower row (highlighted) those obtained with the reversed magnetic field (n = 29 in all cases). Dashed lines indicate the 95% confidence intervals in cases of significant orientation (P < 0.05). Under natural conditions the magnetic North (mN) coincided with the true north direction (nN), whereas in the reversed field mN was opposite to nN. Coloured arrows indicate the direction to the pond (P), the direction from the hibernation area to the pond (HA) and the mean direction along the drift fence (D). In the case of the reversed field, corresponding arrows are placed on the opposite side, in paler colours, to illustrate the directions when following only the magnetic field.

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