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. 2007 Nov 22;274(1627):2901-5.
doi: 10.1098/rspb.2007.0904.

Bats Respond to Polarity of a Magnetic Field

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Free PMC article

Bats Respond to Polarity of a Magnetic Field

Yinan Wang et al. Proc Biol Sci. .
Free PMC article

Abstract

Bats have been shown to use information from the Earth's magnetic field during orientation. However, the mechanism underlying this ability remains unknown. In this study we investigated whether bats possess a polarity- or inclination-based compass that could be used in orientation. We monitored the hanging position of adult Nyctalus plancyi in the laboratory in the presence of an induced magnetic field of twice Earth-strength. When under the influence of a normally aligned induced field the bats showed a significant preference for hanging at the northern end of their roosting basket. When the vertical component of the field was reversed, the bats remained at the northern end of the basket. However, when the horizontal component of the field was reversed, the bats changed their positions and hung at the southern end of the basket. Based on these results, we conclude that N. plancyi, unlike all other non-mammalian vertebrates tested to date, uses a polarity-based compass during orientation in the roost, and that the same compass is also likely to underlie bats' long-distance navigation abilities.

Figures

Figure 1
Figure 1
Experimental chamber in which the bats were exposed to the altered magnetic field.
Figure 2
Figure 2
Hanging positions of bats in response to simultaneous changes in the horizontal and vertical components of the altered field. Animals were exposed for 15 days to the normal field (Nm) before the horizontal and vertical components were simultaneously reversed (Nm+H+V). The reversal was done twice giving a total duration of the experiment of 60 days. Symbols at the periphery of the circle indicate the hanging position of the cluster of bats on each test night. The arrows represent the mean vector with the length proportional to the radius of the circle=1. The inner solid and dotted circles represent the 1 and 5% significance level of the Rayleigh test, respectively. The bats always showed a non-random distribution within the basket: (a) Nm: declination=1.0°, inclination=61.3° and intensity=98.9 μT; r=0.78; α=33.90°±40.52°, p<0.001. (b) Nm+H+V: declination=183.8°, inclination=−60.8° and intensity=100.5 μT; r=0.48; α=174.66°±69.64°, p<0.05. (c) Nm: declination=359.7°, inclination=56.1° and intensity=87.1 μT; r=0.62; α=46.30°±55.82°, p<0.002. (d) Nm+H+V: declination=181.5°, inclination=−61.3° and intensity=98.9 μT; r=0.66; α=184.52°±52.22°, p<0.001. When the horizontal and vertical fields were simultaneously reversed, bats changed their roosting positions significantly: (a,b) Watson U2=0.3458, p<0.005, (b,c) Watson U2=0.3077, p<0.005 and (c,d) Watson U2=0.4636, p<0.005.
Figure 3
Figure 3
Roosting positions of bats during independent reversals of the vertical and horizontal altered fields. Nm represents the normal field, Nm+H the field after the horizontal field was reversed and Nm+V the field after the vertical field was reversed. Experiments were divided into six parts: normal field in the first 15 days, vertical field reversed in the second 15 days, normal field in the third 15 days and horizontal field reversed in the fourth 15 days. Symbols at the periphery of the circle indicate the hanging position of the cluster of bats on each test night. The arrows represent the mean vector with the length proportional to the radius of the circle=1. The inner solid and dotted circles represent the 1 and 5% significance level of the Rayleigh test, respectively. Bats always showed a non-random distribution within the basket: (a) Nm: declination=359.8°, inclination=58.5° and intensity=92.0 μT; r=0.62; α=9.01°±55.97°, p<0.002. (b) Nm+H declination=358.8°, inclination=−64.7° and intensity=114.6 μT; r=0.71; α=26.20°±47.66°, p<0.001. (c) Nm: declination=0.1°, inclination=56.2° and intensity=88.6 μT; r=0.55; α=7.15°±62.95°, p<0.01. (d) Nm+V: declination=180.5°, inclination=61.3° and intensity=100.8 μT; r=0.55; α=183.03°±62.54°, p<0.01. Only when the horizontal field was reversed did the bats significantly change their roosting position: (a,b) Watson U2=0.1203, p>0.10 and (c,d) Watson U2=0.3883, p<0.005.

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