Magnetoreception in birds: different physical processes for two types of directional responses - PubMed (original) (raw)

Magnetoreception in birds: different physical processes for two types of directional responses

Roswitha Wiltschko et al. HFSP J. 2007 May.

Abstract

Migratory orientation in birds involves an inclination compass based on radical-pair processes. Under certain light regimes, however, "fixed-direction" responses are observed that do not undergo the seasonal change between spring and autumn typical for migratory orientation. To identify the underlying transduction mechanisms, we analyzed a fixed-direction response under a combination of 502 nm turquoise and 590 nm yellow light, with migratory orientation under 565 nm green light serving as the control. High-frequency fields, diagnostic for a radical-pair mechanism, disrupted migratory orientation without affecting fixed-direction responses. Local anaesthesia of the upper beak where magnetite is found in birds, in contrast, disrupted the fixed-direction response without affecting migratory orientation. The two types of responses are thus based on different physical principles, with the compass response based on a radical pair mechanism and the fixed-direction responses probably originating in magnetite-based receptors in the upper beak. Directional input from these receptors seems to affect the behavior only when the regular inclination compass does not work properly. Evolutionary considerations suggest that magnetite-based receptors may represent an ancient mechanism that, in birds, has been replaced by the modern inclination compass based on radical-pair processes now used for directional orientation.

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Figures

Figure 1

Figure 1. Orientation of European robins, Erithacus rubecula , in the local geomagnetic field; effects of a broadband high-frequency field and of Xylocain, a local anaesthetic, applied to the skin of the upper beak.

Upper diagrams: compass orientation in the migratory direction under 565 nm green light; lower diagrams: fixed-direction responses under a combination of 502 nm turquoise and 590 nm yellow light. The symbols at the periphery of the circle mark the mean headings of the test birds based on three recordings each. Open symbols: when untreated, solid symbols, with high-frequency fields added or with the local anaesthetic applied, respectively. The arrows represent the corresponding mean vectors, and the two inner circles are the 5% (dotted) and the 1% significance border of the Rayleigh test (Batschelet, 1981).

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