Abstract
Behavioural tests of the magnetic compass of birds and corresponding immunohistological studies on the activation of retinal cryptochrome 1a, the putative receptor molecule, showed oriented behaviour and activated Cry1a under 373 nm UV, 424 nm blue, 502 nm turquoise and 565 nm green light, although the last wavelength does not allow the first step of photoreduction of cryptochrome to the semiquinone form. The tested birds had been kept under ‘white’ light before, hence we suggested that there was a supply of semiquinone present at the beginning of the exposure to green light that could be further reduced and then re-oxidized. To test the hypothesis in behavioural experiments, we tested robins, Erithacus rubecula, under various wavelengths (1) after 1 h pre-exposure to total darkness and (2) after 1 h pre-exposure to the same light as used in the test. The birds were oriented under blue and turquoise light, where the full cryptochrome cycle can run, but not under green light. This finding is in agreement with the hypothesis. Orientation under green light appears to be a transient phenomenon until the supply of semiquinone is depleted.
Highlights
The Radical Pair Model (Ritz et al, 2000) proposed that the avian magnetic compass is based on radical pair processes in the eye
Behavioural experiments testing migratory birds in cages are in accordance with a role of Cry1a in magnetoreception: Australian silvereyes, Zosterops l. lateralis (Zosteropidae), European robins and garden warblers, Sylvia borin (Sylviidae), were found to be oriented in their seasonally appropriate migratory direction under 373 nm UV, 424 nm blue, 502 nm turquoise and 565 green light; under longer
As the first step of photoreduction from the fully oxidized form to the semiquinone cannot take place under green light, we suggested that the time before exposure to green light was crucial: when the birds had been kept under ‘white’ light, a certain supply of the semiquinone was left; this could be photoreduced to the fully reduced form under green light, and re-oxidized, generating the magneto-sensitive radical pairs
Summary
The Radical Pair Model (Ritz et al, 2000) proposed that the avian magnetic compass is based on radical pair processes in the eye. *Present address: Ludwig-Maximillians-University Munich, Department of Earth and Environmental Sciences, Theresienstraße 41/IV, D-80333 Munich, Germany Received 15 July 2014; Accepted 7 October 2014 wavelengths, they were disoriented (Wiltschko et al, 1993; Wiltschko and Wiltschko, 1995; Wiltschko and Wiltschko, 1999; Rappl et al, 2000; Muheim et al, 2002; Wiltschko et al, 2014) This is in agreement with the absorption spectrum of cryptochrome: the oxidized form absorbs UV and blue light up to about 500 nm, the photoreduced semiquinone form absorbs green light up to about 570 nm (Müller and Ahmad, 2011)
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