Magnetoreception in birds: II. Behavioural experiments concerning the cryptochrome cycle

Area of Science:

• Ornithology
• Biophysics
• Neuroethology

Background:

• Avian magnetic compass sense is crucial for navigation.
• Retinal cryptochrome 1a (Cry1a) is the proposed photoreceptor for this sense.
• Previous studies indicated Cry1a activation across various light wavelengths, including green.

Purpose of the Study:

• To investigate the role of cryptochrome 1a in bird orientation under different light conditions.
• To test the hypothesis that a pre-existing supply of cryptochrome semiquinone is necessary for green light-mediated orientation.
• To determine the dependence of the magnetic compass on the cryptochrome photochemical cycle.

Main Methods:

• Behavioral experiments with European robins (Erithacus rubecula).
• Testing orientation responses under UV, blue, turquoise, and green light.
• Manipulating pre-exposure conditions (darkness vs. same light) to control cryptochrome redox state.

Main Results:

• Robins oriented under blue and turquoise light, supporting a functional cryptochrome cycle.
• Orientation was not observed under green light after dark pre-exposure, indicating a requirement for the initial photoreduction step.
• Orientation under green light was transient when birds were pre-exposed to the same light, suggesting depletion of a semiquinone pool.

Conclusions:

• The cryptochrome 1a photochemical cycle, specifically the photoreduction to the semiquinone form, is essential for magnetic compass orientation.
• Green light alone is insufficient to initiate orientation unless a semiquinone supply is already present.
• Avian magnetoreception is dependent on specific light-dependent chemical reactions within the cryptochrome molecule.