Abstract
Short day (8L : 16D), pretreated adult male redheaded buntings were held on various light—dark cycles of 20 to 30 h duration, in which a fixed ultra-short photophase of 3 h was combined with scotophases of varying duration. A photoperiodic testicular response was obtained only in 28- and 30-h cycles (3L : 25D and 3L : 27D). The same photo-period (3 h) in 20- to 26-h cycles (3L : 17D, 3L : 19D, 3L : 21D and 3L : 23D) failed to stimulate testicular growth. The results can be interpreted on the assumption that the positive testicular response in this species, under ultra-short-day light cycles, is the result of an advance in the photosensitive phase of the photoperiodic response system so that it coincides at least partly with the external photophase. The results thus appear to conform with the Bünning hypothesis or external coincidence model.
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