The Circadian Component in the Photoperiodic Mechanism of the House Sparrow, Passer domesticus

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Abstract
As in other photoperiodic species of birds, long days (>ca. 9 h) cause growth and development of the testes in male house sparrows (Passer domesticus). The logarithmic growth rate (k) is essentially a linear function of day length over the range in daily environmental photophases of ca. 9-16 h. Unlike most photoperiodic species, however, k in P. domesticus is an inverse function of day length for very short days (<ca. 7 h). These "ultra-short-day" responses have been investigated by use of light-dark cycles of varying (20-28 h) periods (T) with photophases of fixed duration (3 h) and exploitation of the coupling between the circadian cycles in motor activity and the photosensitivity of the response system in which the phase of the former leads that of the latter by ca. 135° (ca. 9 h in a 24-h cycle). Consistent with general experience with endogenous circadian periodicities, our results demonstrate that the phase-angle difference (Ψ) between the cycle in motor activity and that of the Zeitgeber (environmental photocycle) increases as a function of T. Furthermore, our results show that, as T is increased beyond ca. 24 h, the rate of testicular growth increases as a nonlinear function of Ψ. We interpret these results to mean that the ultra-short-day photoperiodic response of P. domesticus is the result of an advance in the photosensitive phase of the response system so that it becomes at least partly coincident with the environmental photophase. Therefore the ultra-short-day photoperiodic responses as well as the long-day responses of this species can be explained by an external-coincidence model (Bünning hypothesis) that appears widely applicable to photoperiodic species. At this time, at least, we find it unnecessary to invoke a more complex internal-coincidence model. In a further series of experiments, male house sparrows were subjected to continuous darkness (DD) after photostimulation (16L 8D) for either 14 or 30 days. The effect of transfer to DD on the size of the testes was found to be dependent on the size attained at the time of transfer. After 14 days on 16L 8D (28 mg, mean testicular weight), the rate of regression after transfer to DD was the same as in another group transferred to 8L 16D. However, the onset of regression in the former was delayed longer than that of the latter. The levels of luteinizing hormone (LH) in the plasma of both groups declined sharply, but more extensively in the group transferred to DD. In contrast, after 30 days of photostimulation (261 mg, mean testicular weight), regression did not occur after transfer to DD. This is consistent with earlier observations and can be rationalized by an internal-coincidence model. Nevertheless, application of the Law of Parsimony to the currently available information suggests acceptance of a simpler explanation that combines an external-coincidence model and the established "carry-over" function of the neuroendocrine control systems of photoperiodic species of birds.