Entwicklung circannualer Rhythmen tropischer Schwarzkehlchen (Saxicola torquata axillaris) nach Bebrütung und Aufzucht unter konstanten Bedingungen

Abstract

East African Stonechats (Saxicola torquata axillaris) exhibit endogenous circannual rhythms in gonadal size and moult that persist for at least 7 1/2 years under constant conditions of temperature and photoperiod. However, as in other studies on avian circannual rhythms, the experimental birds were transferred to controlled conditions not before they were several days or weeks old. Therefore, the effect of exposure to seasonal environmental changes during early embryonic or postembryonic development could have contributed to the development of a normal circannual system. The present study shows, however, that even Stonechats that were bred and raised by parents that had been living for more than a year in a constant equatorial photoperiod and in constant temperature conditions exhibited circannual cycles, indistinguishable from conspecifics that had developed under less rigidly controlled conditions. The results, therefore, indicate that circannual rhythms of stonechats are not only endogenous in the sense that they actually run under constant conditions but that they also develop spontaneously and “normally” in the absence of seasonal environmental changes.