CIRCANNUAL RHYTHMS IN TROPICAL STONECHATS BRED AND RAISED UNDER CONSTANT CONDITIONS

East African Stonechats (Saxicola torquata axillaris) exhibit endogeno us circannual rhythms in gonadal size and moult that persist for at le ast 7 1/2 years under constant conditions of temperature and photoperi od. However, as in other studies on avian circannual rhythms, the expe rimental birds were transferred to controlled conditions not before th ey were several days or weeks old. Therefore, the effect of exposure t o seasonal environmental changes during early embryonic or postembryon ic development could have contributed to the development of a normal c ircannual system. The present study shows, however, that even Stonecha ts that were bred and raised by parents that had been living for more than a year in a constant equatorial photoperiod and in constant tempe rature conditions exhibited circannual cycles, indistinguishable from conspecifics that had developed under less rigidly controlled conditio ns. The results, therefore, indicate that circannual rhythms of stonec hats are not only endogenous in the sense that they actually run under constant conditions but that they also develop spontaneously and ''no rmally'' in the absence of seasonal environmental changes.