Circannual Rhythms in Gonads and Endocrine Organs of the Great Tit Parus major in South-West Sweden

Abstract

Seasonal studies on gonads, accessory sex organs, adrenals and thyroid glands of adult Great Tits from a population in south-west Sweden showed that testes started to grow rapidly at the end of March and reached maximal size in the middle of April. Testicular regression started in the middle of July. The ovary reached maximal size at the end of April. The cycle of the Leydig cells could be related to annual changes in sexual behaviours. Notable was the complete absence of mature Leydig cells during August and September. Concomitant with a short period of increased sexual interactions in October the Leydig cells reappeared. There seemed to be a time lag in the response of the seminal vesicles to an increased testosterone production from the Leydig cells. Adrenal and thyroid glands, in both sexes, showed cyclic changes in their activity during the year. Maximal cortical activity, in both sexes, was found during the breeding period. The breeding and molting periods were associated with high thyroid activity in both sexes.