Seasonal Changes in Plasma Levels of LH and Gonadal Steroids in Free-Living Willow Tits Parus montanus

Abstract

Seasonal variations in plasma levels of luteinizing hormone (LH), dihydrotestosterone (DHT), testosterone and estradiol (only in females) were studied in free-living adult and juvenile Willow Tits from southwest Sweden. All hormones, except DHT, showed pronounced annual cycles. For both sexes, and both age groups, LH titers showed a typical bimodal pattern with transitional peaks during the breeding period and early autumn. Also, LH levels increased significantly in January and remained elevated throughout the winter. Male plasma levels of testosterone also showed a bimodal pattern with peaks occurring concomitant with LH. The August September peak was, however, caused by some juveniles having very high plasma levels of testosterone, whereas the majority of juveniles and all adults showed low testosterone levels at this time. Female testosterone values did not vary significantly during the year, but in junvenile females testosterone levels showed a transitional peak in August. Estradiol showed only one significant annual peak during the breeding period. The winter increase in plasma levels of LH was not accompanied by changes in testosterone, or estradiol, titers. In spring, male LH and testosterone levels started increasing a month earlier than LH and estradiol in females.