Seasonal Variations in the Ultrastructure of the Leydig Cells and Plasma Levels of Luteinizing Hormone and Steroid Hormones in Juvenile and Adult Male Great Tits Parus major

Abstract

Annual cycles in plasma levels of luteinizing hormone (LH), testosterone and dihydrotestosterone (DHT), as well as seasonal changes in the ultrastructure of Leydig cells, were studies in free-living adult and juvenile male Great Tits from south-west Sweden. Special attention was paid to the autumn/winter period. Adult males have very few Leydig cells during the autumn, whereas such cells occur in large numbers in the testes of juvenile Great Tits. At the end of the autumn period there is an increase in plasma levels of LH, but not in testosterone, in both age groups. This increase is accompanied by changes in number and ultrastructure of the Leydig cells, especially in adults, so that there is finally no longer any differences between adults and juveniles. For both age groups the ultrastructure of the Leydig cells indicate maximal activity during the winter period, but testosterone levels remain basal. There are no differences in plasma levels of testosterone or LH between juvenile and adult males during the autumn/winter period, except during September when the plasma concentrations of these hormones are significantly higher in juveniles. There is a transitory increase, to maximal values for the year, in plasma levels of LH and testosterone in March. Concomitant with this increase the ultrastructure of the Leydig cells change. The breeding period is characterized by moderate plasma levels of testosterone and LH. There is a trend for higher levels of LH, but not for testosterone, in June. Plasma levels of both hormones are down to basal in July. When testes regress in July, most Leydig cells are in a state of dissolution. Plasma levels of DHT did not show an annual cycle. Ecological and behavioural implications of the results are discussed.