Seasonality in Reproduction
Reproductive responses to sexual activity
Our studies show that social interaction with ewes affects reproductive processes in rams. Mating for a few hours stimulates episodic LH release and elevates testosterone secretion in the summer when LH pulse frequency is low. Stressful sexual encounters (e.g., repeated mounting without ejaculation) have the opposite effect on LH and testosterone in the autumn, perhaps due to elevations in prolactin. Rams penned adjacent to or in with ewes have a faster rate of testicular redevelopment in the summer. This appears to be due to greater stimulation of the testes by FSH and LH in the presence of a normal springtime rise in prolactin.
Thyroid supplementation and testicular recrudescence
Poor fertility of rams in the summer may be partly due to reduced thyroid function. We began to test this hypothesis by injecting rams with triiodothyronine (T3) to produce a mild "hyperthyroid" condition in July through October. The testes of these rams enlarged at the normal rate, but were smaller than usual by early autumn. Compared with controls, treated rams ejaculated more spermatozoa and had higher testosterone levels in blood and seminal plasma in mid- to late summer. FSH, prolactin and libido were not affected by T3 injection. Boosting thyroid action in the summer does provide a short-term benefit to the testes.