Research Synopsis 4

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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.

Photograph by Ian Britton

Fig 9: Yearly rhythm in testosterone in Landrace boars kept near or away from gilts

Seasonality in sexual function of boars

Under natural photoperiod (latitude 45ºN), adult Landrace boars have larger and more active testes in late autumn and early winter than in late spring and early summer. Peak activity is preceded by a rise in FSH, but not prolactin secretion. Blood testosterone level is elevated in late autumn, due mainly to more frequent episodic secretions. Steroids responsible for boar taint are not any higher. Ejaculates contain greater numbers of spermatozoa and more protein and citric acid. The latter indicate an increase in seminal vesicle function. The extent of seasonality in some traits depends on how much social contact the boars have with gilts (Fig 9).

Sexual function of roosters and drakes in controlled environments

Sexually mature Leghorn roosters were kept under constant long days (16 hr light: 8 hr dark) for one year. We did not find any evidence of an endogenous "seasonal" rhythm in LH or testosterone secretion, but semen quality and fertility did decline as the birds aged. In a second study, seasonal rhythms (latitude 50ºN) in LH and testosterone were observed in wild mallard and Rouen drakes. The rise in blood testosterone level in late winter and early spring was four-fold greater and of much longer duration in the domestic birds (Fig 10). Daily rhythms and episodic patterns in LH and testosterone secretion were seen in Rouen drakes in both long and short days.