We have studied the antlers and male reproductive organs of thirteen roebucks, shot at approximately monthly intervals throughout the year. The roebuck is a seasonally breeding mammal that is in rut from mid-July to mid-August. Antlers `in velvet' begin to develop in January, and the velvet is shed in March or April. The animal then remains in `hard horn' until November or December, when the antlers are cast.
The testes are at their most inactive state in January, when there is no spermatogenic activity and a very low content of testosterone. By mid-February, the testicular testosterone content has risen considerably, and primary spermatocytes are to be seen in the enlarging seminiferous tubules. The testosterone content of the testis remains high until the beginning of the rut, but falls precipitously towards the end. Spermatogenesis is not finally completed until April or May, and, although it continues for several weeks after the end of the rut, there is a highly significant decline in testis tubular diameter that coincides with the fall in testosterone content. Thus spermatogenesis and androgenesis are closely related at the beginning and end of the sexual cycle, suggesting that fsh and lh secretion by the pituitary gland normally go hand-in-hand.
The seminal vesicles secrete fructose, sorbitol, inositol and citric acid. Although there is a significant correlation between testicular testosterone and vesicular fructose and citric acid, the correlation coefficients are not high. This is probably because the seminal vesicles do not respond until some weeks after the onset of testosterone production in the spring, and their secretion declines less rapidly than testicular testosterone after the end of the rut.
These endocrine changes are in accord with the seasonal changes in antler growth, which are known to be under endocrine control. Shedding of the velvet occurs soon after the testicular testosterone levels have risen in the spring, and casting of the antlers in late autumn coincides with extremely low testicular testosterone levels.