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In order to study the seasonal reproductive changes in the Red deer stag (Cervus elaphus L.), two adult animals were shot per month in 1968.

During May and June the stags were in a state of reproductive quiescence, and the antlers were rapidly growing in velvet. Testosterone was barely detectable in testicular tissue, and the seminiferous tubules were regressed with small numbers of spermatogonia and primary spermatocytes but no spermatids or spermatozoa. The accessory organs were also fully regressed, the concentrations of fructose in the seminal vesicles and ampullae were minimal, and the epididymides were devoid of spermatozoa.

At the end of June, there was a slight increase in the testicular testosterone concentration, coincident with an increase in the number of spermatogonia in the seminiferous tubules. In July, as the testosterone continued to increase, spermatids appeared and the tubules began to enlarge. Histological changes were seen in the epithelia of the epididymides, ampullae, prostate and seminal vesicles, although organ weights and vesicular fructose concentration remained unchanged.

In August, there was a marked increase in testicular testosterone, spermatogenesis was now complete and the tubules reached their maximum diameter. The accessory organs had increased in weight considerably, and the vesicular fructose concentrations were also elevated. The antlers were transformed from velvet to hard horn, and the neck girth had started to expand.

The stags showed rutting behaviour from mid-September to the end of October, when their testicular testosterone concentrations were 1000 times the resting level. All the accessory organs reached their maximal stage of development and the fructose concentrations were highest. The neck girth was maximal, the voice changed to a deep-throated roar, and the urine developed a characteristic rutting odour.

By November, when rutting activity had almost ceased, the testicular testosterone concentrations had declined, the seminiferous tubules had shrunk, and spermatogenesis was much reduced. The accessory organs had also regressed. During the winter and spring, androgenic and spermatogenic activity of the testis became progressively more reduced, while the epididymis remained packed with spermatozoa. In April, the testicular testosterone concentration reached very low levels, spermatogenesis degenerated still further with no division stages maturing beyond primary spermatocytes; the accessory organs became completely involuted, and the antlers were cast.

Even during the period of reproductive quiescence, it is probable that the testis was still secreting some testosterone; the castrated stags grew abnormal antlers, and their accessory organs were more involuted than those of any of the intact stags.

This annual cycle of reproductive activity in the stag was considered to be controlled principally by daylight length, and was associated with changes in the pituitary gland, the adrenal, and the thyroid. There were also pronounced changes in body weight which were a direct consequence of rutting behaviour.