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Hormonal regulation of reproduction and the antler cycle in the male Columbian black-tailed deer Odocoileus hemionus columbianus

University of British Columbia

The hormonal regulation of reproduction and the antler cycle in male Columbian black-tailed deer (Odocoileus hemionus columbianus) was investigated by measuring serum testosterone, testis volume, sperm production and the antler growth cycle of wild deer and of captive deer treated with methallibure and various hormones. A histological examination of the pituitary, adrenals, thyroids, testes and accessory sex glands of normal and methallibure-treated deer was also performed to study the functional relationships of these organs to reproduction and antler growth. Various constituents in the serum and urine of normal and methallibure-treated deer were measured to investigate the effects of methallibure on physiological function. Testosterone secretion, testis volume, sperm production, and the secretory activity of the accessory sex glands were maximal in November, at the height of the reproductive season. During this period the mean serum testosterone level of the adult males was 10 ng/ml, testis volume averaged 30 cm³, and the concentration of sperm in the semen was 100 x 10⁶ to 700 x 10⁶/ml. In winter, the activity of the reproductive organs declined, until a minimum was reached in February or March. The antlers were cast several weeks after the serum testosterone dropped below 1 ng/ml. In spring, a significant increase in spermatogenetic activity occurred, coincident with the initiation of antler growth. Spermatogenesis declined in June and July, but the seminiferous tubules and accessory sex glands were still more active than in late winter. The serum testosterone level however, remained low (<1 ng/ml) throughout the antler growing period. In late August or early September the testosterone level rose above 1.5 ng/ml, and velvet shedding ensued. Sperm production and the secretory activity of the seminal vesicles Increased markedly in the fall. The concentration and viability of sperm was greatest between October and December, although some captive deer produced sperm all year round. These deer also had a higher serum testosterone level, and remained in rut longer than deer in the field. When methallibure, a non-steroidal inhibitor of gonadotrophin secretion, was applied in June, it suppressed reproduction and arrested antler growth. PMS and HCG were capable of stimulating the testes of methallibure-treated deer to secrete enough testosterone to induce velvet shedding, but neither hormone by itself was effective in completely restoring reproductive function. When methallibure was applied in April, antler growth was prevented. The subsequent administration of prolactin, PMS, and some androgenic steroids during June and July failed to stimulate antler growth, but HCG did rejuvenate it in one instance. During the normal period of antler growth, the testes of methallibure-treated deer did not respond to the exogenous administration of HCG by secreting testosterone, whereas both PMS and prolactin were effective in this respect. After the administration of PMS, HCG was capable of stimulating testosterone production, but it was ineffective in the deer previously treated with prolactin. Also, when methallibure treatment was terminated in the fall, testosterone production and spermatogenesis recovered, but the deer that had previously received prolactin did not produce mature sperm until the following spring. Thus, the testicular response may depend on the temporal sequence as well as the type of hormonal stimulation. The application of histochemical stains to the anterior pituitary revealed seven cell types, most of which, underwent cyclic changes that could be related to a seasonal pattern of functional activity. Hormonal release by the gonadotrophic cells was judged to be greatest in the fall, but some activity was also evident in spring and summer. The gonadotrophs of methallibure-treated deer were small and chromo-phobic, whereas other cell types appeared to be affected little or not at all. The results of this study support the hypothesis that a gonadotrophin is responsible for stimulating antler growth.

Text

2010-02-06

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http://hdl.handle.net/2429/19737

Zoology

University of British Columbia

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