In behavior changes the internal mechanisms, both endocrine and nervous, are, of course, at work. External controlling factors are also becoming known with increasing accuracy. Two domestic species, the ewe and the mare, are seasonal breeders. The reader wiU find here a discussion of the fact that the sexual season is induced in the ewe by withdrawal of light, and in the mare by increased hght, as had been previously found in birds. The role of temperature is also definite, for it may delay the onset of the ewe's sexual season, cooling hastening this event. The endocrine changes in pregnancy have led, as is well known, to dramatic discoveries, for example, that of the very high hemal titers in female sex hormone in the pregnant mare (subsequently amazingly found in the stallion) and in rehable pregnancy tests. Perhaps nothing is stranger than the similarity between the mare and women in high pregnancy estrogen and gonadotropin titers. In the mare, unlike the situation in women, the gonadotropin appears to be secreted by maternal structures called endometrial cups and not by the chorionic tissue of her offspring, although estrogens are secreted by the chorion in both forms. Explanations for the sudden great oveφroduction of these gonadotropins are not at hand.
The tremendous importance of artificial insemination in animal breeding has served as a stimulus for intensive research on spermatogenesis, on the biochemistry of semen, and on the factors influencing sexual libido and sperm production. It is perhaps in this area that some of our knowledge in domestic animals compares favorably with, and m some instances suφasses, that available in laboratory animals.
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