The normal reproductive cycle of the female prairie dog and the effects of light

Most species of mammals, including tropical species, breed only once a year. The term "breeding season" is used in this paper for that period of the year during which changes occur in the gonads that result in the production of mature gametes. Thus, in the breeding season one finds the development of secondary glands and organs, of sexual behavior, and quite probably of wide metabolic adjustments, although these are not yet well defined. In the female these morphological and metabolic adjustments begin just prior to proestrus and end with the onset of diestrus. The breeding season is quite precise in females of many monestrous species including the prairie dog. It is prolonged in the females of those species with successive estrous cycles during a particular period of the year and in species which exhibit considerable variability in the onset of estrus in the course of the mating season. The breeding season in the male is considered to be the period of active spermatogenesis. It begins with the first maturation changes in the testis tubule, includes the production of mature sperm and the secretion of the accessory glands and ends with the first processes of degeneration of spermatocytes in the tubules. In the male, the breeding season is much less precise since spermatogenic processes begin earlier and end later than the changes of the female. The males of some species in which the female is a seasonal breeder apparently exhibit spermatogenesis at all seasons of the year. Reproductive activity is initiated in some animals by environmental changes which precipitate the liberation of hormonal secretions. It appears, however, that in some species the onset of the breeding season is controlled by a genetic mechanism which produces hormonal changes regardless of the environment. When such a genetic mechanism is present, the onset of the breeding season is correlated with optimal environmental conditions for the young established by natural selection. The physiological mechanisms in either environmental or genetically induced breeding seasons are probably quite similar and both presumably involve the nervous system. Harris ('55) has suggested some afferent stimuli which could activate neural secretions effecting pituitaq gonadotrophic secretions. As pointed out by Mossman and Judas ('49), few genera have been studied in the group of annual seasonal breeders. Most of the investigations have been based entirely on animals collected in the field. Although such studies may be very complete, for example, Deanesly and Parkes ('33) and Allanson ('33) on the gray squirrel, an experimental analysis of factors involved in the breeding season requires a laboratory colony. Studies using both field and laboratory populations have been made on only 5 species: the ground squirrel, the ferret, the male pocket gopher, and the prairie dog. It is on these few studies that our knowledge of annual breeding rests and they have failed to consider some aspects of reproductive physiology. The most complete studies are those on the ferret ( Allanson, '32; Bissonnett, '32; Marshall, '36) and the ground squirrel (Moore et al., '34; Wells, '35; Nelson, '36; Wells and Overholser, '40; Wells and Zalesky, '40; Simmons, '44a,b). The relationships of