Seasonal Reproductive Events Related to the Pineal Gland

Abstract

Under natural environmental conditions reproduction, and especially birth of the young in mammals, is carefully regulated so that these events coincide with the seasons of the year which will most likely ensure the propagation of the species. The most important physiological event in the annual reproductive cycle is obviously the actual delivery of the young. To optimize the survival of the newborn, animals in their natural habitat characteristically deliver their young in the spring or early summer. It is at these times that environmental temperatures and food availability are such that the young will have the maximal chance of survival. In order for these species to ensure spring delivery of the newborns they must synchronize their breeding habits accordingly. Obviously, if the animals were sexually competent the entire year they would breed indiscriminately with regard to season. Thus, evolution has provided them with a system to restrict their reproductive habits to specific times of the year. In many mammals it is the environmental photoperiod which determines their reproductive capability; when this is the case, the pineal gland serves as the intermediary between the changing daylengths and the neuroendocrine-reproductive system. Dependence on the photoperiodic fluctuations allows animals to not only be in the proper sexual condition at a given time but to actually anticipate the upcoming season and to make the necessary physiological adjustments. In the absence of the pineal gland photosensitive species simply cannot make the necessary alterations and, consequently, they become continual as opposed to seasonal breeders (Reiter, 1973/74). This affords them no advantage under natural conditions and, if it persisted, the species as a whole could become extinct. The data illustrating photoperiodic-pineal-reproductive interactions in rodents are summarized below.