Temporal Organization in Avian Reproduction

Abstract

Recent studies have demonstrated that biological rhythms have important roles in avian reproduction. In the photoperiodic stimulation of the reproductive complex, there are two systems involved in the interpretation of day length. One system is entrained by the photoperiod, probably by dawn. This entrained system in turn entrains two light-sensitive phases which occur later in the day. If the photoperi od is long enough, it is present during the sensitive phases when it can induce the production of luteinzing hormone and follicle stimulating hormone. The interrenal gland appears to be a part of the entraining system inasmuch as injections of corticosterone can set a sensitive phase when light can induce gonadal development. The annual cycle of photosensitivity and photorefractoriness appeals to be controlled by the temporal relations between the daily rhythms of corticosterone and prolactin which change seasonally. When the hormonal relations of photosensitive and photorefractory birds are simulated by injections of the hormones, the appropriate conditions (photosensitivity or photorefractonness) can be produced. Seasonal changes in the hormonal relations are not direct reflections of changes in the photoperiod; they depend on more complex physiological experiences. The systems involved in egg laying and parental behavior may also have a temporal basis of biological rhythms. For example, the intensity of the pigeon cropsac response depends on the time of daily injections of prolactin relative to those of corticosterone. It is apparent that the daily rhythm constitutes the basic structuial unit in the temporal organizationtion of avian reproduction.