Evidence for a Spontaneous Ovarian Cycle in Fish of the Genusxiphophorus

The nature of the ovarian cycle in fish of the genus Xiphophorus was studied by using artificial insemination to precisely fix the moment of sperm introduction and by comparing the time to produce primary broods with subsequent broods. 2. The mean time to the production of primary broods was significantly longer than the time to produce subsequent broods. However, there was not a general lag in the production of primary broods but instead they were seen to be born in two peak periods—an earlier one conforming to the single subsequent brood peak, and a later one. 3. The presence of two primary peaks, the fact that the mean number of days to the production of those broods clustered around the first peak was significantly less than that of their subsequent broods, and the lack of the birth of any broods in the time periods immediately following the second peak suggests the presence of a spontaneous ovarian cycle in these fish. 4. Based on the time factors observed in this laboratory, a model cycle was constructed indicating its length (35 days), gestation period (27 days), and length of time fertile eggs were available in the ovary (11 days).