Biological control of reproduction and fertility is a normal part of the life history of all organisms. Control mechanisms allow timing of reproduction with respect to age, time of day, season, and other periodic environmental events. Further modulation can occur with variations in temperature, rainfall, nutrition, and health status. Interactions with other members of the species, ranging from pherohormonal stimulation of estrus to social delay of puberty and breeding to infanticide, provide further constraints upon fertility and recruitment. All of these processes ultimately act through molecular neuroendocrine mechanisms that are under genetic control and are subject to natural selection and evolutionary change. The basic neuroendocrine pathways are present in all vertebrates and share common neuroendocrine mechanisms. These pathways are subject to external intervention and interruption in individuals.