A specific season of the year in which many animals, including mammals and birds, mate, which ensures that offspring are produced only at a certain time of the year. This timing is important as it enables animals to give birth at a time of the year when environmental conditions and food supply are at their optimum. The breeding season of most animals is in the spring or summer. The stimulus to mate is the result of a photoperiodic response (see photoperiodism), which is thought to be controlled by day length affecting levels of the hormone melatonin. Differences in the timing of breeding season is one isolating mechanism whereby closely related species living in the same area can avoid the risk of mating between individuals of different species.