(in animals), the time of year when mating occurs. The breeding period in many vertebrates is characterized by the development of secondary sexual characteristics in males and the occurrence of special forms of behavior, such as “courtship” of the female by the male and fierce fights between males. In nontropical regions the period has a sharply defined seasonal character. The development of sex glands and the accompanying phenomena are realized on the basis of the internal physiological rhythm of the organism controlled by the influence of external factors (photoperiod, temperature, and so on). In the tropics, reproduction in many animals is not strictly periodic; in arid tropical regions the beginning of the breeding period is determined by the onset of the rainy season. In the majority of invertebrates, reproduction is not accompanied by ritual forms of behavior, and the season of reproduction may only conditionally be called a breeding period.
I. A. SHILOV