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How do seasonal breeding cycles affect ovaries in many vertebrate animals?

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Detailed cross-sectional illustration of a human ovary showing the stages of follicle development, ovulation, and corpus luteum formation, with labeled structures including primordial, primary, secondary, and mature follicles, a ruptured follicle releasing an ovum, and corpus luteum and albicans. An inset at the bottom shows the ovary's location in the female reproductive system. — ovary The steps of ovulation, beginning with a dormant primordial follicle that grows and matures and is eventually released from the ovary into the fallopian tube.

In many vertebrate animals, seasonal breeding cycles exert a profound influence on the physical state and function of the ovaries. Because reproduction is often coordinated with the seasons to ensure that offspring are born during the most favorable environmental conditions, the ovaries undergo significant cyclical changes in size and activity.1

During the breeding season, the ovaries typically enlarge as they mature and produce gametes in response to rhythmic stimulation by gonadotropins secreted by the pituitary gland. In species such as seasonally breeding fishes and amphibians, this growth can be dramatic, with the ovaries distending the body wall as they fill with eggs. Conversely, between breeding seasons, the ovaries diminish in size and become relatively inconspicuous, often consisting primarily of oogonia and immature oocytes.23

These cycles are frequently regulated by photoperiodism, where changes in the duration of daylight serve as an environmental cue that triggers the hormonal pathways governing ovarian development. While temperature and food supply also play roles, the rhythmic environment acts as a primary regulator of gonadal activity across many vertebrate species.43