Induced vs Spontaneous Ovulation: Integrating KNDy Circuitry, Sensory Cues, and Ecological Context

Ovulation links folliculogenesis to fertilization and pregnancy, yet vertebrates have evolved contrasting ovulatory strategies that reflect ecological and social pressures. Spontaneous ovulators rely on internally generated cyclic signals, whereas induced ovulators couple follicular rupture to external stimuli such as copulation or social cues. Despite these differences, both strategies converge on a conserved hypothalamic-pituitary-ovarian cascade in which a GnRH-driven preovulatory LH surge coordinates oocyte maturation, follicle rupture, and corpus luteum formation. Arcuate Kisspeptin/Neurokinin B/Dynorphin (KNDy) neurons function as the GnRH pulse generator, while anteroventral periventricular nucleus/Preoptic Area (AVPV/POA) neurons are the GnRH surge generator by integrating estradiol-mediated positive feedback to gate the timing and amplitude of this surge. This review aims to contrast spontaneous and induced ovulation across vertebrates, emphasizing how both strategies converge on a common final pathway. The review then examines how diverse sensory and environmental cues are wired into this core axis in taxa such as camelids, rabbits, cats, honeycomb groupers, most birds, and zebrafish. Finally, ecological and social contexts are considered, highlighting how ovulation mode shapes paternity monopolization, mating systems, and the adaptive balance between male reproductive success and female control over fertilization.