Structural homeostasis, functional coordination, and spontaneous ovulation in the Giant panda during the...

The periovulatory period is the most critical window for successful conception in the giant panda (Ailuropoda melanoleuca); however, the in vivo structural dynamics of the reproductive tract during this phase-specifically, the temporal patterns of endometrial thickness, ovarian volume, and follicular development relative to the estrogen peak-remain poorly characterized. To address this gap, this study combined noninvasive endocrine monitoring from 23 adult females across five breeding seasons (2021-2025). We demonstrate this period is defined not by linear, hormone-driven morphological changes but by four key features: (i) Structural homeostasis: The endometrial thickness and ovarian volume remained stable across the -10 to 66-hour window relative to the estrogen peak, with no significant linear association with time or concurrent hormone levels. (ii) Spontaneous ovulation: Direct laparoscopic evidence from one individual confirmed ovulation at ~48 hours post-peak without mating stimulation. (iii) Coordinated follicular development: A positive trend between the follicle count and size heterogeneity challenges the conventional resource competition model. (iv) Primacy of interindividual variation: Time phase accounted for only 9.0% of the variance in follicle number (η2 = 0.090), with the remaining 91.0% attributable to other sources of variation, including interindividual differences. This substantial unexplained variance, combined with large absolute differences among individuals at similar time points, indicates that interindividual variation is a major source of phenotypic variance. These findings shift the focus from a linear, hormone-centric model to an integrative framework grounded in individual biological identity, providing a basis for precision conservation breeding tailored to each female's unique reproductive phenotype.