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

Abstract

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.