Bold female zebrafish (Danio rerio) have physiological advantages in reproductive performance.

Research has established that individual behavioral differences (such as the bold-shy axis) can significantly influence reproductive success in various vertebrates. However, there is a notable gap in understanding the physiological mechanisms behind these reproductive outcomes. This lack of research limits our comprehension of how behavioral traits influence reproductive strategies and adaptability via neuroendocrine pathways. The study aimed to investigate the relationship between boldness traits and reproductive physiology in zebrafish (Danio rerio), including the gonadosomatic index (GSI), gonad histology, sex steroid hormones, and expression levels of hypothalamic-pituitary-gonadal (HPG) axis-related genes. Zebrafish were classified into bold females, shy females, bold males, and shy males through the open field test. The results showed that GSI was significantly increased, and a strong non-significant trend was observed for higher plasma 17β-estradiol (E2) concentrations in bold females compared to shy females. No difference in plasma testosterone (T) concentrations had been found between bold and shy males. Bold female ovaries had more mature oocytes, while bold male testes had more mature spermatozoa. The detection of genes related to the gonadotropin pathway, steroidogenesis, and yolk synthesis revealed that significantly upregulated expression levels of gnrh2, cyp19b, hsd17β, fshr, lhr, vtg1, erα, and er2β were observed in bold females. It was found that the expression levels of gnrh2, fshβ, and lhr were significantly upregulated in bold males. These results suggest that bold and shy phenotypes are closely linked to the physiological state of reproduction, establishing a basis for further investigation into the biological significance of boldness in regulating fish reproduction.