Boldness covaries with growth and reproductive traits in female zebrafish

Abstract Life-history theory posits that organisms allocate limited resources among competing demands. For fish with indeterminate growth, the trade-off between reproduction and growth is particularly important, as reproductive investment may directly constrain future somatic growth. However, little is known about how individuals with different boldness traits are associated with life-history traits. This study combined morphological and molecular physiological measures to explore the link between boldness and reproductive investment and somatic growth in female zebrafish (Danio rerio). Female boldness had a significant inverted U-shaped relationship with body length, was unrelated to relative body mass, and was positively correlated with relative abdominal area. After controlling for similar body mass differences, bold individuals exhibited significantly greater body depth, a higher body depth-to-body length ratio, increased abdominal area, and a higher gonadosomatic index (GSI) compared with their shy counterparts, along with elevated expression of mstnb (myostatin b) in muscle. Conversely, shy individuals showed higher expression levels of genes related to fatty acid oxidation, mitochondrial biogenesis, and lipid storage, specifically pparα, cpt1b, pgc1α, and pparγ. These findings highlight a systematic phenotypic differentiation between boldness types in the allocation of resources to reproduction and growth traits. Bold individuals tend to favor current reproductive investment, while shy individuals exhibit enhanced somatic growth and muscle metabolic features. This pattern aligns with predictions from life-history trade-off theory and may be connected to broader ecological functions in fish. Future studies should consider integrating interdisciplinary approaches to further investigate the mechanistic connections between fish personality and life history.