Unravelling the origins of boldness: A common garden experiment with cave fish (Barbatula barbatula)

Many animals show an aversion to bright, open spaces, with significant variability seen across species, populations, and individuals within populations. Although there is much interest in the underlying causes of this behaviour, few studies have been able to systematically isolate the role of heritable and environmental effects. Here we addressed this gap using a common garden experiment with cave fish. Specifically, we bred and cross-bred cave loaches ( Barbatula barbatula ) in the lab, raised the offspring in complete darkness or normal light conditions, and studied their light avoidance behaviour. Cave fish spent much more time in a light area and ventured further out, while surface fish spent considerable time on the edge between light and dark areas. Hybrids behaved most similarly to cave fish. Light treatment and eye size and quality only had a modest effect. Our results suggest light avoidance behaviour of cave fish has a heritable basis and is fundamentally linked to increased boldness rather than reduced vision, which is likely adaptive given the complete lack of macropredators in the cave environment. Our study provides novel experimental insights into the behavioural divergence of cave fish and contributes to our broader understanding of the evolution of boldness and behavioural adaptation. ### Competing Interest Statement The authors have declared no competing interest.