Variation in nest building, aggression, learning, and steroid hormone levels in Betta splendens

•Three groups of Betta splendens were characterized by nest building behavior.•Nest builders show low hormonal levels, low aggressiveness and good learning.•Non-builders are the most aggressive with higher cortisol levels.•Intermediate builders present low cortisol but high testosterone, and good learning.•Different traits confer variability to a group.