Aggressive behavior, brain size and domestication in clonal rainbow trout lines.

Domestication causes behavior and brain size changes in many species. We addressed three questions using clonal rainbow trout lines: What are the mirror-elicited aggressive tendencies in lines with varying degrees of domestication? How does brain size relate to genotype and domestication level? Finally, is there a relationship between aggressive behavior and brain size? Clonal lines, although sampling a limited subset of the species variation, provide us with a reproducible experimental system with which we can develop hypotheses for further research. We performed principal component analyses on 12 continuous behavior and brain/body size variables and one discrete behavioral variable ("yawn") and detected several aggression syndromes. Two behaviors, "freeze" and "escape", associated with high domestication; "display" and "yawn" behavior associated with wild lines and "swim against the mirror" behavior associated with semi-wild and domestic lines. Two brain size traits, total brain and olfactory volume, were significantly related to domestication level when taking total body size into account, with domesticated lines having larger total brain volume and olfactory regions. The aggression syndromes identified indicate that future QTL mapping studies on domestication-related traits would likely be fruitful.