The chapter discusses the evolution of social behavior in female house mice (Mus domesticus) through the defense of their pups against intruding males. Stephen Gammie and colleagues found that some females were more aggressive than others, with about 60% of the variation attributed to genetic differences. They conducted an artificial selection experiment where they randomly assigned mice to control and selected lines. Over eight generations, the selected line showed significantly more aggressive behavior, spending an average of 15 seconds attacking intruders compared to 3.77 seconds in the control line. The brains of the selected mice also showed altered expression of genes, including neurotensin. The results demonstrate that social behavior can evolve through artificial selection, similar to other traits.The chapter discusses the evolution of social behavior in female house mice (Mus domesticus) through the defense of their pups against intruding males. Stephen Gammie and colleagues found that some females were more aggressive than others, with about 60% of the variation attributed to genetic differences. They conducted an artificial selection experiment where they randomly assigned mice to control and selected lines. Over eight generations, the selected line showed significantly more aggressive behavior, spending an average of 15 seconds attacking intruders compared to 3.77 seconds in the control line. The brains of the selected mice also showed altered expression of genes, including neurotensin. The results demonstrate that social behavior can evolve through artificial selection, similar to other traits.