Urinary odors and size protect juvenile laboratory mice from adult male attack

For the portion of their ontogeny, the juveniles of many mammalian species appear resistant to aggressive attack by adult male conspecifics. The possibility that urinary odors and small size contribute to this immunity from attack was investigated with C57/Bl, A/J, and DW/J laboratory mice. The results of the 1st of 6 experiments were that juvenile mice of both genders evoked great curiosity from aggressive resident male adults, but juveniles were rarely attacked. In Experiments II and III, exchanging urine between juveniles and adult males suggested that juveniles possess a distinctive odor, In Experiments, IV-VI, dwarf adult, normal-size adults, and juveniles were placed with aggressive adult male residents. The dwarf males were attacked, but not as severely as normal-sized male intruders (Experiment IV). The dwarf males, however, evoked more aggressive behavior than juveniles (Experiment V), and juveniles with the odors from either normal-sized or dwarf adult males were attacked more readily than non-odorized juveniles (Experiment VI). These data suggest that both odor and size contribute to the juveniles' immunity from attack.