Life-span studies of dominance and aggression in established colonies of laboratory rats.

Six mixed-sex colonies of Long-Evans rats were observed at 100-day intervals from colony formation at 100 days of age until all colony members died. Long-term stable dominance relations were observed among males in four colonies while two colonies which had low initial levels of aggression continued to show low intracolony conflict at all ages and no clear dominance relationships. Agonistic interactions among females and between males and females were relatively infrequent and no dominance hierarchy among females was apparent. The aggression of resident males toward intruders increased from 201-601 days of age, but declined overall at age 701. Nevertheless, older males which fought intruders did so as readily as they had when they were young and the few animals that reached 800 days of age continued to attack intruders. Although no systematic decline in total duration or intensity of offense was found across age, there was some evidence of impaired motor performance by older males.