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.

Male rat behavior, endocrinology and reproductive physiology in a mixed-sex, socially stressful colony

Previous research suggests that social stress in the absence of females disrupts male reproductive functioning. The presence of females often increases intermale aggression and, presumably, the probability that fighting----social stress----hyperadrenalism----reproductive disruption. Colony membership of male rats was manipulated in the present research to provoke high or low aggression and the consequent environments that were characterized as high or low socially stressful. The principle comparisons were between all-male and mixed-sex colonies. Results were that the presence of females predictably increased aggressive behavior in both high and low stress environments, yet the adrenal response was different in the two environments. When females were present, adrenal weights and circulating corticosterone levels of males increased in the low stress setting but decreased in the high stress setting. Males cohabitating with females in both environments, on the other hand, experienced elevated titers of circulating testosterone, increased activity of various androgen-sensitive tissues and greater epididymal sperm reserves. The conclusion is that the profound changes in males with sexual contact can attenuate the stress----reproductive disruption relationship.

The effects of age and strain on aggression in male rats

Aggressive behaviors were compared for male Fischer-344 and Long-Evans rats formed into mixed-sex colonies at 100, 200, or 300 days of age. Within each strain, male aggression did not differ across these ages, nor did it differ for an additional group of 600-day old Long-Evans males: both the form and the incidence of specific aggressive and defensive behaviors remained relatively constant over an age range representing nearly the entire normal life span for this species. However, Fischer-344 males at all ages were reliably lower in aggression than the Long-Evans rats. The specific aggression components seen in the Fischer-344 rats tended to be nondamaging, resembling "play fighting" more than the tissue damaging attack behaviors typical of other strains. Despite the frequent use of this strain in gerontological research, such low levels of aggressiveness counterindicate their use in research in aging and aggression.

Social genetics

Most behavior is expressed within social systems, and the genetic analysis of its variance therefore presents theoretical and technical problems that have been sidestepped in most previous research. The dog presents obvious advantages for studying behavioral interactions between genotypes. Two sets of data are summarized that indicate that the magnitude of genetic differences is related to the differentiation of social roles in a competitive situation, whereas similarity of genotypes is an advantage in a situation demanding coordinated activity. Issues concerning the use of correlational techniques to study social behavior are raised.