Communal reproductive success in rats (Rattus norvegicus): effects of group composition and prior social experience.

We compared the reproductive success and maternal behavior of sibling pairs of female Long-Evans rats (Rattus norvegicus) housed together from birth (familiar) to that of pairs of unrelated females housed apart during development (unfamiliar). Sires either remained in the colonies through weaning of their pups or were removed before parturition. Familiar animals reared more pups to weaning, were more likely to share in caring for their pups, and were less likely to exhibit infanticide than were unfamiliar ones. The presence of males in cages with pups had no direct effect on the reproductive success of females, but female pairs housed with males spent less time than female pairs housed alone caring for pups together in a combined nest. Conflicting evidence for communal rearing in populations of wild rats may reflect differences in the genetic relatedness or early social experience of female rats.

Control of human aggression. A comparative perspective.

Recent animal research has demonstrated that humans are not a uniquely aggressive species and that even in so-called violence-prone animals, aggression is always an optional strategy. Although some form of intraspecific aggression exists in every vertebrate species studied thus far, it is also true that all organisms have coevolved equally potent inhibitory mechanisms that enable them to use an aggressive strategy selectively or to suppress aggression when it is in their interest to do so. Parallel studies of aggression in children, assaultive adults, and even entire societies have suggested that humans are exquisitely sensitive to subtle social controls that could be used to reduce the frequency of individual acts of violence.

Postweaning social experience and adult aggression in rats.

One group of male rats was reared continuously in small, stable colonies of three animals each during a ten-week period beginning when they were 46 days old. A second group of animals was also reared in small groups of three animals each but membership within these colonies was randomly changed at weekly intervals. Thereafter, all subjects were housed in individual cages. A 24 hr test of the subjects' aggression toward an unfamiliar male conspecific at 130-133 days of age indicated that only the heaviest animal in each of the stable colonies was aggressive. In contrast, all animals in the random-composition group were aggressive toward intruders. Scores on a second test of intruder-directed aggression administered when the subjects were nine months old were significantly related to scores on the first aggression test. Subtle variations in social experience during development are critical in establishing long-term patterns of aggression toward conspecifics in laboratory rats.

Intermale aggression of subordinate resident long-evans rats.

Dominant male rats were separated from their colonies and subordinate residents were tested for aggression after a two week period. Subordinate animals showed significant increases in body weight and aggressive behavior toward intruders. Replacement of the previously dominant male led to rank reversals in three of six colonies. To assess whether two weeks of separation from the dominant male was necessary for subordinate males to exhibit aggression towards conspecifics, another intruder test was given following an additional two weeks of social housing. At the conclusion of this test, the more aggressive resident was removed and a naive intruder was immediately introduced into the colony with the subordinate male resident and again at 1, 5, and 10 day intervals. Aggressive posturing by the subordinate resident increased immediately following the removal of the dominant male, and remained at a high level throughout the subsequent intruder tests. In contrast, biting was initially low but increased steadily throughout the period of separation. These findings demonstrated that social or situational changes within a colony can greatly influence the aggression of subordinate males.

Intermale and maternal aggression in adult rats tested at different ages.

In Experiment 1, rat colonies consisting of two males and one female were established when the animals were either 100, 200, or 300 days old. All colony members were reared from weaning in small, stable isosexual groups prior to colony formation. Males that were 200 days old engaged in significantly more fighting at colony formation, and, three weeks later, were more likely to wound an intruder during a 24 hr aggression test than either younger or older animals. These findings demonstrate that colony aggression in male rats varies markedly as a function of age when reared under housing conditions that are employed in most laboratories. In Experiment 2, colony females were individually housed after their second pregnancy and their response to a male intruder was evaluated for a 24 hr period after nine days of lactation. Maternal aggression occurred at all ages, indicating that age may be of lesser importance in nest defense of females than intermale conflict. However, maternal aggression was ineffective in preventing the destruction of a high proportion of the litters by intruders in the three maternal age groups.

The effects of prenatal psychological stress on the sexual behavior and reactivity of male rats.

Male offspring of prenatally stressed rats showed low levels of copulatory behavior during a series of brief tests with estrous females but successfully impregnated female cagemates during long term tests of breeding effectiveness. Sexual performance on the short term tests was inversely related to open field activity with the offspring of prenatally handled mothers exhibiting more sexual behavior and less open field activity. The results support the notion that prenatal stress influences both the sexual behavior and emotional reactivity of male offspring. Deficits observed during short term tests of breeding effectiveness appear to be reversible under optimal conditions.