Wild rats, Rattus norvegicus, (a) trapped as adults or (b) of the second generation in captivity (lab‐wild), and domestic rats of two strains, were studied for 28 days in artificial colonies in large cages with attached nest boxes. Controls were kept in mated pairs in small cages. Each colony consisted of six males and six females. The interactions of the males in six colonies of trapped rats were highly “stressful;” 61% died; and most of the survivors lost weight and had greatly enlarged adrenals. In each colony, however, there was a male (an alpha) that gained in weight and spent much time, during the dark hours, in the open on the floor of the cage; and in three colonies there were also other males (“betas”) that gained in weight. The adrenals of alphas and betas weighed about the same as those of the controls. In one of the 12 colonies of domestic rats one male behaved like a wild male; but in the other colonies the males gained in body weight and their adrenal weights resembled those of the controls. In three colonies of lab‐wild rats 22% of the males died, but there was no evidence of males of different status. The findings confirm that the “agonistic” behavior of domestic rats is usually much attenuated in comparison with that of the wild type; a number of methodologic implications are discussed.
Domestic rats, Rattus norvegicus, aged nearly one year, were studied in artificial colonies in large cages with attached nest boxes. As in previous experiments on younger domestic rats, but not those on wild rats, the colonies were peaceful. Questions concerning the “aggressive” or “agonistic” behavior of domestic rats are further discussed.
Long‐haired rats, Rattus villosissimus, were studied in large cages. Groups of adult rats (each of 3 males, or 1 male and 2 females) were observed during intermittent encounters with a male intruder for up to 9 days. Two further groups, each of 8 males and 8 females, were maintained for 70 days without introduction of intruders. Controls were kept in small cages in which intolerant behavior was rare. Behavior during attack resembled that of male R. norvegicus, but social relationships were less stable. Only observations on males are described in detail.
Some rats collapsed under attack, though unwounded, and died when not removed. Collapse occurred sometimes after a few hours, but sometimes after many days of exposure. Exposure to attack was accompanied by a decline in body weight and by some adrenal hypertrophy. Two kinds of renal pathology are described: focal glomerular hypercellularity (FGH), probably due to glomerulonephritis, and dilated distal convoluted tubules. Neither condition occurred in the controls. FGH occurred in 3 of 12 rats (25%) that remained apparently healthy during the 9 days of continuous exposure, in 21 of 23 intruders (91%) that were exposed to intermittent attack over 9 days or less without becoming debilitated, and in 8 of 11 such rats (73%) that collapsed. All rats examined from 70‐day colonies had FGH, whether collapsed or not. Dilated tubules occurred in 6 of 32 intruders (19%) exposed to intermittent attack, and in 2 of 6 animals that collapsed during 70 days of exposure. Renal pathology, especially glomerulonephritis, was therefore a correlate of social intolerance; but there was no evidence that it was a significant cause of death.
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