Socially induced renal pathology of captive wild rats (Rattus villosissimus)

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.