Decrement in "social stress" among wild rattus rattus treated with antibiotic.

"Black" rats, Rattus rattus, were trapped and studied in colonies of 6 adult males and 6 adult females. Eight colonies had antibiotic (Neoterramycin) in their drinking water, and eight (controls) had not. Twenty mated pairs were kept in small cages; of these, ten received Neoterramycin. The only deaths of males were in the control colonies. In the antibiotic colonies there were also (1) a lower incidence of attack when behavior was directly observed, (2) less skin wounding, (3) less macroscopic evidence of lung lesions. The lowest incidence of skin scars and lung lesions was among the rats kept in small cages. There was no evidence of an effect of the antibiotic on body or adrenal weights, or on kidney pathology. The locomotor activity of rats was separately recorded, with or without antibiotic, in artificial automated environments There was no evidence that activity was reduced by antibiotic. It is hypothesized that diseased rats provoke more attack than healthy ones.