Keeping normally gregarious rats isolated from their own kind boosts their cancer risk, according to Martha McClintock and her colleagues at the University of Chicago in Illinois.
They kept 20 rats alone and 20 rats in groups of five. All 40 were genetically prone to mammary cancer. The lone rats exhibited a 135% increase in the number of tumours, an 8,391% increase in the size of tumours and a 3.3-fold increase in the relative risk of malignancy compared with those kept in groups.
Isolated rats were also more stressed, anxious, fearful and vigilant. The authors suggest that prolonged exposure to large pulses of the stress-related hormone corticosterone may have contributed to tumour origin and growth.