The environment, hormones, and aggressive behaviour: a 5-year-study in guinea pigs.

This paper summarizes persistent consequences of varying rearing and testing conditions on intermale aggression, endocrine responses, and body weight (BW) in guinea pigs. Pairs of unfamiliar males were placed in chronic confrontations for 3-50 days in 2 m2 enclosures. Their behaviour was recorded in more than 1000 h of observation time. About 900 samples were assayed for plasma glucocorticoid (GC), testosterone (T), and norepinephrine (NE) before and at different times after the beginning of the confrontations. The males were reared either in large mixed-sex colonies (CRM) or with a single female (FRM). When FRM males were confronted, high levels of aggressive behaviour occurred and high degrees of stress were found in losers, especially when the environment was familiar and a female was present. In contrast, CRM males confronted in the same situation, but in a nonaggressive way, showed no changes in GC, NE, and BW. These differences between FRM and CRM developed around puberty. A 50-min agonistic experience with an unfamiliar male around this time shifted the subsequent adult behaviour of a FRM to a CRM pattern. These findings suggest a causal relationship between social experiences occurring around puberty, subsequent behaviour as adults, and degree of stress in chronic social encounters.