It has been known for some time now – especially as a result of research in guinea pigs – that the social environment during adolescence, i.e. the period from late childhood through puberty into adulthood, has a considerable impact on how individuals behave in later life. Male guinea pigs, for example, which grew up only with a female during this period, are particularly aggressive towards unfamiliar males. Behavioural biologists at Münster University have now been able to demonstrate for the first time that males are still able to adapt their hormone systems to changes in their social environment in adulthood.