Plasma concentrations of sex steroids and fighting in male Tilapia zillii

In laboratory‐held Tilapia zillii the gonadosomatic index ( I G ) is an accurate predictor of dominance in pairwise fights between males. Immediately after fighting no differences were detected between winners, losers or controls, in plasma concentrations of testosterone (T) or 11‐ketotestosterone (11KT). 11KT was correlated negatively with I G and positively with T. 11KT appeared to be the major androgen in this species, but there was no evidence that simple differences in levels of these sex steroids mediated aggressive behaviour.