| Summary: | Nonmigratory brown trout (Salmo trutta L.) from northern Spain showed no between-sex differences in total length or mass and only moderate, but significant, dimorphism in some morphometric traits. The main dimorphic features were the sizes of the abdomen (larger in females) and head (larger in males, including the upper jaw), relative to each other and also to the post-anal length, the latter being considered a control for body size. Least-squares linear regressions of head length on abdomen length were homogenecus in both the slopes and the intercepts for immature fish of both sexes, but mature males exhibited significantly steeper slopes than mature females. Within each sex the slopes for immature and mature fish were homogeneous, and mature fish of both sexes had higher intercepts than immature ones. Therefore, immature individuals were monomorphic and mature individuals of both sexes diverged from the juvenile pattern, each in a particular fashion: males acquired a greater relative head length (particularly a longer jaw) and females a greater relative abdomen length. Sexual selection for larger head size relative to body size in males and fecundity selection for larger abdomen size relative to body size in females are hypothesized to be important in the evolution of sexual dimorphism in brown trout.
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