Dimorphic male midshipman fish: reduced sexual selection or sexual selection for reduced characters?

Abstract

In most taxa with male dimorphisms, some males are large in body size with exaggerated secondary sexual characters (exaggerated morph), whereas other males in the same population are small and have reduced secondary sexual characters (reduced morph). What selective pressures cause male dimorphisms? Reduced morphologies may result when a) some males develop a morphology that, in the absence of sexual selection pressures for an exaggerated morphology, reduces energetic and developmental costs and/or b) some males opt for an alternative morphology that does well at an alternative behavioral tactic such as cuckoldry. The 2 mechanisms could act together, but each alone is theoretically sufficient to drive dimorphisms. Here, we tested hypothesis “b” (sexual selection for reduced characters) in the plainfin midshipman fish, Porichthys notatus. Behavioral plasticity between territoriality and cuckoldry in an exaggerated male morph (type I) allows for a direct comparison of cuckoldry by exaggerated morph males to cuckoldry by reduced morph (type II) males. Compared with type I cuckolders, type II cuckolders were able to remain near the nest for longer periods before being chased by the territorial type I male, suggesting that the reduced type II morphology allows type II males to prolong the time before attack by territorial males. Combined with other studies showing a role of sexual selection in maintaining the exaggerated morph, the data support the “sexual selection for reduced characters” hypothesis and elucidate how sexual selection can act in different ways on different males to maintain 2 male morphologies within a single species.