The main reason for this is that in most species males potentially reproduce faster than females, with the consequence that males willing to mate outnumber females willing to mate. This surplus of willing male causes competition among the males for females.
Female pipefish compete for males through dominance hierarchies.
In these females, the evolution of sexual signals is constrained by costs of these signals to female condition, survival and fecundity. Fecundity costs are likely to be of utmost importance, but has received very little attention. In internally fertilising nudibranchiates, which are simultaneous hermaphrodites, Anna Karlsson aim to test the idea that ejaculate size is a sexually selected character, signalling quality to the mating partner. Thus, both intersexual selection, in the form of choice for nutritious ejaculates, and intrasexual selection, in the form of sperm competition, may select for signalling ejaculates. In a most cases a secondary sexual character functions both in mate competition and as a cue for partner choice.