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If, in their partner choice, males seek direct benefits (fecund females), the result will be selection for traits indicating female quality rather than for arbitrary (Fisherian) traits. However, the costs of developing and maintaining the sexually selected traits (ornaments) may reduce the resources available to the female for allocation to reproduction and hence result in lower reproductive success per brood. This hitherto unrecognized fecundity cost of sexually selected traits will constrain both the potency of sexual selection mechanisms and the degree of elaboration of sexually selected traits in females, and can also apply to males which invest in their offspring: sexual selection becomes self‐limiting. The fitness implications of these costs are examined for both sexes in a variety of mating and parental care patterns. Sexual selection acting on both sexes may lead to either dimorphism or monomorphism, the latter being the case when the quality indicators chosen by both sexes coincide. Ways of evasion or reduction of these reproductive costs of allocations to sexually selected traits include using different resource components for the ornament and for reproduction, or partitioning the two allocations in time.

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In pipefishes, males brood their offspring on their body or in a brood pouch. In 2 species of pipeflsh, Siphonostorna typhle and Nerophis ophidion, sexual dimorphism, dichromatism, and sex role reversal differed in degree between the species. N. ophidion females were larger than males, whereas in S. typhle sexes were equally large. Furthermore, N. ophidion females have pronounced sexual colourings and possess dorso-ventral skin folds, in contrast to males, whereas sexes in S. typhle look alike. The hypothesis that the males in the more reversed sex dimorphic species, N. ophidion, should invest relatively more energy in their young than the males of the species with smaller differences in sex roles and sex dimorphism was investigated by measuring energy contents in adults and different zygote stages as well as in vitro respiration of zygotes, also of different stages. Our results showed that in both species males provide nourishment to their broods. Contrary to expectation, however, males of N. ophidion invest less energy per zygote than do the females, while in S. typhle the 2 sexes make about equal investments. The strong dimorphism in N. ophidion therefore is likely to have evolved in response to some constraint other than energy investment imposed by males on the female's reproductive success.

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