The costs of reproduction can and do differ between the sexes.

BACKGROUND: Measuring costs of male versus female reproduction in cosexual species is challenging because the currency and timing of allocation can differ between the two sexual functions. In contrast, costs of male versus female reproduction can be measured indirectly in dioecious species in terms of sex-specific life-history trade-offs with growth and survival. Yet despite abundant evidence for differences in life history between males and females, there remains confusion over how such differences should be interpreted.

SCOPE: Here, I address misconceptions in interpreting potential differences in the costs of reproduction between the sexes, drawing attention to the relevance of: (1) theories of sex-allocation versus life-history evolution; and (2) observations of sex-ratio variation.

KEY RESULTS: Sex-allocation theory predicts a mother's investment in sons versus daughters and is thus relevant to primary sex ratios at the seed stage. Life-history theory is relevant to trade-offs between, for example, reproduction and survival, and is thus relevant to secondary sex ratios of adults affected by sex-biased mortality. The preponderance of species with male- in comparison to female-biased secondary sex ratios points to a frequently greater cost of reproduction for females.

CONCLUSIONS: Male and female costs of reproduction often differ, but there remain unanswered questions about why one sex (most often the female function) should often be more expensive than the other. A correct understanding of theoretical predictions will help future research to address such questions.