Parental investment, sexual selection and sex ratios

Abstract

Conventional sex roles imply caring females and competitive males. The evolution of sex role divergence is widely attributed to anisogamy initiating a self-reinforcing process. The initial asymmetry in pre-mating parental investment (eggs vs. sperm) is assumed to promote even greater divergence in postmating parental investment (parental care). But do we really understand the process? Trivers [Sexual Selection and the Descent of Man 1871Man -1971Man (1972, Aldine Press, Chicago] introduced two arguments with a female and male perspective on whether to care for offspring that try to link pre-mating and post-mating investment. Here we review their merits and subsequent theoretical developments. The first argument is that females are more committed than males to providing care because they stand to lose a greater initial investment. This, however, commits the 'Concorde Fallacy' as optimal decisions should depend on future pay-offs not past costs. Although the argument can be rephrased in terms of residual reproductive value when past investment affects future pay-offs, it remains weak. The factors likely to change future pay-offs seem to work against females providing more care than males. The second argument takes the reasonable premise that anisogamy produces a male-biased operational sex ratio (OSR) leading to males competing for mates. Male care is then predicted to be less likely to evolve as it consumes resources that could otherwise be used to increase competitiveness. However, given each offspring has precisely two genetic parents (the Fisher condition), a biased OSR generates frequencydependent selection, analogous to Fisherian sex ratio selection, that favours increased parental investment by whichever sex faces more intense competition. Sex role divergence is therefore still an evolutionary conundrum. Here we review some possible solutions. Factors that promote conventional sex roles are sexual selection on males (but non-random variance in male mating success must be high to override the Fisher condition), loss of paternity because of female multiple mating or group spawning and patterns of mortality that generate female-biased adult sex ratios (ASR). We present an integrative model that shows how these factors interact to generate sex roles. We emphasize the need to distinguish between the ASR and the operational sex ratio (OSR). If mortality is higher when caring than competing this diminishes the likelihood of sex role divergence because this strongly limits the mating success of the earlier deserting sex. We illustrate this in a model where a change in relative mortality rates while caring and competing generates a shift from a mammalian type breeding system (female-only care, male-biased OSR and female-biased ASR) to an avian type system (biparental care and a male-biased OSR and ASR).

The evolution of sex roles in birds is related to adult sex ratio

“…It is important to emphasize that mating behaviour, parenting and sex ratios may have more dynamic relationships than currently acknowledged 15,26,27 . First, ASR can affect sex roles (see above), and conversely, reproductive behaviours can also influence mortalities and thus ASR.…”

“…On the one hand, if mortality from care provisioning is high in a population with male-biased or female-biased care, this would reduce the extent of ASR bias in the population. On the other hand, if sexual selection is costly, then this may generate a positive feedback between ASR and sex roles, so that ASR may shift towards more extreme bias 15 . It is conceivable that populations can be locked in an unusual breeding system, because it is the best response to a biased ASR as generated by the breeding system itself.…”

“…Recent theoretical models put breeding-system evolution in a different perspective by showing that ASR (expressed here as the proportion of adult males in the adult population) has a major influence on mating competition, mating systems and parental behaviour 14,15 . These models predict that the rarer sex is under selection to provide less care; for instance, male-biased ASR should facilitate male-biased parental care (henceforth, male care) and thus reversal of conventional parental roles, whereas femalebiased ASR is predicted to favour female-biased care (henceforth, female care) 14,15 .…”

“…These models predict that the rarer sex is under selection to provide less care; for instance, male-biased ASR should facilitate male-biased parental care (henceforth, male care) and thus reversal of conventional parental roles, whereas femalebiased ASR is predicted to favour female-biased care (henceforth, female care) 14,15 .…”

Frequency-Dependent Social Transmission and the Interethnic Transfer of Female Genital Modification in the African Diaspora and Indigenous Populations of Colombia

“…To better understand the evolutionary dynamics responsible for the emergence and persistence of costly traits such as FGMo, Ross et al (2015) derive formal mathematical models from evolutionary anthropological theories linking population sex ratios, gender roles, power dynamics, and marriage markets (Kokko and Jennions 2008) to cultural practices, such as bride price, dowry, and divorce. These models aim to disentangle the forces hypothesized to be responsible for the origins of the FGMo trait (virginity assurance [Buss 1989] and/or costly signaling [Grafen 1990;Zahavi 1975]) from those that cause its persistence in contemporary populations (frequency dependence [McElreath et al 2008]).…”