Sex Ratio Is Synergistically Modulated by Local Resource Competition and Enhancement: the Case of a Primitively Eusocial Wasp Under Female Philopatry

BackgroundIn animals, the offspring sex ratio is modulated by kin conflict and cooperation, and determining the ratio is a main concern in evolutionary biology. Male competition for access to local mates is predictive of a female-biased sex ratio in the offspring (local mate competition; LMC). Conversely, female competition for access to local resources is predictive of a male-biased sex ratio in the offspring (local resource competition; LRC). However, several factors other than competition should synergistically operate in real-world populations. In the Australian paper wasp Ropalidia plebeiana, LRC and local resource enhancement (LRE) may operate simultaneously. To determine whether this is the case, we evaluated colony sex ratios and examined whether competition and/or enhancement operates at the population level in this species. ResultsIn spring, many foundress queens started their colonies by comb-cutting, in which nest combs from the previous season were divided into several combs to be reused. Genetic relatedness among foundresses did not differ before and after comb-cutting. Relatedness among foundresses was 0.339, whereas relatedness among new foundresses was 0.589, revealing nearly functional monogyny. The global FST value calculated with mtDNA markers was higher than that calculated with microsatellite markers, even after we corrected for differences in effective population sizes between sexes. This finding indicates female philopatry, which was also confirmed by mark–release–recapture before and after the hibernation of new foundresses. The colony sex ratio of reproductives became slightly biased toward males in larger colonies. In addition, both the number of foundresses and number of workers were positively associated with the number of reproductives, which indicates that LRE was also operating.ConclusionsOur results suggest that although the population structure seems to meet the requirements of LRC, the sex ratio is not modulated solely by LRC. Instead, the availability of female helpers at the founding stage likely mitigates the sex ratio predicted by LRC through LRE. Thus, LRC at the founding stage and LRE at the reproductive stage synergistically modulate the colony sex ratio in R. plebeiana.

Sex ratio elasticity influences the selection of sex ratio strategy

There are three sex ratio strategies (SRS) in nature—male-biased sex ratio, female-biased sex ratio and, equal sex ratio depending on the proportion of male offspring being greater than, less than, or equal to ½. The problem was already noted in Darwin’s (1859) “Origin of Species,” and it was R. A. Fisher (1930) who first explained why most species in nature display a sex ratio of ½. Consequent SRS theories such as Hamilton’s (1967) local mate competition (LMC) and Clark’s (1978) local resource competition (LRC) separately explained the observed deviations from the seemingly universal 1:1 ratio. However, to the best of our knowledge, there is not yet a unified theory that accounts for the mechanisms of the three SRS. Here, we introduce the price elasticity theory in economics to define sex ratio elasticity (SRE), and present an analytical model that derives three SRSs based on the following assumption: simultaneously existing competitions for both resources and mates influence the level of SRE in both sexes differently. Consequently, it is the difference (between two sexes) in the level of their sex ratio elasticity that leads to three different SRS. Our analytical results demonstrate that the elasticity-based model not only reveals a highly plausible mechanism that explains the evolution of SRS in nature, but also offers a novel framework for unifying two major classical theories (i.e., LMC & LRC) in the field of SRS research.

Sex ratio elasticity influences the selection of sex ratio strategy

There are three sex ratio strategies (SRS) in nature—male-biased sex ratio, female-biased sex ratio and, equal sex ratio depending on the proportion of male offspring being greater than, less than, or equal to ½. The problem was already noted in Darwin’s (1859) “Origin of Species,” and it was R. A. Fisher (1930) who first explained why most species in nature display a sex ratio of ½. Consequent SRS theories such as Hamilton’s (1967) local mate competition (LMC) and Clark’s (1978) local resource competition (LRC) separately explained the observed deviations from the seemingly universal 1:1 ratio. However, to the best of our knowledge, there is not yet a unified theory that accounts for the mechanisms of the three SRS. Here, we introduce the price elasticity theory in economics to define sex ratio elasticity (SRE), and present an analytical model that derives three SRSs based on the following assumption: simultaneously existing competitions for both resources and mates influence the level of SRE in both sexes differently. Consequently, it is the difference (between two sexes) in the level of their sex ratio elasticity that leads to three different SRS. Our analytical results demonstrate that the elasticity-based model not only reveals a highly plausible mechanism that explains the evolution of SRS in nature, but also offers a novel framework for unifying two major classical theories (i.e., LMC & LRC) in the field of SRS research.

Predicting cross-cultural patterns in sex-biased parental investment and attachment

If parenting behavior influences attachment, then parental investment (PI) theory can predict sex differences and distributions of attachment styles across cultures. Trivers-Willard, local resource competition, and local resource enhancement models make distinct predictions for sex-biased parental responsiveness relevant to attachment. Parental investment and attachment probably vary across cultures in relation to “local fitness currencies” for status, wealth, and well-being.