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Reformulation of Trivers-Willard hypothesis for parental investment.

ABSTRACT: The Trivers-Willard hypothesis (TWH) plays a central role in understanding the optimal investment strategies to male and female offspring. Empirical studies of TWH, however, yielded conflicting results. Here, we present models to predict optimal comprehensive multi-element parental strategies composed of primary sex ratio, brood size, resource allocation among offspring, and the resultant secondary sex ratio. Our results reveal that the optimal strategy depends on sex differences in the shape of offspring fitness function rather than in fitness variance. Also, the slope of the tangent line (through the origin) to the offspring fitness function can be used to predict the preferred offspring sex. We also briefly discuss links between the model and the empirical research. This comprehensive reformulation of TWH will offer a thorough understanding of multi-element parental investment strategies beyond the classical TWH.

SUBMITTER: Choi J

PROVIDER: S-EPMC9018816 | biostudies-literature | 2022 Apr

REPOSITORIES: biostudies-literature

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Reformulation of Trivers-Willard hypothesis for parental investment.

Choi Jibeom J Roh Hyungmin H Lee Sang-Im SI Kwon Hee-Dae HD Kang Myungjoo M Jablonski Piotr G PG

Communications biology 20220419 1

The Trivers-Willard hypothesis (TWH) plays a central role in understanding the optimal investment strategies to male and female offspring. Empirical studies of TWH, however, yielded conflicting results. Here, we present models to predict optimal comprehensive multi-element parental strategies composed of primary sex ratio, brood size, resource allocation among offspring, and the resultant secondary sex ratio. Our results reveal that the optimal strategy depends on sex differences in the shape of ...[more]

PMID: 35440689

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Project description:The Trivers-Willard hypothesis (TWH) states that parents in good condition tend to bias their offspring sex ratio toward the sex with a higher variation in reproductive value, whereas parents in bad condition favor the opposite sex. Although the TWH has been generalized to predict various Trivers-Willard effects (TWE) depending on the life cycle of a species, existing work does not sufficiently acknowledge that sex-specific reproductive values depend on the relative abundances of males and females in the population. If parents adjust their offspring sex ratio according to the TWE, offspring reproductive values will also change. This should affect the long-term evolutionary dynamics and might lead to considerable deviations from the original predictions. In this paper, I model the full evolutionary dynamics of the TWE, using a published two-sex integral projection model for the Columbian ground squirrel (Urocitellus columbianus). Offspring sex ratio is treated as a nonparametric continuous function of maternal condition. Evolutionary change is treated as the successive invasion of mutant strategies. The simulation is performed with varying starting conditions until an evolutionarily stable strategy (ESS) is reached. The results show that the magnitude of the evolving TWE can be far greater than previously predicted. Furthermore, evolutionary dynamics show considerable nonlinearities before settling at an ESS. The nonlinear effects depend on the starting conditions and indicate that evolutionary change is fastest when starting at an extremely biased sex ratio and that evolutionary change is weaker for parents of high condition. The results show neither a tendency to maximize average population fitness nor to minimize the deviation between offspring sex ratio and offspring reproductive value ratio. The study highlights the importance of dynamic feedback in models of natural selection and provides a new methodological framework for analyzing the evolution of continuous strategies in structured populations.

Project description:Study questionIs there a negative relationship, as predicted in the Trivers-Willard hypothesis (TWH), between the intensity of maternal stress and sex ratio at birth (SRB)?Summary answerUsing a comprehensive data set with multiple indicators of maternal stress, most measures of stress show no statistically significant association with SRB over a period spanning 243 years, indicating no support for the TWH.What is known alreadyEvolutionary biologists have proposed a widely discussed hypothesis that women in poor and stressful conditions during pregnancy are more likely to give birth to girls, and exposure to stressful events may therefore lead to a reduction in sex (male-to-female) ratio at birth. The empirical evidence so far is mixed.Study design, size, durationAnnual time series data, spanning 243 years between 1749 and 1991 for Sweden at the national level, were drawn from multiple sources. The outcome is defined as the percentage of male births relative to all births in Sweden in a given year. The covariates include a set of economic and climatic variables as proxies for maternal stress.Participants/materials, setting, methodsWe conduct a series of ARIMA (autoregressive integrated moving average) models to examine the relationship between maternal stress and SRB during three periods: 1749-1991, 1749-1861 and 1862-1991.Main results and the role of chanceIn 1749-1991, economic proxies for maternal stress showed no statistically significant association with SRB. In 1749-1861, two indicators were significantly associated with SRB, but the coefficients were opposite in direction to the TWH. In 1862-1991, five out of six covariates showed no significant association with SRB. An additional analysis found no significant correlation between sex ratio of stillbirths and all covariates in 1862-1991. Our results are incompatible with the TWH and suggest that previous findings in support of the TWH are not robust.Limitations, reasons for cautionThis study provides population-level evidence that may not necessarily reflect the nature of all individuals due to the ecological fallacy. The time series analysed in this study are annual data, and we cannot examine the potential seasonality due to the lack of disaggregated monthly data. Our findings may not be generalised to the contexts of extreme maternal stress conditions such as famine and war.Wider implications of the findingsThe results from existing studies in this topic may be speculative, and additional research with more comprehensive design, data and covariates is needed to reconsider the robustness of previous findings.Study funding/competing interest(s)The author receives no external funding and has no conflict of interest to declare.Trial registration numberN/A.

Project description:Background and objectivesThe thrifty phenotype hypothesis proposes that at resource limitation, the growth of some organs/tissues is selectively spared to preserve more critical ones, such as the brain or lungs. The Trivers-Willard hypothesis (TWH) predicts that boys are more vulnerable in the case of resource limitation than girls. Both hypotheses were tested in children from disrupted families, differing in the extent of deprivation/adversities imposed on them.MethodologyIn a retrospective cohort study in the mid-20th century Estonia (Juhan Aul's database), different types of orphans and children of divorced parents (treatment groups; n = 106-1401) were compared with children from bi-parental families (control groups; n = 2548-8648) so that children from treatment groups were matched with control children on the basis of sex, age, year of birth, urban versus rural origin and socioeconomic position.ResultsChildren in orphanages suffered strong growth suppression, best explained by psychosocial deprivation. Their feet were on average 0.5 SD shorter than the feet of the controls, followed by height, leg/torso ratio and cranial volume that differed from controls by ca 0.4 SD. Weight difference was 0.2 SD units, while body mass index did not differ from controls. The growth of boys and girls in orphanages was suppressed to the same extent. Boys whose mothers were dead were relatively smaller and less masculine than girls from such families. Fathers' absence was unrelated to growth suppression. Sons of divorced parents had broader shoulders than boys whose fathers were dead.Conclusions and implicationsPrediction of TWH about the greater vulnerability of male growth may hold under some conditions but not universally. Predictions of the thrifty phenotype hypothesis were partly supported: trunk growth was spared at the expense of leg growth; however, no evidence for brain sparing was found. Comparison of children of divorced versus dead fathers may appear useful for indirect assessment of sexual selection on offspring quality.Lay Summary: Boys and girls in orphanages suffered similarly strong growth suppression, best explained by psychosocial deprivation. Boys whose mothers were dead were relatively smaller and less masculine than girls from such families. The occurrence of sex-specific associations between family structure and children's growth depends on the type of family disruption.

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Reformulation of Trivers-Willard hypothesis for parental investment.

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Reformulation of Trivers-Willard hypothesis for parental investment.

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