Sex Ratio Dynamics in Reproductive Strategies
Summary
Sex ratio dynamics refer to the evolutionary and ecological processes by which organisms bias the proportion of male and female offspring. At the heart of this field lie theoretical frameworks such as Fisher’s principle, which explains the emergence of an approximately equal sex ratio under frequency-dependent selection, and the Trivers–Willard hypothesis, which predicts facultative biasing towards the sex with the higher potential reproductive payoff. Empirical research has revealed that both intrinsic factors (parental condition, hormonal milieu) and extrinsic factors (adult sex ratio, environmental stressors, resource availability) can influence sex allocation decisions. These adjustments occur through diverse mechanisms including differential fertilisation, biased embryo development, selective offspring mortality and sperm sex-ratio modulation. Understanding these dynamics has implications for conservation biology, livestock management and human demography, and offers insights into adaptive strategies across taxa and ecosystems.
Research from Nature Portfolio
Recent studies have demonstrated that adult population structure can trigger adaptive shifts in brood composition. In a wild avian system, researchers found that females bias their hatchlings towards the rarer sex in response to local adult sex ratios, providing the first evidence of facultative adjustment of offspring sex under natural conditions. Complementing this, a longitudinal analysis of historical birth records spanning periods of environmental upheaval showed that male births declined during times of heightened stress, consistent with greater male vulnerability, while indicators of maternal health had no systematic effect on birth sex ratios. Together, these findings underscore the roles of demographic context and environmental harshness in shaping primary and secondary sex ratios.
Sex Ratio Dynamics in Reproductive Strategies publication trend
The graph below shows the total number of articles in sex ratio dynamics in reproductive strategies across all publications each year (not limited to Nature Index journals).
Technical terms
Facultative sex allocation: Adaptive adjustment of offspring sex ratio in response to environmental or parental cues.
Primary sex ratio: Proportion of male to female offspring at the time of conception or egg laying.
Secondary sex ratio: Proportion of male to female individuals at birth or hatching.
Adult sex ratio: Ratio of adult males to adult females within a population, influencing mating systems and sex allocation.
Reproductive value: Expected future contribution of an individual or sex class to the gene pool, used in predicting optimal sex ratios.
Trivers–Willard hypothesis: Evolutionary model proposing that parental condition influences a bias towards producing the sex with potentially higher reproductive success.
References
- A possible case of offspring sex manipulation as result of a biased adult sex ratio. Scientific Reports (2024).
- Sex ratios at birth vary with environmental harshness but not maternal condition. Scientific Reports (2019).
- Exposure to male‐dominated environments during development influences sperm sex ratios at sexual maturity. Evolution Letters (2019).
- Why so many Agta boys? Explaining ‘extreme’ sex ratios in Philippine foragers. Evolutionary Human Sciences (2019).
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