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Ashley Atkins Coleman, Kelly D. Miller, Kelly L. O’Neil, Rin Pell, Shelby R. Green, and E. Keith Bowers: Read the article
Atkins Coleman et al. investigate effects of parental care on offspring fitness and the extent to which adult offspring resemble their genetic parents in parental investment, revealing selective pressures favoring offspring sex-ratio adjustment in a number of complex ways
The question may seem bizarre, but in nature, it’s important in evolutionary strategy. The balance between sons and daughters—known as the sex ratio of offspring—varies wildly across and within species. Sex ratios often appear to be finely tuned by parents based on conditions that influence the survival and reproductive success of male versus female offspring. In a recently published article in The American Naturalist, Atkins Coleman et al. (2024) provide evidence that Carolina wrens—a socially monogamous bird species—may tailor their offspring’s sex ratios in response to the quality of the parents’ environment, but in a far more nuanced and complex manner than once thought.
The researchers showed that mothers who paired with high-providing fathers—those who brought more food to the nest—were more likely to have sons. These sons then benefited from this rich environment and grew larger, increasing their ability to compete for better territories and provide more food for their offspring. This aligns with a long-standing hypothesis in behavioral ecology, termed the “Trivers-Willard Hypothesis,” which posits that parents in good conditions will (in this case) invest in sons over daughters. Due to their size and competitive ability, high-quality sons that can control good territories promise greater reproductive returns than equally well-conditioned daughters, who would likely find a mate regardless of their quality.
Interestingly, the researchers also found that high-quality mothers laid larger-than-average clutches that were biased toward daughters. They suggest that this choice likely serves a long-term strategy: daughters inherit their mothers’ high-quality traits and go on to raise robust broods of their own. This passing of quality traits mother-daughter-granddaughter, termed the “Advantaged Matriline Hypothesis,” reinforces the family’s success by focusing investment on daughters, who, regardless of their future partner’s quality, are more likely to pass down beneficial traits directly.
This study highlights the complexity of the set factors that go into answering our initial question: What ratio of sons to daughters should you have? Rather than following a single, straightforward rule, Carolina wrens appear to balance multiple, sometimes opposing, pressures when determining the ratio of sons to daughters. This deeper understanding of sex-ratio strategies not only enriches our knowledge of avian biology but also suggests that similar hidden complexities could be at work in other species, potentially offering new insights into the evolutionary pressures that shape family dynamics across the animal kingdom.
Iain Moodie is a doctoral student at Lund University, Sweden. He is working with Stephen De Lisle on questions that link evolutionary biology and ecotoxicology, using macro-evolutionary models, field population surveys, and microcosm experiments. Outside of work, Iain loves bike touring, electronic music, and cooking.
