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Sex allocation theory predicts that females should bias their offspring sex ratios when the fitness benefits of producing sons or daughters differ depending on rearing environment. The Trivers-Willard hypothesis proposes that whether females produce more sons or daughters depends on food availability via both intrinsic maternal condition and differing reproductive potential (typically from mating system structure) for sons versus daughters. However, tests of its key predictions are often based on untested, implicit assumptions that are difficult to quantify, especially in migratory animals. In a 5-year study, we manipulated food availability in low- and high-elevation forest to test the Trivers-Willard hypothesis in the migratory black-throated blue warbler (Setophaga caerulescens). We found that the population-wide offspring sex ratio was significantly male-biased (population mean: 0.58), which was driven by an overproduction of sons in high-elevation forest (high-quality habitat mean: 0.59). Yet, we found no effect of food availability on offspring sex ratio from either natural variation or supplemental feeding. Sex-specific developmental costs did not differ for sons and daughters reared under low and high food availability. These results suggest that female black-throated blue warblers do not manipulate offspring sex ratios in response to food availability and are not consistent with the predictions of the Trivers-Willard hypothesis. This study highlights challenges of examining mechanisms driving patterns in offspring sex allocation in migratory species for which both the costs of rearing and relative fitness benefits of sons and daughters cannot be tracked into adulthood.
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This content will become publicly available on October 17, 2026
Parental age effects on offspring fitness in a wild population of a short-lived reptile
Abstract As organisms age, the fitness of the offspring they produce can decline, which is often attributed to parental senescence. However, few studies have tested for effects of parental age on offspring fitness in wild populations or in short-lived vertebrates, and only recently have studies begun to examine such effects in male and female offspring independently. Here, we use five generations of mark-recapture and genetic parentage data from an island population of a short-lived lizard, the brown anole (Anolis sagrei), to test for effects of maternal and paternal age on the survival to adulthood, first-year reproductive success, longevity, and lifetime fitness of their offspring. When comparing parents of different ages within the same offspring cohort, survival to adulthood increased with paternal age in sons, but we found no effects of maternal or paternal age on any component of fitness in daughters and no evidence that parental age effects differed based on the sex of the parent or the offspring. When considering repeated measures of individual parents sampled at multiple ages, we found that first-year reproductive success of sons decreased with paternal age, but longevity of sons increased with maternal age. However, when pooling sons and daughters, we found no overall effects of parental age on any component of offspring fitness, and little evidence that parental age effects differed between sons and daughters. Our study adds to the growing literature suggesting that negative effects of parental age on offspring fitness may not be as prevalent as once thought, particularly in wild populations.
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- PAR ID:
- 10686543
- Editor(s):
- Bilde, Trine; Moore, Michael
- Publisher / Repository:
- Oxford Academic
- Date Published:
- Journal Name:
- Journal of Evolutionary Biology
- Volume:
- 39
- Issue:
- 2
- ISSN:
- 1420-9101
- Page Range / eLocation ID:
- 200 to 211
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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