Stress-induced sex ratios in ground squirrels: support for a mechanistic hypothesis
Bibliographic record
Abstract
Systematic deviations in sex ratio may be adaptive in the face of the prevailing environmental or physiological cues experienced by the mother; yet some theoretical and mechanistic hypotheses remain at odds and are rarely examined noninvasively under natural conditions. Conventional interpretations of the Trivers and Willard (Trivers RL, Willard DE. 1973. Natural selection of parental ability to vary the sex ratio of offspring. Science 179:90–92) hypothesis (TWH) predict that higher stress should be associated with female-biased litters; yet, several mechanistic hypotheses predict the opposite. We tested the predictions of the TWH and 2 stress-related mechanistic hypotheses in a free-living polygynandrous sciurid, Richardson’s ground squirrel, Urocitellus richardsonii. We examined the relationship between litter sex ratio and indicators of maternal condition and investment. These included litter size, juvenile mass at emergence, as well as maternal age, changes in maternal body mass, and maternal fecal glucocorticoid (cortisol and corticosterone) levels during gestation, lactation, and post-weaning. Males born of small litters were significantly heavier at emergence than males from larger litters, which were female biased, whereas females showed no significant change in mass with litter size. Mothers with higher fecal cortisol levels during the gestation period (but not during lactation or post-weaning) were more likely to produce male-biased litters, whereas females producing larger litters showed significantly higher cortisol levels during lactation (but not gestation or post-weaning) than mothers producing smaller litters. Our results provide support for both the proximate glucose-mediated mechanistic model of Cameron (Cameron EZ. 2004. Facultative adjustment of mammalian sex ratios in support of the Trivers-Willard hypothesis: evidence for a mechanism. Proc R Soc Ser B Biol Sci. 271:1723–1728) and the broad-sense TWH, where social and life-history characteristics play an integral role.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.001 | 0.002 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.001 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.002 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".