Adaptive fetal sex allocation in elk: Evidence and implications

Population managers of large ungulates often assume that approximately even numbers of male and female offspring are produced. In contrast, evolutionary theory predicts that maternal investment in polygynous mammals should depend on the relative resource status of the female in relation to sex-specific demands of the offspring (i.e., females should bias sex allocation depending on their body condition). Examination of elk (Cervus elaphus) reproductive tracts from Oregon revealed that fetal sex ratios Vary with maternal body condition (kidney fat index [KFI]). Females with low maternal body condition were more likely to produce daughters than females with high KFI levels. Adult females with high KFIs conceived earlier and produced significantly more sons than daughters in the first 2 weeks of the reproductive season than females with lower body condition. Because reproductive data from elk in Oregon support the hypothesis of adaptive sex ratios according to the Trivers and Willard hypothesis, I evaluated the effects of adaptive sex ratios on offspring production in a hypothetical elk population. Using the observed relations between body condition, pregnancy, and fetal sex ratio, I examined the effects of maternal body condition on the production of male and female offspring in a population. I used a gamma distribution to simulate different maternal body condition (KFI) distributions and estimated the resultant fetal sex ratios. These simulations indicated that equality of sex ratios in offspring is a rare occurrence and is limited to average environmental conditions. With increasing body condition in the herd, the model predicted that more sons were conceived and that fetal sex ratios increasingly deviated from unity. Fetal sex ratios from annual and regional samples in Oregon supported this result and were positively correlated with mean KFI at the population level. If females in populations at or near carrying capacity suffer from generally low body condition, intrauterine and neonatal mortality may further reduce the number of sons produced and produce a significant bias toward daughters. Thus, adaptive sex ratio adjustment in elk may have significant importance for elk population management and the production of males.