| For well over a century, ecologists have attempted to understand the evolutionary pressures shaping the allocation of resources to offspring of different sexes but support for competing theories remains inconclusive overall. Most empirical studies with mammals have been performed on free-ranging animals where researchers are unable to control environments or to trace the line of individuals over multiple years. At Washington State University and Starkey Experimental Forest and Range in Oregon, we set up nutritional trials to test the effects of body mass, body condition, and diet on the birth sex ratios of two species of polygynous mammals: mule deer (Odocoileus hemionus) and elk (Cervus elaphus). Despite carefully controlling and measuring nutritional resources, we found little evidence that forage quality or body fat near breeding influences birth sex ratio (BSR) of female mule deer and elk, or the probability that a neonatal elk or mule deer would be a certain sex. However, in our sample of 76 mule deer fawns, we found that digestible protein intake influenced the probability of having a male fawn. Similarly, although BSR of females treated with GC was absolutely higher than those treated with a placebo (0.71 vs. 0.50), BSR were not statistically different and corticosteroid treatment did not predict the sex of a fawn. In addition to analyzing the preexisting data, we used another herbivore, Grammia nevadensis, to test how growth rate, population density and diet affect sex ratio of offspring. We used high protein, low fiber kale and a low protein, high fiber pelleted diet to explore how these various diets affect other life-history traits of G. nevadensis. Despite the benefits of using G. nevadensis as a model species to test sex allocation hypotheses, we found little evidence that forage quality, average number of days till pupation or average number of days to emergence affected population sex ratios. Forage quality did, however, affect growth rates and the average number of days till pupation or emergence. Our results illustrate the difficulties in testing evolutionary hypotheses such as TWH in wild ungulates and herbivores in general. |
