9th International Deer Biology Congress

AUTHORS: Duane R. Diefenbach, U.S. Geological Survey, Pennsylvania Cooperative Fish and Wildlife Research Unit, Pennsylvania State University; Gary L. Alt, Lagunitas, CA; Bret D. Wallingford, C. S. Rosenberry – Pennsylvania Game Commission; E. S. Long, Seattle Pacific University

ABSTRACT: White-tailed deer (Odocoileus virginianus) populations throughout North America have endured harvest rates as high as 80% of antlered males resulting in a female-biased sex ratio throughout the 20th century with no adverse effects on overall abundance. In 2002, Pennsylvania implemented statewide harvest regulations that increased the survival rate of males by implementing antler point restriction (APR) regulations and reduced the survival rate of females by increasing the allocation of antlerless licenses.1 These changes in harvest regulations created an older age structure in the male population (percentage of antlered males >2.5 years old increased from 1 year old) male:female ratio that was less skewed towards females (from 1:3.7 to 1:2.1). During 1999–2006 we examined road-killed females to monitor pregnancy rates, conception dates, productivity (embryos/female), and embryo sex ratios to test if changes in sex-age structure of the population influenced timing and synchrony of breeding and productivity. We predicted the shift to an older age structure of males and a less skewed sex ratio would result in the average conception date occurring earlier, greater breeding synchrony (reduced variance in conception dates), increased productivity, and more male offspring produced.2,3 We examined 2,209 adult females (>2 years old), 1,397 yearling females (>1 year old), and 2,647 fawns for embryos and we calculated date of conception for 3,107 female deer (207–523 per year). We developed a hierarchical model with wildlife management unit (WMU) and year treated as random effects, in which both the date of conception and the variance around the date of conception changed after APRs were implemented. We failed to detect a change in conception date over years (mean date = 13 November) and any consistent change in the SD of conception dates. In contrast to our predictions, reproductive rates were greater before APRs were implemented (Yearlings: 1.51 embryos/female, SE = 0.053; Adults: 1.65 embryos/female, SE = 0.043) and declined to 1.39 embryos per yearling female (SE = 0.046) and 1.56 embryos per adult female (SE = 0.030). Also, we found no evidence that pregnancy rates for female fawns increased. Furthermore, we found no evidence that offspring sex ratios changed after APRs were implemented. The best linear mixed effects model, treating WMU as a random effect, included only an intercept term. During 1999-2006, yearling females produced 1.13 males per female (SE = 0.093) and adult females produced 1.09 males per female (SE = 0.031). We believe there are several reasons for the lack of change in breeding behavior and demography. First, breeding in white-tailed deer is not dominated by older age classes so increasing the proportion of older males in the population is unlikely to change who dominates breeding.4 Second, there may be limited flexibility regarding when breeding can occur. The timing of breeding should result in an optimal birth date for fawns such that it is not too early that they are exposed to environmental conditions that elevate the risk of mortality (e.g., cold temperatures) yet early enough to attain a body condition in autumn that maximizes winter survival. Synchrony of breeding in white-tailed deer increases with latitude but Pennsylvania is located within the portion of the range of white-tailed deer that exhibit little variation. Third, productivity has been adversely affected in ungulates when sex ratios were >25 females per male.3 In white-tailed deer, given typical reproductive rates, it is difficult to achieve a breeding age (≥1.5 year old) male:female sex ratio skewed beyond 1:4 unless adult male annual mortality rates exceed 0.90 and adult female annual mortality rates are