Factors associated with antler size of white-tailed deer in Iowa
Antler size of an individual cervid is a result of age, genetic, and environmental factors. Antlers are physiologically costly to produce and dependent upon condition, as nutritional requirements for body maintenance and growth take precedence over antler growth, indicating only individuals with access to adequate nutrition and in good health can afford to allocate resources for maximum antler growth. Therefore, biologists and managers are interested in antlers as possible indicators of condition of individuals and populations. To date, most studies of relationships between environmental factors and antler size have been conducted in the Southeastern U.S., but findings from these studies may not be generalizable to deer elsewhere in North America where environmental conditions are different. In order to identify associations between antler size and environmental factors in the Midwest U.S., I sampled hunter-harvested white-tailed deer in the row-crop dominated state of Iowa. I collected antler measurements, age, and location for 1,575 deer harvested between 2012–2018 to identify relationships between environmental factors and antler size. The first component of this thesis was an evaluation of methods for aging white-tailed deer. Because of the close relationship between antler size and age, controlling for the influence of age is important when attempting to identify environmental factors associated with antler size. I examined the congruence of age estimates obtained from two different methods for aging white-tailed deer: tooth replacement-and-wear (TRW) and counting cementum annuli (CA). I also examined the precision of the CA method using paired CA age estimates from two incisors from the same deer. Congruence rates of CA and TRW ages differed among age classes (80% congruence in yearling TRW age classification, 65% with 2-year-olds, 78% with ≥ 3-year-olds) and the precision of CA aging was influenced by the level of certainty assigned to the age estimate as well as the batch in which the teeth were aged. These findings suggest managers are best served by using TRW to age adult deer as yearlings or ≥ 2-years-old, as the TRW method does not accurately age deer ≥ 2-years-old to a single age. If additional ages are required, CA aging is likely to be more, but not perfectly, accurate. The second component of this thesis was to, after controlling for age, identify environmental factors associated with antler size of Iowa white-tailed deer and quantify variation in antler size across Iowa. Age was the most important factor explaining variation in antler size of white-tailed deer in Iowa. The amount of agricultural area in the area (typically ≈ 23.3 km2) the deer was harvested and the average summer temperature the year the deer was born both had a positive influence on antler size, while the amount of forested area and the winter severity while the deer was in utero both had a negative influence. I observed regional differences in antler size among older age classes where, generally, deer from the southwestern part of Iowa had larger antler sizes than deer from the northern regions of Iowa. Environmental influences and aging error were among the possible explanations for these differences. However, the magnitude of the differences was smaller than previously observed in other studies examining differences in antler size metrics between physiographic regions (e.g., < 1–13% differences between landform regions in Iowa compared to 3–31% for deer sampled in Mississippi). Through the sampling of harvested deer, I identified that the primary factor responsible for the antler size of an individual white-tailed deer in Iowa is age, with small, but statistically significant, influence from environmental factors. These environmental factors may partially explain regional differences in antler size across Iowa. The third component of this thesis was an analysis of spatial and temporal trends in the Iowa Department of Natural Resources’ Iowa Trophy Deer record book that included entries from 1939–2017. While I did not identify any temporal trends within the records, negative latitudinal trends were observed within a category of the records. These trends are similar to the regional differences in antler size from my contemporary sample. Although I observed some interesting trends from the record book data, biases associated with record books, such as their reliance on self-reporting by hunters, lack of age information for the submitted specimen, and focus on larger deer, may limit their usefulness for making inferences about a population. Monitoring a more representative sample of a population may better serve managers looking to observe trends in antler attributes of a population. Overall, the environmental factors I identified influenced antler size of white-tailed deer in Iowa. However, antler size, when controlling for age, did not vary as greatly across the state compared to previous studies conducted outside of the Midwest. These findings suggest the Iowa landscape offers deer adequate nutritional resources for antler growth, such that age is the primary factor explaining antler size in Iowa white-tailed deer.