Changes in antler characteristics from harvested white-tailed deer across 50 years.

Abstract: Current and past geographic variation of antler size of white-tailed deer (Odocoileus virginianus) across Missouri is unknown. Our objectives were to determine spatio-temporal patterns and changes of antler characteristics in Missouri between the periods 1951-1970 and 1997-2001. For the early and recent time periods mean beam circumference and mean number of points were determined by county for 1.5-year-old deer. Regression of number of points and latitude/longitude indicated latitude was important during the earlier and latter time periods ([r.sup.2] = 0.75, p < 0.0001; [r.sup.2] = 0.52, p < 0.0004, respectively). Beam circumference also was related latitude during both time periods ([r.sup.2] 0.73, p < 0.0001; [r.sup.2] 0.44, p < 0.0028, respectively). Geographic distribution of antler characteristics changed over time. Variation of antler size became less over time and shifted from a north-south distribution to a northwest-southeast distribution. Hypothesized factors influencing recent characteri stics include density, nutrition associated with land use practices, and genetics as related to selective harvest criteria.

Introduction

Geographic variation of morphological characteristics of mammals is extensive (Wiig and Andersen 1989, Fandos and Reig 1993, Moncrief 1993, Derocher and Stirling 1998, Arita and Figueroa 1999, Wehausen and Ramey 2000). Morphologic studies focus on cranial characteristics most often. Antler size, as a cranial characteristic, is related to body size (Geist 1998). White-tailed deer body size increases with latitude (Baker 1984), and antler size is expected to increase with latitude as well. Geographic variation studies of white-tailed deer antlers, however, are limited (Maffei et al. 1988, Molina and Molinari 1999).

Many factors are related to antler characteristics in white-tailed deer (McEwen et al. 1957, Short et al. 1969, Roseberry and Klimstra 1975, Cowan and Clark 1981, Jacobson 1984, Sauer 1984, Strickland and Demarais 2000). Among the factors, soil types provide the basis for potential habitat quality, and habitat quality is considered a proximate factor that affects antler development. At the landscape level, soil types vary spatially and are associated with differences among wildlife populations (Albrecht 1944, Denny 1944, Crawford 1950, Williams and Caskey 1965, Hill 1972, Strickland and Demarais 2000).

Basic geographic relationships of antler characteristics have received limited attention (Strickland and Demarais 2000), and have not been examined in the Midwest. More specifically, how deer antler development varies across the state of Missouri is unknown. Geographic Information Systems (GIS) technology has expanded to a great number of ecological and conservation applications (Grinder and Krausman 2001, Strittholt and Dellasala 2001) such that it is well suited to address morphological patterns at the landscape scale. Our study objectives were to: 1) determine geographic patterns of antler characteristics across Missouri; and, 2) determine if antler characteristic patterns have changed in Missouri over time.

Methods

Antler characteristics (circumference (mm) and number of points) and age (Servinghaus 1949) data were collected from harvested deer at established county check-stations by Missouri Department of Conservation (MDC) personnel and volunteers during 2 time periods, 1951-1970 (early) and 1997-2001 (recent); comparable data were not available during 1971-1996. We assumed the county of harvest was the county in which the animal was checked. An unquantifiable amount of bias may have been in the data, as it was very likely not all yearlings harvested were checked. We believe, however, the bias was consistent and sufficient data were collected to explain the variation. Yearlings comprised the majority of harvest and provided the greatest potential for variation. Therefore, only 1.5-year old males were used in analyses. Because management units used by MDC have changed over time, counties were used as experimental units and years as replicates.

Mean circumference and number of points by county were determined for each time period. Geographic relationships across the state were determined by interpolating data using inverse distance weighting. Linear regressions for each time period were performed using antler characteristics as dependent variables and the latitude and longitude of the center of each county as independent variables. Spatially, counties were well represented across the state except for the extreme west-central and southeastern counties. Comparison of slopes using 95% confidence intervals was conducted to determine if changes occurred over time. All statistical analyses were conducted using SAS (SAS Inst., Inc. 2001) and alpha was set at 0.05 unless otherwise noted.

Results

Latitude related significantly (p < 0.0001) to the number of points (Points = -25.747 + 0.758(latitude) - 0.019(longitude),[r.sup.2] = 0.75, p <0.0001, d.f. = 36) during the early period. During the recent period, latitude related significantly (p = 0.0012) to points, but accounted for less variation (Points = -18.544 + 0.286(latitude) - 0.133(longitude), [r.sup.2] = 0.52, p = 0.0004, d.f. = 23). Longitude, while unimportant during the early time period (p =0.8089), became increasingly important during the recent time period (p = 0.0667). Beam circumference also related significantly to latitude (p < 0.0001) during the early period (Beam Circumference = -139.500 + 4.521(latitude) - 0.0320(longitude), [r.sup.2] = 0.73, p < 0.0001, d.f. = 37) and the recent period (Beam Circumference = -77.485 + 2.305(latitude) - 0.584(longitude), [r.sup.2] = 0.44, p = 0.0028, d.f. = 22). The effect latitude had on the number of points became significantly less (95% C.I.: early, 0.586 < [b.sub.1] <0.917; recent, 0.127 < [b.sub.1] < 0.445) overtime, but was not found to differ over time for beam circumference (95% C.I.: early, 3.442 < [m.sub.1] < 5.687; recent, 1.069 < [m.sub.1] < 3.602). Longitude was not found to be related to beam circumference during either time period.

Interpolation of number of points from 1951-1970 showed harvested yearlings north of the Missouri river with 5-7 points and 2-5 points south of the Missouri river. Distribution across the state during the early time period followed a north-south distribution (Figure 1). Interpolation of average number of points from 1997-2001 indicated harvested deer in the northwestern corner of the state with 5-6 points. Yearling bucks with 3-5 points represented the rest of the state. Distribution across the state during the recent time period shifted to northwest-southeast (Figure 1). A difference between interpolations of number of points between the periods indicated a decrease in the north by 1-2 points, and a decrease south of the Missouri River from 0-1 points. However, isolated locations in southern Missouri increased by up to 1 point (Figure 2).

Interpolation of beam circumference for 1951-1970 indicated harvested deer north of the Missouri river had the largest beam circumferences. Distribution of beam circumference across the state also followed a north-south pattern (Figure 3). Interpolation of beam circumference from 1997-2001 showed the largest beam circumferences occurred in the northwest corner of the state. Distribution across the state also was northwest-- southeast (Figure 3). A difference between interpolations of the beam circumference between periods displayed a decrease in the north by up to 9 mm and an increase in the south by up to 6 mm (Figure 4).

Discussion

Antler characteristics in Missouri tended to follow a north-south dine during the early time period. Body size is correlated with antler size, and body size increases with increases in latitude (Baker 1984). Superabundant seasonal forage increases with latitude and may have provided for the relationships observed. Environmental differences across the state may have been sufficient to support such a dine during the early time period.

A reduction in the [r.sup.2] value in the recent time period regression indicated latitude was not as important during the recent time period. At least three factors may be hypothesized to explain the decline in variation related to latitude. First, density increased through time, and white-tailed deer history in Missouri is typical of many southeastern and mid-western states. Competition with livestock, logging, and market hunting contributed to the decline of deer prior to 1900 (Torgerson and Porath 1984). A deer restoration program began in 1938 and continued through 1957. Approximately 90% of transplanted animals were from Missouri. A harvest season was allowed in 1944 as restoration efforts succeeded. Conservative limits were gradually eased and seasons expanded as the deer population increased. Although deer are difficult to census, population simulations put Missouri's deer herd at over 1,000,000 animals. Improvements in deer mass and antler development are expected after deer herd reduction, though r esults have been mixed (Roseberry and Klimstra 1975, Shea et al. 1992, Ditchkoff et al. 1997, McCullough 2001). Antler size increased in some portions of southern Missouri despite increasing density, suggesting other factors may be more influential.

Second, nutrition as related to land use may have provided not only the increase in abundance of forage needed for antlerogenesis, but the differences exhibited in a north-south gradient. Oak (Quercus spp.) and hickory (Carya spp.) forests of the Ozarks are found in the southern third of the state and more agricultural lands are found in the northern third of the state. Transition between the two occurs in the middle of the state. Agricultural crops have been related to increased body size and antler characteristics, and large forested areas have been suggested as sub-optimal habitat for antler growth (Richie 1970).

Genetics as affected through harvest is the third hypothesized factor potentially affecting antler characteristics. Each year Missouri deer hunters collectively spend about 3 million mandays pursuing deer (Hansen, unpubl. data). With such effort, what selective pressure hunting has on antler characteristics over time is unknown. Selective removal has received limited study (Strickland et al. 2001), and most genetic studies are under penned conditions (Lukefahr and Jacobson 1998, Williams et al. 1994).

Further research is needed to determine the relationships among factors influencing antler characteristics at the landscape scale. Hypotheses supporting these relationships include: increased deer densities decrease antler size; land use changes, which affect nutrition, alter antler size; and hunting selects for smaller antler characteristics.

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