Conformation to Bergmann's rule in white-tailed deer can be explained by food availability.
Much of the discussion in ecology and wildlife biology concerning variability in animal body size at multiple taxonomic and geographic scales has focused on Bergmann's rule, which states that members of species or closely related taxa are larger in colder parts of their range (Bergmann, 1847; Geist, 1987; Meiri and Dayan, 2003; Meiri et al., 2004; Rodriguez et al., 2006). There are numerous examples in which Bergmann's rule appears to hold and others in which it does not (see summary in Meiri et al., 2004). There are also several competing explanations for the underlying cause of the Bergmann pattern, such as contingencies of thermoregulation, responses to seasonality, responses to food availability and starvation resistance in different habitats. Informed choice of an appropriate explanation requires study of variables that influence body size within the taxon of concern and their relationship to body size across diverse habitats and a range of latitude.
We favor Geist's (1987, 1998) hypothesis that food availability-per-animal during the growing season is the most important factor governing intraspecific variability in body size. This hypothesis has received minimal attention for at least three reasons. First, many studies examine body size at a taxonomic scale (e.g., family or higher taxonomic units) that ignores and/or obscures factors related to variability in ontogenetic growth rate. Studies of growth rate and body size in distinct populations of a species allow examination of a host of proximate factors, such as habitat variability and differences in population density that influence growth rate and body size. Second, the linear decline in the length of the growing season with latitude is assumed to mean that food availability also decreases with latitude. This relates to the third reason, which is that the conventional understanding of the latitudinal distribution of net primary productivity (NPP) is incorrect. Recent analyses suggest that growing-season NPP, which we term ecologically relevant NPP or eNPP, is highest at temperate latitudes (Huston and Wolverton, 2009), as surmised by Geist (1987, 1998) in his interpretation of the geographic distribution of North American cervid body size.
The relationship between population density and body size in animals has received much attention in the paleozoological, biogeographic and ecological literature during the last several decades (Blackburn et al., 1993; Calder, 1984; Cotgreave, 1993; Damuth, 1981, 1991; Gaston and Blackburn, 1995; Greve et al., 2008; Huston, 1994; Johnson, 1998; Lyman, 2004; Meiri et al., 2004; Peters, 1983; Peters and Raelson, 1984; Purdue, 1989; Rosenzweig, 1968; White et al., 2004, 2007; Woodward et al., 2005). Although there appears to be a universal relationship between body size and population density, the mechanisms that drive regional and global patterns in animal body size are not clearly understood (Blackburn and Gaston, 1996, 1997; Gaston and Blackburn, 1995; Greve et al., 2008). Thus, it is important to shift to a finer taxonomic scale to understand the mechanisms underlying phenotypic variability in body size (e.g., Meiri et al., 2007).
A host of ecological factors influence animal body size within a species (Calder, 1984; Peters, 1983; White et al., 2004; Woodward et al., 2005) including metabolic rate, reproductive rate, competition, home range size and food availability. We assume that body size (other than relatively short-term [e.g., seasonal] fluctuations in soft-tissue mass) in animals with determinate growth is a function of the ontogenetic growth rate during the time period prior to reaching maturity (sensu Leberg et al., 1989; Zullinger et al., 1984). We also assume that ontogenetic growth rate is a function of food availability per individual animal. If this is the case, then two independent factors are important proximate causes of differences in ontogenetic growth rate--(l) net primary productivity (NPP), which is the amount of plant biomass produced per unit time and area in a specific habitat or ecosystem, and (2) the population density of the animal population, which influences the rate at which plant forage is available to individual animals. Figure 1 is a conceptual model of these variables and their relationship to body size. Although we present the relationship between NPP and population density using four quadrants (Fig. 1A) this portion of the model is simply heuristic. Figure 1B portrays the model as a topographic continuum with an increase in food per animal, ontogenetic growth rate and body size at high plant productivity and low population density.
[FIGURE 1 OMITTED]
In this paper, we examine the influence of the X-axis variable (Fig. 1)--plant productivity--at the regional scale in south central North America and the influence of population density at a local scale (central Texas) on body size in white-tailed deer. This species is ideal for examining these relationships for several reasons. First, its geographic range is large, so it is found in a range of habitats and latitudes. Second, it is a generalist "concentrate feeder" that browses on the high-quality portions of plants of a large variety of species (Demarais et al., 2000; Hesselton and Hesselton, 1982). Third, data are readily available from both contemporary wildlife harvests and archaeological sites. We also examine the influence of the Y-axis variable (population density) on white-tailed deer body size using a temporal, historic dataset from central Texas where population density is known to have decreased through time.
Body size in white-tailed deer in North America is expected to respond to variation in food supply (net primary productivity) and also to population size and density, particularly in areas of low habitat quality where populations are at or near environmental carrying capacity (Geist, 1987, 1998; Kie et al., 1983; Lesage et al., 2001; McCullough, 1979, 1984; Simard et al., 2008; Teer et al., 1965). Net primary productivity is very sensitive to variation in environmental conditions that affect plant growth, particularly the availability of mineral nutrients and water in the soil, and also temperature. While only a fraction of the total NPP in most ecosystems is available to terrestrial vertebrate herbivores, we assume that in most cases, the NPP available to deer will be positively correlated with the total NPP of the local ecosystem. We emphasize ecologically relevant NPP (eNPP), which is the rate at which plant material is produced when animals (as well as plants) are growing and reproducing. This contrasts with the standard measurement of NPP as an annual rate, which ignores the strong seasonal variability found in many environments and produces an average value that is useless for understanding the growth patterns of plants and animals (Huston and Wolverton, 2009). There is strong evidence for relatively high eNPP at temperate latitudes in terms of tree size, forest mass, leaf production relative to wood production and wood density as well as other measures of NPP (Huston and Wolverton, 2009). Because data based on these measures are much coarser in spatial scale than our white-tailed deer size data, we focus on a surrogate measure, crop productivity, which exhibits a similar latitudinal pattern as eNPP (Huston, 1993) and for which data exist at smaller spatial scales. Here we use the terms ecologically relevant NPP and food supply interchangeably.
MATERIALS AND METHODS
Data collected during managed harvest of white-tailed deer at Fort Hood, Texas during the last three and half decades are used to assess the effects of population thinning via harvest on body size. White-tailed deer were reintroduced to the fort during the mid-twentieth century, and age, sex and body mass (field dressed weight) have been collected for each deer harvested at Fort Hood since 1971. A decrease in population density is evaluated using coarse scale spotlight survey data and harvest per decade data (Table 1). Fort Hood stands in stark contrast to many other areas of central Texas where white-tailed deer populations are relatively unhunted by sport hunters and where large predators have been exterminated. In those areas, population densities are high and deer are small (Teer et al., 1965; Teer, 1984; Walton, 1999; Wolverton et al., 2007).
Mid to late Holocene and modern assemblages of white-tailed deer astragali (ankle bones) from central Missouri, central Texas and southeast Texas are used to compare deer body size from areas with different NPP (Table 2). Paleozoological assemblages are from multiple Holocene archaeological contexts that date primarily to the last 5000 y BP, such as rockshelter sites and open-air sites (Arnold Research Cave [23CY64] in Missouri, Eagle's Ridge in southeast Texas [41CH252], and Kincaid Shelter [41UV2] and other sites in central Texas [Wolverton, 2007]). The paleozoological assemblages studied here are time-averaged agglomerations likely to reflect a host of accumulation and depositional processes. Astragalus size (Fig. 2; Table 3) is a useful proxy of age-independent body size in that it matures by 6 mo of age in white-tailed deer (Purdue, 1987, 1989; Wolverton et al., 2007; Wolverton, 2008:186, Fig. 3). Further, the astragalus is common in archaeological and paleontological faunal assemblages because it is a dense bone that preserves well through time.
We use modern crop productivity as an index of local variation in NPP in central Missouri, central Texas and southeast Texas where native forest and grassland ecosystems have been modified by modern urban, suburban and rural development (Table 3). While the NPP of agro-ecosystems is typically higher than the NPP of natural ecosystems for several obvious reasons (e.g., use of fertilizers), both types of vegetation within a local area experience the same climate and same basic soil properties (soil texture, clay content, parent material, initial organic matter content, cation exchange capacity, etc.). These inherent differences in climate and soil properties are expected to produce correlations between agricultural production and NPP of the mixed ecosystems that support deer populations. It is well-known that agricultural crops can form an important component of deer diet where there is a mixture of cropping and woodland habitats (Hansen et al., 1997; Nixon et al., 1991; Seagle, 2003). While total crop NPP is rarely measured, there are extensive data on crop yield as mass or volume of the harvested product. We have summarized the standard yield data for several crops in bushels per acre for counties included in each of the three study areas (Table 3). Although crop productivity is only a surrogate for ecologically relevant NPP, a distinct advantage of its use here is that crop productivity estimates are available at approximately the same spatial scale as our deer astragalus samples--the county scale. Because the three crops for which we have data varied greatly in the total area planted in each of the three regions, and thus vary in the degree to which they represent the local environment, we have created a standardized index by dividing the per acre yields of each crop by the maximum yield for that crop (central Missouri yields were highest for all four crops), and then averaging across the four crops (Table 3).
[FIGURE 2 OMITTED]
VARIATION IN BODY SIZE AND POPULATION DENSITY OVER TIME
White-tailed deer body size increased significantly during the last three and a half decades at Fort Hood as population density decreased (Wolverton, 2007, 2008). Census records do not exist for Fort Hood for prior to the 1980s, and the dataset is patchy thereafter. However, spotlight-survey estimates indicate higher deer density from 1981 to 1991 than from 1997 to 2005 (Table 1). Average body mass is relatively high in the later period as well mirroring the increase in body size depicted in Figure 3. The decrease in population density is also reflected in a dramatic decrease in deer harvested per decade: in the 1970s 13,289 deer were harvested; in the 1980s 6306, in the 1990s 4293 and from 2000 to 2005, 1912 deer were harvested at Fort Hood. The decrease in number of deer harvested reflects a substantial reduction in population density in most areas of the fort produced by sustained harvest (Table 1), which has also produced a change in age structure through time (Wolverton, 2008; Wolverton et al., 2008).
At Fort Hood, age structures become old-adult depleted through time, which is a hallmark of sustained harvest pressure (Caughley, 1977; Festa-Bianchet et al., 2003). The body size increase through time is age- and sex-independent (Fig. 2); however, the slope of increase in bucks is steeper than for does. In Odocoileus bucks tend to have larger home ranges and higher energetic requirements than does but also greater phenotypic plasticity in body size (Beier and McCullough, 1990; Comer et al., 2005; Lesage et al., 2001; Purdue et al., 2000; Weckerly, 1993) and literally "have more to lose" at high population densities especially in unfavorable habitat (Lesage et al., 2001; Relyea et al., 2000).
[FIGURE 3 OMITTED]
SPATIAL VARIABILITY IN BODY SIZE
Modern and prehistoric astragali are significantly smaller in central Texas than in central Missouri (Fig. 3), and in southeast Texas prehistoric astragali are significantly smaller than in central Texas (Fig. 3b; Table 4). Four crops, corn, wheat, soybean and sorghum, are used to assess differences in food supply between these three regions (Table 3), of which only sorghum is common to all three areas. Crop productivity is consistently highest in central Missouri and low in southeast Texas, and astragalus size increases with standardized relative crop yield across the three areas (Fig. 4).
Population density is an important factor determining body size in white-tailed deer in areas where populations are at or near carrying capacity. In central Texas, deer have reached pest population levels (Walton, 1999) and their body size appears to be stunted as a result (Geist, 1998; Teer et al., 1965; Wolverton et al., 2007). Sustained harvest at Fort Hood has caused a reduction in population abundance that has resulted in a body size increase during the last few decades (Fig. 3). Population abundance remains high and deer are small in many areas of central Texas where hunting is uncommon and large carnivores are rare (Wolverton et al., 2007). Clearly, population density is an important factor mediating white-tailed deer body size in this setting.
It is also clear, however, that population density (crowding) is not the only determinant of white-tailed deer body size in central Texas. This is illustrated by the higher rate of body size increase in bucks than in does at Fort Hood over the past 35 y (Fig. 3). Assuming that bucks have larger home ranges, which appears to typically be the case in Odocoileus (Beier and McCullough, 1990; Comer et al., 2005; Purdue et al., 2000; Relyea et al., 2000; Weckerly, 1993), an increase in population density restricts their use of space more severely than for does, especially in relatively low-productivity habitat. As a result, bucks should experience greater stunting when populations are at or near environmental carrying capacity. This reflects a sexually dimorphic phenotypic-plasticity pattern common in cervids that relates to epigenetic evolution in morphology (Geist, 1978, 1998).
Body size maintenance requirements are higher for male than female cervids in general, but size is also more phenotypically plastic in males (Lesage et al., 2001; Simard et aL, 2008). In addition, it is generally understood that winter is a period of energetic maintenance for cervids and that the annual growing season is a period of ontogenetic growth and body size recovery (Bergerud, 2000; Coady, 1982; Ferguson, 2002; Langvatn and Albon, 1986; Reimers et al., 1983; see Lesage et al., 2001 for white-tailed deer). For females, however, the growing season overlaps with the birthing season, which contributes to sexual dimorphism in body size. That is, during the growing season, females experience high food/energy requirements during gestation and lactation. At any level of food availability, does can be expected to be less phenotypically plastic and smaller in body size than bucks because of the demands of gestation and lactation. In conditions in which food-per-animal is high (Fig. 1), bucks literally "have more to gain" because they do not experience the same reproductive-energetic tradeoffs during the growing season as does. At Fort Hood, as population density decreased, food availability per animal increased for both bucks and does, but because bucks are more phenotypically plastic in terms of body size, the slope of increase (Fig. 3) is steeper in bucks than in does.
At a larger geographic scale, the effects of population density are out-weighed by landscape changes in ecologically relevant NPP and food supply for deer. Unlike at Fort Hood, the history of harvest pressure on modern white-tailed deer is poorly documented for Missouri and southeast Texas. However, the prehistoric samples from the mid to late Holocene in each area represent a period prior to predator extermination and during which Native American subsistence hunters heavily exploited white-tailed deer (Baker, 1998; McMillan and Klippel, 1981; Smith, 1974; Wolverton, 2005). Thus, it is unlikely that prehistoric size differences among these areas reflect differences in harvest pressure. However, these prehistoric size differences are correlated with modern crop productivity, which serves as a proxy for deer food supply (Fig. 5). Soil properties are unlikely to have changed significantly over the past 5000 y, so current soil properties and eNPP are probably similar to those during the mid to late Holocene. The size differences in prehistoric deer between these three areas are likely to be the result of differences in eNPP and thus food availability for deer (sensu Geist, 1987, 1998).
[FIGURE 4 OMITTED]
[FIGURE 5 OMITTED]
It is difficult to distinguish the separate effects of population density, NPP, latitude, altitude and climate gradients on animal body size at the coarse ecological (community) and taxonomic (greater than family) levels (e.g., Rodriguez et al., 2006). Each of these variables can be more precisely examined at the intraspecific scale (e.g., Meiri et al., 2007; Simard et al., 2008; Wolverton, 2008), which is why we focus on variability in body size of white-tailed deer (other examples include Ashton, 2004; Chown and Klok, 2003; Geist, 1987; Kennedy and Lindsay, 1984; Kennedy et al., 2002; Langvatn and Albon, 1986; Maehr and Moore, 1992; Maehr et al., 2001; Wolverton and Lyman, 1998). In addition, effects of food availability per animal on body size may account for some of the exceptions to the widely observed negative relationship between individual body size and population density of a species at coarse ecological and taxonomic scales (sensu Meiri et al., 2007). Similarly, we suspect that latitudinal distributions of body size in species that conform to Bergmann's rule actually relate to the macrogeographic distribution of food availability per animal through the response of ontogenetic growth rates to ecologically relevant NPP (sensu Geist, 1998). For example, body size of white-tailed deer varies with food supply, which varies with latitude, leading to the appearance of an instance of Bergmann's rule. If food availability drives variability in body size in this species, then this latitudinal pattern, however, has little to do with factors classically related to Bergmann's rule, such as purported thermoregulatory constraints on body size (Geist, 1987). While the productivity hypothesis has been summarily dismissed as an explanation for Bergmann's rule (cf, Geist, 1987; Meiri et al., 2007), we believe that this important factor has been erroneously discounted because of a widespread misunderstanding of the global patterns of NPP, and the failure to recognize the advantages of eNPP over total annual NPP for understanding ecological and evolutionary patterns (Huston, 1994; Huston and Wolverton, 2009).
Acknowledgments.--The US Army Corps of Engineers allowed access to astragali from site 41CH252 in southeast Texas. Fort Hood, Directorate of Public Works, Natural Resources Branch provided white-tailed deer harvest data. This research also received in-kind support from Orion Research and Management Services, Incorporated, and the University of North Texas Institute of Applied Sciences, Department of Geography and Department of Biological Sciences. Lee Lyman read and commented on early and late drafts of this paper and provided insightful comments. We also thank Beau Bush, Julie Densmore, Ben Fullerton, George Gilchrist, Jeff Johnson, Ernie Lundelius, Lisa Nagaoka, Charles Randklev and Scott Turrentine for their various avenues of support.
SUBMITTED 16 JUNE 2008
ACCEPTED 8 NOVEMBER 2008
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STEVE WOLVERTON (1)
Department of Geography & Institute of Applied Sciences, University of North Texas, 1155 Union Circle #305279, Denton, Texas 76203
MICHAEL A. HUSTON
Department of Biology, Texas State University, San Marcos, Texas 78666
JAMES H. KENNEDY
Department of Biological Sciences, Institute of Applied Science, University of North Texas, PO Box 310599, Denton, Texas 76203
Directorate of Public Works, Natural Resources Branch, Fort Hood, Building 1939 Rod and Gun Club Loop at 53rd, Fort Hood, Texas 76544
JOHN D. CORNELIUS
Orion Research & Management Services, Inc., 21 Cedar Trail, Belton, Texas 76513; and Directorate of Public Works, Natural Resources Branch, Fort Hood, Building 1939 Rod and Gun Club Loop at 53rd, Fort Hood, Texas 76544
TABLE--1.--Estimated population density and body size for two periods at Fort Hood Period Deer/1000 acres Avg. dressed weight (lbs) 1081-1991 36.03 63.09 1997-2005 25.36 78.42 % change -30% +24% TABLE 2.--Loci of white-tailed deer astragalus assemblages by region Region n Astragali Age Central Missouri Boone County 97 Modern Callaway County 59 Prehistoric Central Texas Bell County 82 Modern Comal County 9 Prehistoric Coryell County 6 Prehistoric Hays County 2 Prehistoric Hill County 15 Prehistoric Travis County 2 Prehistoric Uvalde County 17 Prehistoric Val Verde County 7 Prehistoric Southeast Texas Chambers County 147 Prehistoric TABLE 3.--Crop yield and white-tailed deer data for three regions Central Missouri Central Texas SE Texas Crop Data * n counties 2 8 1 % area in crops 39.1 16.3 21 Corn acres 35,904 120,902 0 bushels/acre 92.6 91.9 0 Wheat acres 26,737 118,522 0 bushels/acre 48.0 32.5 0 Sorghum acres 7,863 135,638 976 bushels/acre 90.3 62.6 33.0 Soybean acres 92,683 483 3465 bushels/acre 33.8 17.0 20.7 Mean Relative Yield 1.00 0.72 0.25 White-tailed deer Data Prehistoric n astragali 59 58 141 Thickness mean 22.6 21.3 19.3 S 1.7 1.2 1.5 CV (%) 7.5 5.6 7.8 Length mean 33.5 29.9 28.6 S 2.3 1.4 1.6 CV (%) 6.9 4.7 5.6 Modern n astragali 97 82 -- Thickness mean 23.3 21.1 -- S 1.3 1.2 -- (%) 5.6 5.7 -- Length mean 33.2 29.9 -- S 1.6 1.5 -- CV (%) 4.8 5.0 -- * Data from 1997 USDA Agricultural Census. TABLE 4.--Results of Student's t tests on astragali samples Test t-statistic p-value Modern Missouri vs. Central Texas Length 13.97 <0.0001 Thickness 11.97 <0.0001 Prehistoric Missouri vs. Central Texas Length 10.34 <0.0001 Thickness 4.65 <0.0001 Central vs. SE Texas Length 5.61 <0.0001 Thickness 9.31 <0.0001
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|Author:||Wolverton, Steve; Huston, Michael A.; Kennedy, James H.; Cagle, Kevin; Cornelius, John D.|
|Publication:||The American Midland Naturalist|
|Date:||Oct 1, 2009|
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