The University of Notre Dame Acorn Preference and Habitat Use in Eastern Chipmunks Author(s): Sanjay Pyare, Julie A. Kent, Diane L. Noxon and Michael T. Murphy Source: The American Midland Naturalist, Vol. 130, No. 1 (Jul., 1993), pp. 173-183 Published by: The University of Notre Dame Stable URL: https://www.jstor.org/stable/2426285 Accessed: 23-10-2024 17:00 UTC JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org. Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at https://about.jstor.org/terms The University of Notre Dame is collaborating with JSTOR to digitize, preserve and extend access to The American Midland Naturalist This content downloaded from 128.227.1.13 on Wed, 23 Oct 2024 17:00:24 UTC All use subject to https://about.jstor.org/terms Am. Midl. Nat. 130:173-183 Acorn Preference and Habitat Use in Eastern Chipmunks SANJAY PYARE, JULIE A. KENT, DIANE L. NOXON AND MICHAEL T. MURPHY Department of Biology, Hartwick College, Oneonta, New York 13820 ABSTRACT.-We used field and laboratory trials to examine the effects of size and species of acorn on food choice by eastern chipmunks (Tamius striatus) from central New York. Initial trials suggested that chipmunks preferred the low tannin, white oak (W; Quercus alba) acorns over the high tannin, red oak acorn (R; Q. rubra). However, the smaller size of the W acorns, and therefore greater ease of handling, possibly confounded the affects of tannin content on acorn choice. Additional field and laboratory experiments confirmed that chipmunks preferred smaller acorns, but W acorns were still preferred to equal-sized R acorns. Frequency of live capture of chipmunks during the period of acorn drop (mid- September to mid-November) at the field site was related directly to the total basal area (P -0.02) but not number of W trees. Capture frequency was also correlated negatively with the number (P = 0.04) but not basal area of R trees in the canopy. Our results suggest that chipmunks prefer to eat W acorns and that chipmunk activity is in part influenced by this preference. Heavy chipmunk harvesting of the preferred W acorns may be influencing the successional patterns in the forest at our study site. INTRODUCTION Eastern chipmunks (Tamius striatus) are one of the principal mammalian granivores of eastern deciduous forests. Many ecological and behavioral studies have documented aspects of foraging ecology (Elliot, 1978; Kramer and Nowell, 1980; Shaffer, 1980), social behavior (Elliot, 1978; Yahner, 1978; Mares et al., 1982; Getty, 1987; Mares and Lacher, 1987) and population biology (Tryon and Snyder, 1973). Little information is available regarding chipmunk food preferences. Knowledge of food preferences is important for understanding foraging decisions (Smallwood and Peters, 1986; Briggs and Smith, 1989) and possibly habitat use (Drickamer, 1976). Preferences may also affect seed dispersal and therefore patterns of plant regeneration and succession (Vander Wall and Balda, 1977; Stapanian and Smith, 1984; Johnson and Adkisson, 1985; Webb, 1986). Chipmunks larder-hoard and secondarily scatter-hoard mast (both Fagus and Quercus) and other seeds (Ewer, 1968; Shaffer, 1980). Caching behavior is critical for winter survival and maintenance of pregnancy during early spring food shortages (Wrazen and Wrazen, 1982). Given that cached foods must remain edible during storage periods, and that nuts vary in quality (i.e., size, lipid vs. protein content and presence of plant chemicals), it is expected that animals may exhibit food preferences (Scarlett and Smith, 1991). The large size, high nutrient content and (in mast years) abundance make acorns (Quercus spp.) a valuable resource for many animals. However, some acorns also contain large quantities of tannins (Martin et al., 1951; Short, 1976), which presumably reduce their attractiveness to some foraging animals. Tannins are a heterogenous group of water-soluble, phenolic secondary plant products that appear to adversely affect the palatability and/or digestibility of plant materials (Has- lam, 1981). Tannins form complexes with salivary and digestive enzymes, interfering with the metabolism of several classes of consumers (Smallwood and Peters, 1986). Acorns of the red (Erythrobalanus) and white oak (Lepidobalanus) subgenera have tannin concentrations 173 This content downloaded from 128.227.1.13 on Wed, 23 Oct 2024 17:00:24 UTC All use subject to https://about.jstor.org/terms 174 THE AMERICAN MIDLAND NATURALIST 130(1) TABLE 1 -Size categories of acorns used in the choice experiments Medium Large White oak (W) Small Red oak (R,) Red oak (RM) Red oak (RL) xR mass (SD) 2.42 (0.55) 2.46 (0.99) 4.94 (0.67) 7.04 (0.90) Range 1.20-3.86 0.46-3.95 3.96-5.90 5.91-9.15 n 402 182 127 124 SD = standard deviation that vary between 5-10% and 0.5-2.5% of dry mass, respectively (Smallwood and Peters, 1986). Indeed, lower tannin content has been suggested to be the basis of a preference for white oak acorns by gray squirrels (Sciurus carolinensis; Martin et al., 1951; Short, 1976) and white-footed mice (Peromyscus leucopus; Drickamer, 1976). Other evidence suggests, however, that acorns may be selected on the basis of size and lipid content (Smith and Folmer, 1972; Lewis, 1980, 1982; Smallwood and Peters, 1986) or prior experience (Briggs and Smith, 1989; Scarlett and Smith, 1991). Furthermore, Robbins et al. (1992) showed that the inhibitory effects of tannins cannot be generalized. In particular, some mammals that commonly eat plant products that contain tannins produce salivary proteins that bind to and reduce the inhibitory effects of tannins. The preferred food of chipmunks is apparently beech nuts (Fagus grandifolia) (Vander Wall, 1990), but in their absence acorns are eaten. Beyond this, little is known regarding chipmunk food preferences and factors affecting preferences. The purposes of our study were to (1) determine if chipmunks preferred white (Quercus alba) to northern red oak (Q. rubra) acorns; (2) assess the influence of acorn size on choice, and (3) determine whether or not acorn preference had measurable impacts on habitat use by chipmunks. METHODS Study site.-Our field studies were performed at the Forest Management Trail site of Hartwick College's Biological Field Station located in West Davenport, Delaware Co., N.Y. (42026'N, 74055'W). Information on small mammal activity, tree community structure and field trials of acorn choice were all collected from the same site in the autumn of 1991. Six animals that were used for laboratory choice experiments were captured at this site. These animals were held in captivity until January 1992, when trials were conducted. They were then released the following April. Acorn collection and storage. -We attempted to obtain acorns of both species from trees at the study site. Red oak (R) acorns were relatively abundant on the forest floor, but virtually no white oak (W) acorns could be found. We therefore collected dropped W acorns from two trees located 15 km away on the campus of Hartwick College, Oneonta, N.Y. W acorns were relatively homogenous in size and weight. In contrast, R acorns were highly variable in size and, after some initial trials, were separated into three size classes (Table 1). Group Rs acorns were about the same size as W acorns, RM were slightly larger than W acorns, and group RL were considerably larger than W acorns. All collected acorns were washed with tap water, air-dried and weighed. Acorns invaded by seed predators or showing evidence of rotting were excluded from analysis. We refrigerated all acorns until trials were run. Field and laboratory choice experiments. -Two series of choice trials were conducted in the field. The first series (Ra.,) ignored size differences among R acorns and provided chipmunks with a choice of equal numbers of W and R acorns. Ten W and R acorns were This content downloaded from 128.227.1.13 on Wed, 23 Oct 2024 17:00:24 UTC All use subject to https://about.jstor.org/terms 1993 PYARE ET AL.: CHIPMUNK FOOD SELECTION 175 presented in an 18-cm diam tray divided into four equal-sized sections. Acorns of each species were arranged in alternate fashion around the tray, with each section holding five acorns of a single species. A single tray was placed at or within 5 m of the grid point (see below) likely to have chipmunk activity (i.e., near burrows or fallen logs). Observers were located 15 m or more from the tray to note any interference by gray squirrels, in which case the trial was terminated. After 1 h, the numbers of W and R acorns remaining were counted. Once we recognized that acorn size may influence choice, we initiated a second series of trials in which we matched 10 W acorns against 10 R acorns of a single size class (Rs, RM, or RL)- Other conditions were identical to those described above. Controlled laboratory experiments were conducted in January 1992. The six chipmunks captured in October 1991, had been maintained on a diet of sunflower seeds, commercial rat chow and apples until the experiments. The trials were conducted in a 1.5 m x 0.9 m x 0.9 m rectangular cage enclosed in white paper. Food was removed from the chipmunks 24 h before testing (some chipmunks may have eaten food stored within their cage). The following day, four acorns of each of the four types (W, RS, RM, RL) were placed separately in glass petri dishes in the test chamber. A chipmunk was then released at the opposite end of the cage and permitted to interact with the acorns for 1 h. We considered a choice to have been made when a chipmunk made an obvious attempt to eat and/or store an acorn. Sniffing, handling or displacement of an acorn was not considered to be a choice. Each of the six animals was tested six times and the order with which the four types of acorns were presented (left to right) was varied randomly among trials. Habitat measurements and live trapping. -In conjunction with and simultaneous to the field trials of acorn preference, we also sought to determine if chipmunk activity within the Forest Management Trail was influenced by the distribution of oak trees. We established a 100-point grid (10 x 10, 20 m spacing) in the autumn of 1991. At each of the 100 grid points, we censused all the woody vegetation greater than or equal to 2.54 cm diam breast height (DBH; 1.3 m aboveground) within a circle of 8-m radius (area = 200 m2) centered on the point. Each tree or shrub was identified to species, categorized into a height class, and permanently tagged with a numbered aluminum tree tag. We recognized four height classes of trees. Dominant trees [height class (HC) = 1] were those that stood above the top layer of the canopy and received light on the top and sides. Codominants (HC = 2) were found within the main canopy and received direct sunlight from above, but only small amounts of direct sunlight on their sides. Intermediate (HC = 3) trees grew into the main canopy but only received small amounts of light from above and no direct sunlight on their sides. Overtopped (HC = 4) individuals were part of the canopy but received no direct sunlight. Within the understory, we recognized an upper and lower understory. The upper understory individuals remained below the canopy, but were more than half way to the bottom of the canopy. Lower understory individuals (shrubs and saplings) were less than half way to the bottom of the canopy. One Sherman live trap (25 cm x 9 cm x 9 cm) was placed at each grid point. From early October until mid-December, we trapped small mammals weekly. Each trap was baited with sunflower seeds, and was then covered with a wooden board and provided with insulation to minimize deaths from hypothermia. Traps were baited and set between 1500 and 1800 h and checked between 0630 and 0900 h the next morning. We identified all captured individuals to species. With the exception of Peromyscus and Clethrionomys, all individuals were released immediately at the capture site. The two named species were measured and/or marked, and then released at the capture site. Seven species of small mammals were captured, but chipmunks made up 42% of all captures (n = 419). Thus, despite the facts that (1) chipmunks are diurnal and (2) many animals had already become This content downloaded from 128.227.1.13 on Wed, 23 Oct 2024 17:00:24 UTC All use subject to https://about.jstor.org/terms 176 THE AMERICAN MIDLAND NATURALIST 130(1) TABLE 2.-Total number of individuals, total basal area and frequency of occurrence of the 10 most important species of trees found in the canopy layer of the Forest Management Trail at Pine Lake, Delaware Co., N.Y. Importance values are given for the canopy trees and for the understory woody vegetation (parentheses) Basal Frequency Importance Species Number area (m2) (no. plots) value (%)a Red oak (Quercus rubra) 460 25.9 88 33.0 (5.5) Red maple (Acer rubrum) 469 11.0 91 24.6 (26.2) White oak (Q alba) 116 6.6 64 11.8 (1.6) Black birch (Betula lenta) 163 2.3 43 8.6 (5.8) American beech (Fagus grandifolia) 101 1.4 32 5.7 (16.6) Eastern hemlock (Tsuga canadensis) 37 3.2 16 4.0 (2.7) Bigtooth aspen (Populus grandidentata) 36 2.4 19 3.8 (0.0) White pine (Pinus strobus) 41 1.8 16 3.3 (6.1) Hophornbeam (Ostrya virginiana) 13 2.4 13 1.8 (4.8) White birch (B. papyrifera) 11 0.4 16 1.1 (0.0) aImportance value = relative density + relative dominance + relative frequency (Cox, 1990). Based on 100 sample points (see Methods) Other species found infrequently at the site included American chestnut (Castanea dentata), shadbush (Amelanchier arborea), yellow birch (B. lutea), black cherry (Prunus serotina), pin cherry (P. pennsyl- vanzca), sugar maple (A. saccharum), white ash (Fraxinus americana) torpid by late November, we caught nearly as many chipmunks as white-footed mice (43% of all captures). Therefore, chipmunks were in all probability the most abundant small mammals at the site. We assumed that the frequency of capture of chipmunks was a direct measure of the level of activity in the field. Analysis.-We used Ivlev's electivity index (Ivlev, 1961) to measure preference in acorn choice. Electivity (E) is calculated as: E = (Ri - Pi)/(Ri + Pi), where Ri is the relative occurrence of the acorn type in the chipmunk diet and Pi is the relative occurrence of the acorn type in the available food supply. Electivity ranges from -1 (complete avoidance) to + 1 (maximum attraction), with 0 equivalent to random choice. Comparisons of the number of acorns taken were based on square-root transformations of the data. We added 0.5 to all counts to account for zeros, and compared means using 95% confidence intervals (=ci). We report the back-transformed values (Sokal and Rohlf, 1981). Our census of woody vegetation produced a large data set that we reduced by first calculating importance values (Cox, 1990) for all species of trees with individuals in the canopy (height classes 1 through 4). An identical but separate analysis was done for the woody understory vegetation. Importance values were calculated for each species based on (1) the total number of individuals; (2) the frequency of plots in which the species was found, and (3) total basal area (determined from DBH). We recorded 17 species of trees, but 10 species comprised 98% of the importance value scores (Table 2). We examined the relationship between chipmunk activity and forest community structure by comparing the number of chipmunk captures at all 100 grid points to the (1) total number of individuals and (2) total basal area for the 10 most important species of trees on the grid using multiple linear regression (general linear models procedure; SAS, 1985). The numbers of captures of the other small mammals were also included as independent variables. All variables that This content downloaded from 128.227.1.13 on Wed, 23 Oct 2024 17:00:24 UTC All use subject to https://about.jstor.org/terms 1993 PYARE ET AL.: CHIPMUNK FOOD SELECTION 177 were correlated with chipmunk activity at a level of P < 0.05 were entered into the regression model. However, for a variable to be retained in the model we required that it remain statistically significant (P < 0.05) when entered into the model as the last variable in the regression (type III sum of squares). Additional specific tests are described in the results. For statistical significance to be established, we required a probability of type I error of 0.05 or less. RESULTS Field choice trials.-Fifty-five of the 100 grid points were observed easily and were therefore used in the field trials. In our first series of 17 experiments (Ral; R acorns not sorted by size), no acorns were removed five times, and an equal number of W and R acorns were chosen once, but in 11 trials (64.7%) more W acorns were chosen than R acorns. R acorns were never chosen more frequently than W acorns. Excluding the five trials in which no choices were made, W acorns were taken more commonly than expected (X2 = 24.75, df = 2, P < 0.001). In addition, the mean number of W and R acorns chosen per trial differed. Based on the 12 trials in which acorns were removed, an average of 10.1 (95% ci = 9.65-10.58) and 2.6 (95% ci = 1.18-4.64) W and R acorns, respectively, were taken per trial (P < 0.05). Mean electivity for W (E = 0.20, SD = 0.10, n = 12) and R (E = -0.44, SD = 0.32, n = 12) acorns differed significantly (Kruskal-Wallis, H = 17.04, df = 1, P < 0.001), and in addition, both differed significantly from zero (zero was not included within the 95% confidence interval of either mean). An additional 42 trials were conducted with R acorns separated by size. In all cases, 10 W acorns were matched against 10 R acorns of one of the three R size classes (Table 1). The results of the experiments, broken down into the number of trials in which (a) no choices were made; (b) an equal number of R and W acorns were taken; (c) more W were taken, and (d) more R were taken, again showed that W acorns were taken more commonly than expected regardless of the size of the R acorns they were matched against (Fig. 1). R acorns were never taken more often than W acorns, and the only time that R and W acorns were taken with equal frequency was when W acorns were presented along with equal- sized RS acorns. The mean number of W acorns taken per trial was fairly constant. An average of 7.1 (95% ci = 4.93-9.73, n = 10), 8.2 (95% ci = 6.58-10.1, n = 12) and 7.1 (95% ci = 4.99-9.52, n = 10) W acorns were taken when matched against RS, RM and RL acorns, respectively. However, the mean number of R acorns taken per trial declined as R size class increased (RS = 3.0, 95% ci = 1.55-4.90, RM = 2.3, 95% ci = 1.09-1.92; RL = 0.7, 95% ci = 0.46-1.08). The relative preference for W acorns (=mean number of W taken/ mean number of R taken) therefore increased as R acorn size class increased (relative preference = 2.33, 3.65 and 23.0 for W vs. RS, RM and RL acorn size classes, respectively). Mean electivities for W and all three R acorn types (Fig. 2) indicated that W acorns were in all cases preferred to R acorns (Kruskal-Wallis Test, P < 0.001 for all three comparisons), and given that all electivities differed from 0 (zero was outside the 95% ci), it was apparent that chipmunks did not choose acorns randomly. The avoidance of R acorns became more pronounced as R size class increased (i.e., electivities approached values of -1.0; Fig. 2). Laboratory trials. -A total of 36 trials were run in the laboratory (six per animal) and a total of 95 acorns were chosen. The majority (62%) were W acorns. Of the remaining 38%, Rs, RM and RL made up 19%, 15% and 4%, respectively. Not surprisingly, the mean number of W, Rs, RM and RL acorns taken over each animal's six trials differed significantly (repeated measures analysis of variance, F = 24.16, df = 3, 20; Table 3). No acorns were taken in eight trials. Mean electivities for the remaining 28 trials (Table 3) indicated a preference for W acorns and a general avoidance of R acorns, especially in the large size class (repeated This content downloaded from 128.227.1.13 on Wed, 23 Oct 2024 17:00:24 UTC All use subject to https://about.jstor.org/terms 178 THE AMERICAN MIDLAND NATURALIST 130(1) 15 U Nothing Taken W>R R>W 0W=R CZ 10 0 -0 E :3 5 z Wvs. Rs Wvs.RM Wvs. RL Trial Series FIG. 1.-Acorn selection in field trials that pitted W acorns against either RS (n = 15 trials), RM (n = 14) or RL (n = 13) acorns. The number of times that (left to right) (a) nothing was taken; (b) more W were taken; (c) more R were taken, or (d) equal numbers of W and R were taken is shown for all three comparisons. Corresponding chi-square and P-values are: W vs. Rs; x2 = 20.0, P < 0.001; W vs. RM; X2 = 24.0, P < 0.001; W vs. RL; X2 = 10.4, P < 0.01 measures analysis of variance, F = 20.42, df = 3, 20, P < 0.001). The latter test showed that electivity did not vary among individuals or over the course of the experiment (P > 0.50). Forest structure and chipmunk activity. -The canopy at the Forest Management Trail was dominated, in order of decreasing importance, by red oak, red maple (Acer rubrum) and white oak (Table 2). A negative correlation between the abundance of red oaks and red maples (r = -0.577, df = 98, P < 0.001) described the major vegetational gradient within the site. White oak abundance was not correlated significantly with either red oak (r = 0.036) or red maple (r = -0.013) abundance. Chipmunks were captured at 89 of the 100 points, but number of captures at the 89 points varied from 1 to 4 (63 of the 89 points had only 1 or 2 captures). The number of captures at each point was negatively and significantly correlated with the number of Peromyscus captures (Table 4). Of all of the tree variables (total number of trunks and total This content downloaded from 128.227.1.13 on Wed, 23 Oct 2024 17:00:24 UTC All use subject to https://about.jstor.org/terms 1993 PYARE ET AL.: CHIPMUNK FOOD SELECTION 179 025 - () -0.25 C: W vs. Rs W vs. RM W vs. RL Trial Series FIG. 2.-Mean electivities for field tests in which chipmunks were presented with 10 W acorns and either 10 RS, RM or RL acorns. The bars above and below the histograms indicate ?2 SE basal area for the 10 most important trees), the number of chipmunk captures was correlated only with the number of hemlock and red oak trunks in the canopy (both negative) and the basal area of white oaks (positive; Table 4). Total chipmunk captures were not related significantly to R basal area (r = -0.045, ns). To control for possible covariation among the independent variables and to determine the relative strengths of the correlations with the chipmunk captures, we further analyzed the data using multiple regression analysis. The results (Table 4) were qualitatively unchanged from the univariate comparisons, but the relative importance of the four variables shifted. Most apparent was the decreased importance of Peromyscus in the multivariate analysis. The type III sums of squares suggest that Peromyscus was the least important of the four statistically significant relationships (Table 4). The correlations with the three tree variables were of approximately equal strength (Table 4). Although the regression model was highly significant (P = 0.0003), and all four variables made significant contributions, in combination they accounted for only 19.9% of the variation in the number of chipmunk captures per site. TABLE 3.-Results of the laboratory choice experiments presented as the mean number of acorns taken in the six trials, and mean electivities of each acorn type Acorn type W RS RM RL Number (SD) 9.9 (5.3) 3.0 (2.7) 2.0 (1.1) 0.7 (1.2) Electivity (SD) 0.43 (0.33) -0.55 (0.58) -0.57 (0.57) -0.90 (0.28) SD = standard deviation This content downloaded from 128.227.1.13 on Wed, 23 Oct 2024 17:00:24 UTC All use subject to https://about.jstor.org/terms 180 THE AMERICAN MIDLAND NATURALIST 130(1) TABLE 4.-Results of the multiple regression analysis relating the number of chipmunk captures to features of the forest community Type III Independent variable r (P) F-value (P) White oak basal area 0.239 (0.017) 4.99 (0.028) Number of hemlock trunks -0.223 (0.026) 5.38 (0.022) Number of red oak trunks -0.212 (0.034) 5.35 (0.023) Number of Peromyscus captures -0.274 (0.006) 4.22 (0.043) r-refers to the simple correlation between number of chipmunk captures and the variable indicated P-the probability value for each correlation and F-value DISCUSSION Acorn preference. -Results from the field and laboratory trials demonstrated clearly that acorns were not chosen randomly by chipmunks. In field trials, the average number of W acorns chosen per trial was significantly greater than the number of R acorns, and in almost every trial, W acorns were chosen more frequently than R acorns, regardless of the R acorn size class they were matched against. Even more convincing was the consistent selection of W acorns in the laboratory despite the 3: 1 numerical bias against them. All six chipmunks adhered to this preference throughout the sequence of trials. The randomization of the position of the acorns in different trials guarded against the chipmunks learning the location of the W acorns. Furthermore, in none of the laboratory trials did chipmunks select the first type of acorn encountered or even handled. Hence, it is apparent that chipmunks discriminated among the acorn types and preferred W over R. They appeared to also prefer smaller acorns. This is indicated by the increasing discrimination against R acorns as size increased (Figs. 1 and 2; Table 3). However, given that W acorns were preferred when presented with equal-sized Rs acorns, size was of secondary importance. Although the preference for W acorns can hardly be disputed, the basis for the preference has yet to be established. The lower tannin content of W acorns (Martin et al., 1951) is certainly the leading candidate, but admittedly, we did not measure the tannin content of the acorns in our study. And, although it does not seem likely, we do not know whether the off-site W acorns that we used had a tannin content that differed from the on-site W acorns. Moreover, despite the fact that tannins are known to be distasteful and/or impede mammalian digestion (Haslam, 1981), it is worth noting that neither of these conditions is known to be true in chipmunks. Robbins et al. (1992) have shown that mammals that normally encounter tannins in the diet often produce salivary proteins that bind and inhibit tannin activity. Thus, although a tannin-free diet yields more digestible protein, salivary proteins greatly reduce the quantity of protein that would otherwise be lost for a tannin- containing, but equally rich protein diet. Preference for W acorns by chipmunks may also reflect a foraging strategy that is "designed" (sensu Pyke et al., 1977) independently of tannin content. For instance, both W and R acorns are dropped during the autumn season, but W acorns sprout before winter whereas R acorns remain dormant until the following spring (Spurr and Barnes, 1980). Smith and Folmer (1972) noted that gray squirrels rejected acorns with sprouted radicals, and Fox (1982) reported that extended taproots were indigestible. Smallwood and Peters (1986) therefore suggested that gray squirrels eat early-germinating W acorns before sprout- ing occurs and only then remove the late-germinating R acorns. In essence, a perishable resource is eaten first. Chipmunks may follow a similar strategy (see also Reichman, 1988). This content downloaded from 128.227.1.13 on Wed, 23 Oct 2024 17:00:24 UTC All use subject to https://about.jstor.org/terms 1993 PYARE ET AL.: CHIPMUNK FOOD SELECTION 181 Size, as it influences handling ability, may also influence food choice. Our data suggests that chipmunks prefer smaller R acorns. Although size is the most likely explanation for the difference in selection among R acorns, we cannot discount the possibility that smaller R acorns had detectably lower tannin levels. Assuming this was not the case, then the preference for small size may reflect an evolved or learned behavior geared towards max- imizing efficiency of seed storage during critical prewinter foraging periods (Wrazen and Wrazen, 1982). Chipmunks store a wide range of seeds (references in Vander Wall, 1990), but appear to focus less on acorns than beech nuts (Table 7.4 in Vander Wall, 1990). Given that most beech trees at our site are in the understory (Table 2), chipmunks are apparently forced to feed on acorns and prefer the small acorns. Nonetheless, we note that the total biomass of RM acorns taken (number x mean mass) slightly exceeded the total biomass of RS acorns taken in both the field and laboratory trials (in both cases, the total biomass of R acorns remained below the total biomass of W acorns). So even though RL acorns were clearly avoided, the discrimination between RS and RM acorns may not have been as strong as the simple comparison of number of acorns taken would indicate. Larger acorns may be avoided because they are harder to store in cheek pouches, allowing fewer acorns to be transported to cache sites or burrows. Additionally, smaller acorns should pack more densely in caches, allowing more acorns to be stored. A number of tightly packed caches containing a large number of acorns theoretically increases the odds that an adequate food supply will remain viable through the winter hibernation period when chipmunks rely on food reserves for survival (French, 1988). Habitat use.-Our third purpose was to attempt to place the food choices of chipmunks in an ecological context. In this case it appears that chipmunk preference for W acorns had a measurable influence on activity during the period of acorn drop. Although we accounted for only 20% of the variation in the number of chipmunk captures, our results conform strikingly to the demonstrated preference for W and avoidance of R acorns. The greatest number of captures were found at sites with high basal areas for W trees and few R trees. Plots with a high W basal area were characterized by a few, large W trees that belonged to the dominant and codominant classes. Assuming that basal area is an accurate measure of acorn production, we believe that the high activity of chipmunks at these plots was due to the availability of W acorns. The lack of a correlation between chipmunk activity and R basal area and the significant negative correlation with the number of R trunks suggests an avoidance, or at the least a neutral treatment, of plots with high R abundance. We can only speculate as to the underlying basis for the correlation between chipmunk activity and the other variables. The negative association with eastern hemlocks is very likely related to either the lack of food near hemlocks or a lack of ground cover. Sites dominated by hemlocks had few oaks of either species, and the dense canopy formed by the hemlocks also prevented light from reaching the forest floor. As a consequence, little woody understory vegetation grew beneath hemlocks. Hence, foraging chipmunks probably only infrequently entered hemlock-dominated plots because foraging profitability was low due either to a lack of food or cover while foraging. It is not clear why numbers of white-footed mice and chipmunk captures varied inversely. Given the size differences (20 g vs. 80 g), and the fact that they had nonoverlapping activity periods (diurnal vs. nocturnal), it seems unlikely that interspecific competition was a factor. In general, competition is not viewed as a major factor in the structure of small mammal communities in eastern deciduous forests (Morris, 1983; Seagle, 1985). The slight subdivision of habitat use that we detected possibly reflects an intrinsic difference in habitat preference (Dueser and Shugart, 1979). On the other hand, Peromyscus forage heavily on acorns (Briggs and Smith, 1989) and intense exploitation of W acorns by chipmunks may force Peromyscus to move to obtain sufficient This content downloaded from 128.227.1.13 on Wed, 23 Oct 2024 17:00:24 UTC All use subject to https://about.jstor.org/terms 182 THE AMERICAN MIDLAND NATURALIST 130(1) food. Removal experiments will be required to determine whether Peromyscus habitat use is truly dependent upon chipmunk activity. As a final point, we note that the chipmunk food preference and its influence on habitat use may contribute to successional trends at our study site. Oaks, and especially the white oaks, are being replaced by maples, beech and white pine (Pinus strobus; Table 2). Surely the shade intolerance of oak seedlings (Table 14.2 in Spurr and Barnes, 1980) is an important contributing factor. 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Mammal., 59:324-329. SUBMITTED 20 AUGUST 1992 ACCEPTED 25 JANUARY 1993 This content downloaded from 128.227.1.13 on Wed, 23 Oct 2024 17:00:24 UTC All use subject to https://about.jstor.org/terms

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Acorn Preference and Habitat Use in Eastern Chipmunks