1 Journal of Mammalogy, 91(5): , 2010 Coexisting desert rodents differ in selection of microhabitats for cache placement and pilferage MARYKE J. SWARTZ,* STEPHEN H. JENKINS, AND NED A. DOCHTERMANN Department of Biology, University of Nevada, M/S 314, Reno, NV 89557, USA (MJS, SHJ, NAD) Present address of MJS: San Diego Zoo s Institute for Conservation Research, San Pasqual Valley Road, Escondido, CA 92027, USA * Correspondent: Seed caching by desert rodents in the family Heteromyidae is an important behavioral adaptation for animals living in environments with limited and unpredictable food resources. Heteromyids cache seeds throughout their home ranges, either concentrated in 1 location (larder hoard) or in multiple, small seed piles (scatter hoards). To minimize cache pilferage by other rodents, coexisting species may scatter hoard seeds in distinct microhabitats. We examined interspecific differences in caching and pilfering behaviors of 3 coexisting heteromyid rodents, Merriam s kangaroo rat (Dipodomys merriami), the pale kangaroo mouse (Microdipodops pallidus), and the little pocket mouse (Perognathus longimembris), to determine if caching microhabitat affects likelihood of pilferage. In outdoor enclosures we tracked cache placement and measured pilferage of artificial caches using radiolabeled Indian ricegrass (Achnatherum hymenoides) seeds. M. pallidus and P. longimembris placed seed caches mostly under shrubs, whereas D. merriami placed caches predominately in open microhabitat. However, D. merriami showed a significant preference for pilfering caches under shrubs, whereas P. longimembris did not show a significant preference for pilfering caches in either open or undershrub microhabitats. The 2 species pilfered similar numbers of caches. Coexisting heteroymid rodent species may contribute to spatial heterogeneity of available resources by caching in different microhabitats, thereby reducing but not eliminating cache pilferage by other species. DOI: /09-MAMM-A Key words: cache placement, coexistence, Dipodomys merriami, Heteromyidae, Microdipodops pallidus, microhabitat, Perognathus longimembris, pilferage, scatter hoarding, seed caching E 2010 American Society of Mammalogists Seed caching by desert rodents in the family Heteromyidae is an important behavioral adaptation for animals living in environments with unpredictable food resources (Giannoni et al. 2001). Andersson and Krebs (1978) proposed that food caching could be evolutionarily stable only if the individual caching food has a higher probability of recovering its caches than other individuals. However, Vander Wall and Jenkins (2003) showed that heteromyid rodents pilfer as much as 28 51% of artificial caches per day, and Vander Wall et al. (2006) showed that rodents pilfer artificial caches and real caches at similar rates. To reduce cache pilferage different species of heteromyids may cache seeds in distinct microhabitats. We examined interspecific differences in caching and pilfering behavior to determine if caching microhabitat affected likelihood of pilferage. To maintain seed resources during times of food scarcity heteromyids store seeds in burrows (larder hoards) or in multiple, small seed piles buried in shallow soil (scatter hoards). Most species of heteromyids both larder hoard and scatter hoard (Jenkins and Breck 1998; Jenkins et al. 1995; Price et al. 2000). We focused on microhabitat selection for scatter hoarding, however, because scatter-hoarded caches potentially can be pilfered by all rodent species, but larder hoards made by small-bodied species may be unavailable to larger animals (Jenkins and Breck 1998). Studies using trapping and bait stations suggest that heteromyids partition microhabitats during foraging. Quadrupedal pocket mice (Perognathus spp. and Chaetodipus spp.) more readily take seeds from shrub-covered microhabitats, whereas bipedal kangaroo rats (Dipodomys spp.) and kangaroo mice (Microdipodops spp.) more readily exploit open areas (Daly et al. 1992; Harris 1984; Lemen and Rosenzweig 1978; Price 1978a, 1978b), although Leaver and Daly (2001) did not find microhabitat preferences exhibited by foraging Dipodwww.mammalogy.org 1261
2 1262 JOURNAL OF MAMMALOGY Vol. 91, No. 5 omys merriami and Chaetodipus. Although acquiring seeds is important for desert rodents, maintaining these limited resources also is essential for survival. Resource management has 4 stages: harvesting seeds, caching seeds, sometimes rearranging stored seeds (Jenkins et al. 1995), and retrieving stored seeds. Researchers have studied the 1st and last of these stages in heteromyid seed management and their implications for species coexistence (Leaver and Daly 2001; Price et al. 2000), but little is known about caching and pilfering behaviors of heteromyid rodents in the field. Therefore, we compared preferred microhabitats for caching seeds and for pilfering caches of coexisting species in the field. Caches made by rodents are at risk of pilferage by other rodents (Daly et al. 1992; Leaver and Daly 2001; Murray et al. 2006; Preston and Jacobs 2001), and heteromyids use behavioral strategies to prevent pilferage, including optimal spacing of caches (Daly et al. 1992) and changing foodhoarding strategies from scatter hoarding to larder hoarding in response to pilferage by conspecifics (Preston and Jacobs 2001). Animals also can avoid pilferage by transporting food to areas or microhabitats where potential pilferers are absent or uncommon (Suhonen and Alatalo 1991). We assessed how heteromyids might manage these risks based on both caching and pilfering behavior. We compared caching and pilferage behaviors of 3 coexisting heteromyid rodents, Merriam s kangaroo rats (Dipodomys merriami), pale kangaroo mice (Microdipodops pallidus), and little pocket mice (Perognathus longimembris). Specifically, we compared the microhabitat distribution of scatter hoards and pilfered artificial caches among these species. We hypothesized that heteromyid species would select distinct microhabitats for caching and the same microhabitats for pilfering to minimize pilferage by heterospecifics. We predicted that heteromyids would select microhabitats for cache placement and pilferage that reflected their microhabitat preferences for space use and seed harvesting as reported in the literature. Thus, we predicted that pocket mice (P. longimembris) would scatter hoard and pilfer more seed caches under shrubs, whereas kangaroo rats (D. merriami) and kangaroo mice (M. pallidus) would use open microhabitat more often for scatter hoarding and pilfering. MATERIALS AND METHODS Study site. We conducted this study during the summers of 2007 and 2008 in outdoor enclosures at the Hot Springs Mountains,,14 km east of Fernley, Nevada (Universal Transverse Mercator: E, N). Perennial plants at this sandy site included Indian ricegrass (Achnatherum hymenoides), Sarcobatus vermiculatus, Atriplex confertifolia, A. canescens, Psorothamnus polydenius, Tetradymia spinosa, and T. tetrameres, and cheatgrass (Bromus tectorum) and Russian thistle (Salsola paulsenii and S. tragus) were abundant annuals. The rodent community consisted of several nocturnal heteromyids and 1 diurnal sciurid rodent, Ammospermophilus leucurus. Trials were conducted in 2 enclosures of approximately m. Although much smaller than the home ranges of heteromyid rodents, the enclosures were much larger than arenas used in previous laboratory studies of caching and pilfering behavior of heteromyid rodents (Jenkins and Breck 1998; Preston and Jacobs 2001, 2005; Price et al. 2000). The enclosures were built in 1994 using 4 corner poles and fencing of reinforced 0.6-cm-mesh hardware cloth. Fences extended 0.75 m aboveground and 0.5 m belowground. Along the top of the fences 30-cm-wide strips of sheet aluminum were attached to prevent rodents from entering and escaping (Longland et al. 2001). We buried artificial burrows outside enclosures. For trials with the larger species, D. merriami, we buried an 18.9-liter bucket with 2 attached polyvinyl chloride tubes (55 cm long cm in diameter) that extended into the enclosure and rested on top of the sand. We used a 7.6-liter bucket with 2 polyvinyl chloride tubes 65 cm long cm in diameter in trials with P. longimembris and M. pallidus. To prevent harvester ants (Pogonomyrmex salinus) from entering the enclosures and removing seeds, 30-cm-wide strips of sheet aluminum were buried approximately 45 cm away from the fence and stood approximately 20 cm aboveground. We removed an ant colony in 1 enclosure by shoveling the ants and sand away from the enclosure and applied commercial insecticide (Amdro Ant Block; Ambrands, Atlanta, Georgia) on the ground between the fence and sheet aluminum at both enclosures. The vegetation inside the enclosures included A. canescens, B. tectorum, P. polydenius, A. hymenoides, S. tragus, and S. paulsenii. We removed Indian ricegrass and Russian thistle seedlings to enhance differences in vegetation structure between shrub-covered and open microhabitats. We also trimmed seed heads from Indian ricegrass plants within the enclosures to decrease the likelihood that rodents would harvest seeds from natural sources. We estimated the diameter of every plant in an enclosure by averaging length and width measurements and then estimated area by assuming that each plant was circular. Areas of individual plants were summed to get the total vegetation cover for each enclosure. Although vegetation cover included mature Indian ricegrass and Russian thistle plants, which were nonwoody, we refer to cover as shrub cover because these herbaceous plants provided substantial vertical structure under which rodents could forage. In 2007 the percent cover of vegetation was 11.4% in 1 enclosure and 13.3% in the 2nd. In 2008 corresponding values were 22.6% and 19.2%. The difference in vegetation cover between years was due to new growth of Salsola spp. Although the amount of vegetation cover was different between years, it was similar between enclosures in each year. Seed bank. Seed density can affect microhabitat selection of a rodent during seed harvesting (Price and Heinz 1983). Similarly, seed densities also can affect where rodents place or pilfer caches. To compare the seed banks in open and undershrub microhabitats prior to scatter-hoarding trials, we randomly collected 20 soil samples (10 from open microhab-
3 October 2010 SWARTZ ET AL. RODENT CACHE PLACEMENT AND PILFERAGE 1263 itats and 10 from under shrubs) in each enclosure. Each sample was collected using a 165-ml metal container and was placed into a Ziploc bag (SC Johnson, Racine, Wisconsin). In the laboratory we drained each sample through a sieve with a 1-mm opening. Using a dissection microscope, we identified and counted seeds in each sample. Based on known heteromyid preferences for seed species, only Indian ricegrass (A. hymenoides), cheatgrass (B. tectorum), and Russian thistle (Salsola spp.) were quantified (Kelrick et al. 1986; Longland 2007; McAdoo et al. 1983). We detected no difference in total number of seeds in the seed bank between enclosures (F 1, , P ) or between microhabitat types (F 1, , P ) and found no significant interaction between enclosure and microhabitat type (F 1, , P ) based on a factorial analysis of variance (ANOVA). Thus, we did not expect cache placement or cache pilferage to be affected by the background availability of seeds in each microhabitat type. Soil moisture. Because soil moisture can affect the ability of a rodent to find buried caches (Vander Wall 2000), it may influence where rodents place scatter hoards and where they pilfer caches. On 15 and 21 August 2008 we collected 2 soil samples in open microhabitats and 2 in undershrub microhabitats in each enclosure using a 165-ml metal container. We placed each sample in a Ziploc bag and weighed them at the laboratory. We then dried the 16 soil samples in an oven at 80uC for 3 days. The samples were reweighed after the drying period. Because of an apparent measuring error, 1 sample was removed from the analysis. We detected no difference in percent soil moisture between open and undershrub microhabitats (F 1, , P ) or between enclosures (F 1, , P ), and the interaction between microhabitat type and enclosure also was not significant (F 1, , P ) based on a factorial ANOVA. On average (6 SD), soil from both enclosures contained 0.47% % water. In addition, the average monthly precipitation was only mm from June to August 2007 and mm from June to August 2008 (Western Regional Climate Center 2008). Because microhabitats did not differ in soil moisture under these very dry conditions, we did not expect soil moisture to affect selection by rodents of microhabitats for caching and pilfering. Radiolabeling seeds. To locate and map caches in scatterhoarding and pilfering trials, commercial Nezpar Indian ricegrass seeds (Granite Seed, Lehi, Utah) were labeled using scandium-46, a biologically inactive, gamma-emitting radionuclide with a half-life of 84 days. The decay product is titanium-46, which is neither radioactive nor toxic (Emsley 2001; National Nuclear Data Center 2009). We used a Geiger counter to find each cache location. This technique has been used in several rodent-caching studies, including in our study area, with no apparent harmful effects on the rodents (Hollander and Vander Wall 2004; Longland et al. 2001; Vander Wall 2000). Indian ricegrass seeds were labeled with the radioisotope at the University of Nevada, Reno, by sealing them in containers with ScCl 3 and distilled water for approximately 1 h. After seeds adsorbed the moisture they were dried for 48 h. We transported radiolabeled seeds to the field site following radiation safety protocols created by the Radiation Safety Officer at the University of Nevada, Reno ( ). Scatter-hoarding trials. In May 2007, before scatterhoarding trials were conducted, we removed rodents from the outdoor enclosures during 3 nights of livetrapping. We followed guidelines of the American Society of Mammalogists (Gannon et al. 2007) for all small mammal trapping sessions, and protocols were approved by the Institutional Animal Care and Use Committee at the University of Nevada, Reno (A06/07-51). We obtained subjects for scatter-hoarding trials the night before each trial on trapping grids near the outdoor enclosures. Each animal was tagged to ensure that it was used only in 1 trial. For each trial we transported an individual trapped on the grid to a nearby enclosure and placed it in the artificial burrow outside the enclosure. A trial began when an animal was placed in the artificial burrow, which occurred shortly after dawn. We gave each individual,3,120 (12 g) radiolabeled Indian ricegrass seeds. This is an abundant native plant in the study area, and its seeds are highly preferred by heteromyid rodents (McAdoo et al. 1983). We set out 4-g piles of seed (,1,040 seeds per pile) at 3 random locations, 1 in an open area, 1 under a shrub, and 1 along the periphery of a shrub. Placing seed piles in different microhabitats controlled for the possibility that the placement of caches by rodents could be the result of rapid sequestration near the source of a food bonanza (Jenkins and Peters 1992), which might lead to inaccurate assessment of microhabitat selection for caching. Each individual was allowed 24 h to cache seeds. The day following a caching trial we located caches using a Geiger counter, removed the caches using a spoon and sieve, and counted the seeds in each cache. In addition, we measured the distance from a cache to the edge of the nearest shrub canopy or fence, whichever was closest. Caches along the edge or under a shrub canopy were considered undershrub and caches beyond the shrub edge were in open microhabitat. Animals were trapped and returned to their original location on the trapping grid. Successful trials were conducted with 4 P. longimembris (3 females and 1 male), 5 D. merriami (3 females and 2 males), and 2 M. pallidus (1 male and 1 female). Pilfering trials. Between June and August 2008 we conducted trials to compare cache pilferage behavior of D. merriami and P. longimembris in the same outdoor enclosures used in We obtained individuals from nearby trapping grids as in We did 7 successful pilfering trials with D. merriami (4 females and 3 males) and 6 successful trials with P. longimembris (2 females and 4 males). M. pallidus was not captured in 2008 and therefore was not used in pilfering trials. Before releasing a subject in an enclosure, we made 12 artificial caches to mimic real caches, 6 in open microhabitat and 6 in undershrub microhabitat. Conducting studies using artificial caches is informative because any removal of seeds can be attributed to pilferage (Vander Wall and Jenkins 2003).
4 1264 JOURNAL OF MAMMALOGY Vol. 91, No. 5 Furthermore, Vander Wall et al. (2006) found that artificial caches and real rodent caches were pilfered at similar rates by Tamias amoenus. We made the artificial caches using Indian ricegrass seeds labeled with a scandium isotope (see above). We determined locations of artificial cache sites by using random coordinates within the enclosure. All caches were spaced at least 0.5 m apart to reduce the chance of a pilferer finding an additional cache after discovering one (Leaver 2004). Caches in open microhabitat were placed at least 10 cm from the fence or edge of a shrub. Caches in shrub microhabitat were placed anywhere from the edge of the shrub to the base or stem. If a random location did not fit these criteria, a new location was chosen. We created artificial caches containing,130 (0.5 g) radiolabeled Indian ricegrass seeds at a depth of 1 cm. This is approximately the mean size of Indian ricegrass caches made by heteromyid rodents (Longland and Clements 1995) and the depth at which Dipodomys spp. and Perognathus spp. can detect caches (Johnson and Jorgensen 1981). Using gloves, we then patted down each artificial cache to remove any visual evidence of disturbance of the sand. Each D. merriami or P. longimembris individual was given 24 h to pilfer the artificial caches. We then removed the pilferer and located caches using a Geiger counter. For each cache we recorded the coordinates, counted the number of seeds, and measured the distance to the edge of the nearest shrub or fence. Subjects were exposed to a range of moonlight intensities during caching trials in 2007 and pilfering trials in Because light intensity may deter heteromyid rodents from foraging in open microhabitats (Longland and Price 1991), we used a moon calendar to calculate effective darkness on nights of trials as time between sunset and moonrise + (1 2 moon illumination fraction) 3 time between moonrise and moonset + time between moonset and sunrise. We found no difference in average effective darkness during caching trials with the 3 species in 2007 (F 2, , P ) nor during pilfering trials with D. merriami and P. longimembris in 2008 (F 1, , P ). For kangaroo rats and pocket mice, no differences existed between caching trials in 2007 and pilfering trials in 2008 (D. merriami: F 1, , P , P. longimembris: F 1, , P ). Data analysis. For scatter-hoarding trials, mean cache sizes were compared in open and undershrub microhabitats for 4 D. merriami using a paired t-test, for M. pallidus using a 2- sample t-test, and for P. longimembris using a 1-sample t-test. Mean distances of caches from a shrub edge or fence were compared between species using a 1-way ANOVA. All pairwise comparisons between species using Tukey s test were conducted with an experiment-wise error rate of For pilfering trials, a 2-sample t-test was used to compare the proportion of caches pilfered between species. To compare cache removal from the 2 microhabitats for each species, each individual s 12 caches were ranked by the number of seeds remaining. If an individual harvested more seeds from each of the 6 caches in one microhabitat than from any of the 6 in the other, then rank sums would be and , respectively. The rank sums were then compared between the 2 microhabitat types for each species using paired t-tests. Data from recaching in pilferage trials in 2008 were combined with the caching data from 2007 and were analyzed using a 2-way ANOVA to compare mean cache distances between years and species. Parametric tests were used in all cases because these are robust to nonnormality of data distributions (Gotelli and Ellison 2004), and our data were not markedly skewed or bimodal. All statistics were computed using the program Minitab 15 (Minitab, Inc., State College, Pennsylvania). The criterion for statistical significance was P, RESULTS Rodent community composition. In 490 trap nights in 2007 we captured the following rodent species: D. merriami (85.2% of 250 total captures), Dipodomys deserti (8.4%), P. longimembris (2.4%), Dipodomys microps (2.0%), Peromyscus maniculatus (1.2%), M. pallidus (0.4%), and A. leucurus (0.4%). The community composition was similar in 2008 based on 720 trap nights, except that M. pallidus and P. maniculatus were not captured and 1 Chaetodipus formosus was captured. Although 85.2% of the captures in 2007 were D. merriami, only 50% of 116 total captures were D. merriami in 2008, and 20.7% were P. longimembris, 15.5% were D. deserti, 11.2% were D. microps, 1.7% were A. leucurus, and 0.9% were C. formosus. Scatter-hoarding trials. Dipodomys merriami made caches of similar size in the 2 microhabitats (t , P ; Table 1), although 1 D. merriami made caches only in open microhabitat. Only 1 M. pallidus and 1 P. longimembris placed caches in the open. M. pallidus made similar-sized caches in both microhabitats (t , P ). Examination of the data for P. longimembris suggests that larger caches were made under shrubs compared to the single cache it made in the open (t , P ); however, it is a marginally nonsignificant outcome (0.05, P, 0.10) and sample size was too small to conclude this. The mean distance (6 SD) of caches from a shrub edge or fence was cm for D. merriami, cm for M. pallidus, and cm for P. longimembris. Negative distances for M. pallidus and P. longimembris indicate that caches were placed under shrubs. These mean distances were significantly different among species (F 2, , P ; Fig. 1). Even when cache distances were weighted by numbers of seeds (because caches varied in size), species differed in cache placement (F 2, , P ). Because enclosures differed slightly in shrub cover and logistical constraints prevented use of enclosure as a blocking factor in the experimental design, the interpretation of these results might be compromised by differences in numbers of trials of the 3 species in the enclosures. Four of 5 trials with D. merriami were done in enclosure 2 with greatest shrub cover, both trials with M. pallidus were done in enclosure 2, but only
5 October 2010 SWARTZ ET AL. RODENT CACHE PLACEMENT AND PILFERAGE 1265 TABLE 1. Mean number of seeds per cache (cache size) and mean number of caches in different microhabitats for individuals of 3 heteromyid rodents. SDs are given in parentheses. An asterisk (*) indicates that only 1 individual had a cache (or caches) in the microhabitat. Microhabitat Species n Cache size No. caches Cache size No. caches Dipodomys merriami (167.7) 4.6 (1.9) (122.3) 2.5 (1.7) Microdipodops pallidus (*) 3 (*) 56.9 (37.4) 5.5 (3.5) Perognathus longimembris (*) 1 (*) 92.0 (56.2) 5.8 (5.5) Open Undershrub 1 of 4 trials with P. longimembris was done in enclosure 2. Therefore, P. longimembris cached more seeds under shrubs than the other 2 species even though it had slightly less shrub cover available on average. All pairwise comparisons showed that D. merriami differed in cache placement from M. pallidus and P. longimembris, but M. pallidus did not differ significantly from P. longimembris. Thus, D. merriami placed caches away from shrub edges, whereas P. longimembris and M. pallidus placed caches under shrubs. Pilfering trials. On average (6 SD), individual Merriam s kangaroo rats pilfered seeds from 65.5% % of artificial caches, whereas little pocket mice pilfered seeds from 52.8% % of available caches. Caches were considered pilfered if,120 seeds remained. Because SD seeds comprised 24 samples of 0.5 g, the chance of recovering fewer than 120 seeds without pilferage was if counts of Indian ricegrass seeds in 0.5 g are normally distributed. Therefore, this is a conservative assessment of pilferage. The 2 species did not differ in the proportion of caches pilfered (t , P ). Dipodomys merriami showed a significant preference for pilfering caches under shrubs (t , P ; Fig. 2). P. longimembris did not show a significant preference for pilfering caches from open or undershrub microhabitats (t , P ). Four individual P. longimembris and 4 D. merriami made new caches during pilfering trials. The mean distance of new caches from a shrub edge was cm for D. merriami and cm for P. longimembris. Mean cache distance from the nearest shrub did not differ significantly between years (F 1, , P ), nor was the species by year interaction significant (F 1, , P ), but cache distances differed significantly between D. merriami and P. longimembris when these data from recaching in pilferage trials in 2008 were combined with the caching data from 2007 (F 1, , P ). As in 2007, more of the 2008 trials with D. merriami were done in the enclosure with slightly greater shrub cover and vice versa for P. longimembris, so the greater use of open microhabitat for recaching by D. merriami cannot be accounted for by greater availability of this microhabitat. DISCUSSION Merriam s kangaroo rats scatter hoarded more seeds in open microhabitat, whereas little pocket mice and pale kangaroo FIG. 1. Cache placement by 3 heteromyid rodent species in relation to the nearest shrub or fence edge. Open circles indicate mean distances for each individual, black diamonds indicate species means, and the dotted line indicates the shrub or fence edge. Negative distances indicate that caches were underneath shrubs, and positive distances indicate that caches were in the open. FIG. 2. Rank sums of seeds remaining in artificial caches provided to 7 Dipodomys merriami and 6 Perognathus longimembris. Each individual searched for 6 caches of approximately 130 seeds in open and undershrub microhabitats. The line within each box represents the median, the upper boundary of the box represents the 75th percentile, and the lower boundary represents the 25th percentile.
6 1266 JOURNAL OF MAMMALOGY Vol. 91, No. 5 mice scatter hoarded seeds mostly under shrubs. For kangaroo rats and pocket mice this finding is consistent with their microhabitat preferences for harvesting seeds and their space use in general (Daly et al. 1992; Harris 1984; Lemen and Rosenzweig 1978; Price 1978a, 1978b). This result also is consistent with the demonstration by Hollander and Vander Wall (2004) that Panamint kangaroo rats (Dipodomys panamintinus) preferred open microhabitat, whereas Great Basin pocket mice (Perognathus parvus) preferred undershrub microhabitat for scatter hoarding. However, kangaroo mice in this study scatter hoarded more seeds under shrubs despite their preference for harvesting seeds in the open (Harris 1984). Preferences for harvesting microhabitats have been attributed to heteromyid locomotion and antipredator behavior; bipedal kangaroo rats and kangaroo mice, which are more closely related to each other than to pocket mice (Hafner et al. 2008), use their erratic jumping ability to evade predation attempts more effectively in the open than do quadrupedal pocket mice (Longland and Price 1991). The 2 M. pallidus we studied tended to cache near the edges of shrubs, which still would allow them to use their erratic jumping ability to escape predators. Another possibility is that kangaroo mice might be excluded from open microhabitat by competitive interference from kangaroo rats (Blaustein and Risser 1976; Lemen and Freeman 1986). Thus, the smaller size of kangaroo mice and pocket mice compared to kangaroo rats could be as important as locomotion in determining space use. Similarly, digging costs vary with body size (Morgan and Price 1992; Price and Heinz 1983), so interspecific differences in body size could cause differences in preferred microhabitats for caching seeds if microhabitats differ in soil structure. However, the soil was very sandy in both microhabitat types in our enclosures. Another possibility is that kangaroo mice not only use shrub edges to minimize pilferage by heterospecifics but also make smaller caches to make their caches less apparent to foragers. Despite the impressive olfactory abilities of heteromyids (Johnson and Jorgensen 1981), caches with fewer seeds have a lower chance of being discovered and removed by pilferers, especially in dry soil (Geluso 2005). Distinguishing between these potential explanations is difficult because kangaroo mice have not been studied as extensively as kangaroo rats and pocket mice. More studies of the foraging behavior of kangaroo mice would help decide this issue. Although Merriam s kangaroo rats and little pocket mice selected distinct microhabitats for caching, they pilfered caches in the other or both microhabitats. Thus, preferences for caching microhabitats might be due to factors such as reduction of predation risk, avoidance of interference competition, or suitability of soil for digging rather than avoidance of pilferage by heterospecifics. The finding that Merriam s kangaroo rats preferred to pilfer caches under shrubs has at least 2 alternative explanations. First, kangaroo rats could find caches under shrubs if they use this microhabitat to harvest seeds or make scatter hoards. However, most studies suggest that they primarily use open microhabitats during seed acquisition (Daly et al. 1992; Harris 1984; Lemen and Rosenzweig 1978; Price 1978a, 1978b), and this study demonstrated that they also cache more seeds in open microhabitat. The alternative explanation, which is consistent with our results, is that kangaroo rats prefer to eat or move caches from undershrub microhabitat to open microhabitat, thereby maintaining and increasing their own resources. Kangaroo rats in this study did move caches into the open. If more caches are available for a kangaroo rat in the open, a microhabitat that might be more costly for pocket mice to use, the kangaroo rat could increase its fitness, especially when seed production is low. Similar to kangaroo rats, pocket mice moved pilfered caches into the microhabitat that they preferred for space use and caching under shrubs. However, some individual pocket mice pilfered caches in the open despite the greater risk of predation (Longland and Price 1991). Also, pocket mice and kangaroo rats in this study pilfered at similar rates. These rates of cache removal could be affected in natural conditions (outside of enclosures) by competitive interference because kangaroo rats dominate access to artificial seed patches by chasing away pocket mice (Leaver and Daly 2001). Because Merriam s kangaroo rats pilfered more seeds in undershrub microhabitat and little pocket mice showed no microhabitat preference, caches placed under shrubs would be at a greater risk of pilferage. Thus, we would expect kangaroo rats eventually to have access to more resources and outcompete pocket mice. However, pocket mice might have an advantage over kangaroo rats in their ability to pilfer larder hoards in burrows of other pocket mice and kangaroo rats. It has been suggested that larger-bodied kangaroo rats are unable to enter burrows of pocket mice (Jenkins and Breck 1998). Unfortunately, because individuals did not have access to their own burrow, we were unable to draw reliable inferences regarding pilferage from larder hoards. Merriam s kangaroo rats could have an advantage when pilfering scatter hoards, but little pocket mice might have an advantage when pilfering larder hoards and when larder hoarding their own seeds to protect them from larger heterospecifics. Although Preston and Jacobs (2001) demonstrated in laboratory trials that microhabitat use of cache makers shifts after pilferage events, pilferage might occur so frequently in the field that continuously changing caching strategies would not be beneficial. Cache pilferage might be tolerated. Communities with high pilferage rates, including heteromyid rodent communities, can be stable if the exchange of seeds is a reciprocated behavior between conspecifics or perhaps between heterospecifics (Vander Wall and Jenkins 2003). Thus, reciprocal pilferage could occur between species that use the same microhabitats. The rate of pilferage was.50% by both Merriam s kangaroo rats and little pocket mice. Although this rate could have been amplified by rodents not having their own caches to recover in our enclosures, artificial cache pilferage occurs at a similar rate as pilferage of real rodent caches (Vander Wall et al. 2006). Despite the possibility that rates of pilferage are high under natural conditions, a caching rodent still has a
7 October 2010 SWARTZ ET AL. RODENT CACHE PLACEMENT AND PILFERAGE 1267 recovery advantage over a naïve pilferer because the cacher can use its spatial memory to retrieve caches (Jacobs 1992; Vander Wall et al. 2006). Moreover, some scatter-hoarding rodents prefer to recover their own caches rather than expend energy locating (and pilfering) other rodents caches (Vander Wall et al. 2008). The ability to find and recover stored seeds is critical for survival of scatter-hoarding rodents during times of little or no seed production. To retrieve caches rodents can remember precise locations of cache sites (Jacobs 1992; Vander Wall et al. 2006) or use a subset of microhabitats for cache sites and search only those types of locations. Studies of tits and chickadees (Parus spp.) have shown that birds preferentially use certain microhabitats to increase the probability of cache recovery (Brodin 1994; Petit et al. 1989), and some species avoid caching in the same sites as other species (Suhonen and Alatalo 1991). Similarly, rodents might be selecting their scatter-hoarding microhabitats to recover a higher proportion of food and lower the probability of caches being detected and pilfered. Differences in caching strategies (Price et al. 2000), cache recovery, and pilferage ability (Price and Mittler 2003) have been suggested as mechanisms to facilitate coexistence (Leaver and Daly 2001). Price and Mittler (2003, 2006) used models to demonstrate that species coexistence was facilitated when cache exchange occurred between heteromyids and when species exhibited differences in the ability to pilfer caches and in other behaviors. 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