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Behavioral Ecology Vol. 11 No. 5: 544-549
© 2000 International Society for Behavioral Ecology
The influence of environmental conditions on cache recovery and cache pilferage by yellow pine chipmunks (Tamias amoenus) and deer mice (Peromyscus maniculatus)
Department of Biology and the Program in Ecology, Evolution, and Conservation Biology, Fleischmann Ag. Bldg., University of Nevada, Reno, NV 89557, USA
Address correspondence to S. Vander Wall. E-mail: sv{at}unr.med.edu .
Received 10 June 1999; revised 31 January 2000; accepted 28 February 2000.
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ABSTRACT |
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I conducted a field experiment in 10 x 10 m enclosures to explore how seed and soil moisture levels influence the ability of knowledgeable and naive rodents to find natural caches of Jeffrey pine (Pinus jeffreyi) seeds. Subjects were yellow pine chipmunks (Tamias amoenus) and deer mice (Peromyscus maniculatus) searching for caches that they had made, caches made by other individuals of the same species, or caches made by individuals of the other species. Subjects that made caches (knowledgeable subjects) relied on spatial memory to find many of their own caches during recovery sessions, and their ability to locate caches was not affected by water content of seeds or soil. Naive subjects found few caches under dry conditions, but under wet conditions, they located as many caches as did the rodents that made them. Naive subjects apparently relied on olfaction to find caches, a sensory modality that works more effectively under moist conditions. Subjects had as much success foraging for caches made by members of their own species as for caches made by the other species. I present a hypothesis that predicts how foragers could modify predominately memory-based search to predominately olfactory-based search as the weather changes from dry to wet. When foragers rely on spatial memory, those foragers find only their own caches, but when they can also use olfaction, they pilfer caches made by other individuals. Consequently, the nature of competitive interactions among members of the seed-caching guild may change as the weather changes.
Key words: foraging tactics, granivory, olfaction, pilferage, scatter hoarding, seed caching, Sierra Nevada, spatial memory.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONSTUDY AREAMETHODSRESULTSDISCUSSIONREFERENCES |
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A variety of animals scatter hoard food items in soil, later retrieving and eating some of those items. The cacher has at least three means of finding buried food. Rodents have been shown to use olfaction to locate hidden seeds (Howard and Cole, 1967
;
Howard et al., 1968;
Johnson and Jorgensen, 1981;
Lockard and Lockard, 1971;
Reichman and Oberstein, 1977).
Moisture plays an important role in seed detection by rodents because the
strength of the olfactory signal emanating from buried seeds is much stronger
when seeds or the substrate around the seeds are moist
(Johnson and Jorgensen, 1981;
Vander Wall, 1993,
1995,
1998). This method of locating
buried seeds appears to be relatively unimportant for foraging birds because
granivorous birds are unable to detect buried seeds by the sense of smell
(Kamil and Balda, 1985;
Vander Wall, 1982). Second,
there is considerable evidence that spatial memory is very important in
assisting the cacher to find buried food. Corvids, parids, and rodents can use
the positions of local visual landmarks as cues by which to remember the
positions of items they have hidden (Cheng
and Sherry, 1992; Clayton and
Krebs, 1994; Collett et al.,
1986; Gould-Beierle and Kamil,
1996,
1998;
Jacobs, 1992;
Jacobs and Liman, 1991;
Kamil and Balda, 1985;
Kamil and Jones, 1997;
Sherry, 1992; Vander Wall,
1982,
1991). This method of search
is, of course, available only to the hoarder. And third, rodents and birds can
use exploratory digging or probing to find hidden items (e.g.,
Kamil and Balda, 1985;
McQuade et al., 1986;
Vander Wall, 1991). This
method employs seemingly random digging, and, at least in experimental
situations, is often ineffective in finding cached food, but may be used in
combination with spatial memory or olfaction to raise the likelihood of
locating hidden food.
For hoarding to evolve, the individual that caches an item must have a
greater probability of recovering that item than any other animal
(Andersson and Krebs 1978;
Stapanian and Smith, 1978).
This is not an easy condition to meet because the hoarder shares its home
range with dozens of seed-eating animals and the pilferage pressure on its
buried food reserves may be great. The scatter hoarding of food items at
inconspicuous, widespread sites within the hoarder's home range
(Clarkson et al., 1986;
Stapanian and Smith, 1978,
1984) and a detailed spatial
memory that permits the hoarder to return to the locations of its hidden
caches (Bednekoff et al., 1997;
Clayton and Krebs, 1994;
Jacobs and Liman 1991;
Kamil and Balda, 1985; Vander
Wall, 1982,
1991) are two traits that seem
to ensure that the hoarder will retrieve a sufficient share of its buried
food. Despite the employment of these tactics, the pilferage of cached food is
widespread (Brodin, 1994;
Brodin and Ekman, 1994;
Burnell and Tomback, 1985;
Hampton and Sherry, 1994;
Härdling et
al., 1995). However, the conditions under which pilferage occurs
and the effect it has on the hoarder are still poorly understood. A better
understanding of how the foraging methods of the individual that makes a cache
(the hoarder) differ from those of pilferers is essential if we are to
understand how the hoarder manages to retrieve a sufficient proportion of its
cached food.
The objectives of this study were to determine the relative importance of
spatial memory and olfaction in locating cached seeds and to determine when
these different search methods are employed. We studied the cache-finding
behavior of isolated individual rodents in large enclosures in the field. This
approach permitted us to ascertain the advantage of individual knowledge
(spatial memory) of the cacher in relocating cache sites, to assess how the
physical environment (soil moisture conditions) might facilitate or constrain
the foraging of naive animals, and to reveal the nature of intra- and
interspecific interactions (e.g., cache pilferage) focused on the cached food.
We selected yellow pine chipmunks (Tamias amoenus) and deer mice
(Peromyscus maniculatus) for these experiments because these are two
of the most abundant rodent species in the semi-arid yellow pine forests of
the east slope of the Sierra Nevada. Both species scatter hoard Jeffrey pine
(Pinus jeffreyi) seeds and the seeds of other shrubs and trees and
potentially compete for seeds before and after they cache them in the soil.
However, they differ in activity period and may differ in the ways they search
for and manage seed caches. Yellow pine chipmunks are avid scatter hoarders
(e.g., Vander Wall, 1992,
1995), and deer mice are also
known to scatter hoard seeds in the soil
(Vander Wall et al., in
press).
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STUDY AREA |
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TOPABSTRACTINTRODUCTIONSTUDY AREAMETHODSRESULTSDISCUSSIONREFERENCES |
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We conducted this study in the George Whittell Forest and Wildlife Area in Little Valley, Washoe County, Nevada. Little Valley is in the Carson Range of extreme western Nevada, about 30 km south of Reno, Nevada, USA (39°15'10'' N, 119°52'35'' W). The elevation is 1975 m. The environment consists of semi-arid Jeffrey pine (Pinus jeffreyi) and lodgepole pine (Pinus contorta) forests with a shrub understory of antelope bitterbrush (Purshia tridentata), greenleaf manzanita (Arctostaphylos patula), tobacco bush (Ceanothus velutinus), and Sierra chinquapin (Castanopsis sempervirens). Experimental enclosures were in bitterbrush shrubland along the Jeffrey pine-lodgepole pine ecotone. Soils consist of decomposed granite. The region experiences a long summer dry season from June to October (Vander Wall, 1998
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METHODS |
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TOPABSTRACTINTRODUCTIONSTUDY AREAMETHODSRESULTSDISCUSSIONREFERENCES |
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We conducted all trials inside five 10 x 10 m rodent-proof enclosures. Walls of the enclosures extended
75 cm above ground and 45
cm below ground and consisted of 6-mm wire mesh hardware cloth supported on a
wooden frame. The tops of the walls had 20-cm-wide aluminum flashing on the
inside and outside to prevent rodents from leaving and entering the
enclosures.
Each enclosure had a plastic, 20-1 nest bucket buried in the ground such
that the lid was level with the ground surface. The nest bucket was
partitioned horizontally into three levels with two plywood dividers. Holes 4
cm in diameter in the partitions permitted rodents to move freely between
levels. The bottom level had soil 2 cm deep and cotton nest material. A
nearly horizontal segment of PVC pipe (25 mm diameter, 50 cm long)
connected the side of the upper level of the nest with the ground surface,
permitting rodents to enter and exit the nest bucket. The complexity of the
nest permitted us to more easily isolate and remove rodents from the nest when
needed. Most subjects readily accepted the buckets as nest sites.
We placed a feeder box with a wooden bottom, top, and two sides measuring 40 x 30 x 10 cm near the center of each enclosure. The design excluded birds but permitted rodents to enter along two sides to remove seeds. We placed a tin can containing water several meters from the feeder.
We conducted 20 trials. Each trial consisted of four phases: (1) caching,
(2) search for caches by the cacher, (3) search for caches by a naive forager
of the same species as the cacher, and (4) search for caches by a naive
forager of the other species of rodent. To begin the caching phase of a trial,
we captured a subject (either chipmunk or deer mouse), weighed it, determined
its gender, marked it with a numbered ear tag, and released it into the
entrance of the nest bucket. Then we placed 150 color-marked, radioactively
labeled Jeffrey pine seeds (see Vander Wall,
1992,
1993,
1998) in the feeder. We
allowed chipmunks 24 h to cache seeds, but most deer mice cached at a
slower pace so we gave them up to 3 days to cache seeds. At the end of phase
1, we isolated the caching rodent in the nest bucket and removed it and any
larder-hoarded seeds and seed hulls. We removed unharvested seeds from the
feeder and counted them. Then we searched the enclosure with a Geiger counter
to locate scatter-hoarded seeds and hulls of eaten seeds. We carefully
excavated each cache to determine cache depth and number of seeds per cache.
We also collected cache microsite data (distance to shrub edge and shrub
center), substrate data, and Cartesian coordinates (to the nearest cm) of each
cache. Then we replaced the seeds at the exact site and buried them 5 mm deep,
a typical cache depth for these rodents
(Vander Wall, 1993;
Vander Wall et al., in
press).
Phase 2 began when we reintroduced the rodent that made the original caches and gave it 3 days to forage for cached seeds. No other pine seeds were available in the enclosure, but other types of food (e.g., arthropods, seeds of shrubs and forbs) were available. At the end of this period, we captured the rodent, checked its ear tag number, weighed it, and released it outside the enclosure. Then we searched the nest bucket for eaten and larder-hoarded seeds and checked each of the original caches to see if any had been removed. If one or more caches was missing, we surveyed the entire enclosure for eaten seeds and new caches. We excavated new caches and counted the seeds.
Before beginning phases 3 and 4, we reestablished all caches to the condition at the end of phase 1, the caching phase. Phases 3 and 4 of the experiment were identical to phase 2, but the foragers were naive about the presence and location of scatter-hoarded seeds. For each trial, one deer mouse and one yellow pine chipmunk was tested in these two phases, and the order of testing was arbitrary. No subject was ever used more than once in the experiments; the total number of subjects involved was 30 chipmunks and 30 deer mice.
Before beginning phases 2-4, we made 10 artificial caches (five pairs of two caches) inside the enclosure at depths and microsites similar to the rodent caches and protected them from rodent foragers with 20 x 20 cm pieces of wire mesh. On days 1 and 3 of each phase, we determined seed and soil moisture content by collecting five artificial seed caches and five soil samples (from around the cached seeds) and sealing the samples separately in Whirl-pack® plastic bags. We placed all of these bags in a larger plastic bag, sealed it, returned it to the lab, and stored it in a refrigerator. Within two days, we sieved the soil to remove large pebbles and plant litter, weighed the fresh samples, dried the samples in an oven at 100°C for 48 h, and reweighed them.
In half of the 20 trials, a yellow pine chipmunk was the initial cacher and
in half a deer mouse was the initial cacher. To determine the effect of
substrate water (Vander Wall,
1998), we conducted half of the trials under dry conditions (5
chipmunks and 5 mice) and half under wet conditions (5 chipmunks and 5 mice).
We used natural rainfall events whenever possible, but usually we wetted the
enclosure artificially with water from a nearby stream. Initially we used
175 l of water, but our initial waterings proved inadequate because on
sunny, hot, dry summer days, most of the 175 l of water evaporated before it
could soak into the ground. Therefore, we increased the water to 350 l
(equivalent to 3.5 mm of water over the 100-m2 enclosure) and reran
the earlier wet trials (with different subjects). We watered enclosures in the
late afternoon to minimize evaporation. We only watered before search phases
(phases 2, 3, and 4). All trials were conducted in the autumn of 1996, and
summers and autumns of 1997 and 1998, during the dry season characteristic of
the region.
We analyzed the results using a three-way ANOVA with type of forager (knowledgeable forager, naive forager of same species, or naive forager of other species) nested within condition (dry or wet), nested within species of initial cacher (chipmunk or deer mouse), and the proportion of caches found (arcsine transformed) within a 3-day foraging session as the dependent variable. Data on the fate of seeds (number of new caches made and number of seeds eaten) taken from discovered caches are presented but not analyzed statistically, because these data are not independent of the number of caches found. Other comparisons were made using unpaired t tests.
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RESULTS |
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Enclosure soils under dry conditions contained 0.47 ± 0.03% water (mean ± 1 SE) at the beginning of the recovery phases of dry trials and 0.42 ± 0.04% water at the end of the recovery phases of dry trials (n = 23). Jeffrey pine seeds cached in soil under these conditions contained 3.9 ± 0.3% water at the beginning and 3.6 ± 0.3% water at the end of the experiments (n = 18). Following the application of 350 l of water to the enclosures (or a similar amount of rainfall), soil water increased to 4.2 ± 0.6% at the beginning of trials and then decreased to 1.9 ± 0.4% water at the end of trials (n = 15). Jeffrey pine seeds in the moistened soil rapidly absorbed water (19.3 ± 1.6%) and then gradually lost moisture over the next 2 days to 13.0 ± 1.7% water by the end of the trial (n = 15).
During phase 1, chipmunks made 32.2 ± 2.6 caches per trial and deer mice made 37.9 ± 10.4 caches per trial. The chipmunks made a similar number of caches during dry and wet trials, but the deer mice made more caches during wet trials (57.6 ± 16.6 caches) than during dry trials (18.2 ± 3.9 caches). This difference is significant (unpaired t = 2.304, df = 8, p =.050), but the relationship is spurious because the mice prepared the caches before we treated (or did not treat) the enclosures with water. During phase 2, when these same subjects foraged for their own caches, chipmunks relocated 20.6 ± 4.2 caches under dry conditions and 20.2 ± 3.6 caches under wet conditions, whereas deer mice relocated 11.6 ± 3.4 caches under dry conditions and 20.4 ± 5.1 caches under wet conditions. The proportions of available caches that were recovered under dry and wet conditions were not significantly different (unpaired t-tests of arcsine transformed proportions of caches recovered; chipmunk t = 0.18, df = 8, p =.862; deer mouse t = 0.791, df = 8, p =.452).
When naive subjects searched for buried seeds under dry conditions, they found very few (Figure 1). During ten foraging sessions, naive chipmunks found a total of only 7 caches of 240 available, and deer mice also found only 7 caches of 240 available. Neither species did any better when searching for caches made by their own versus the other species. But under wet conditions, naive foragers did much better (Figure 1); naive chipmunks located 273 caches and naive deer mice located 238 caches of the 424 caches available. The three-way ANOVA found a significant condition (dry versus wet) effect, a significant forager (knowledgeable versus naive) effect, and a significant condition x forager interaction (Table 1), reflecting the fact that naive subjects improved their foraging success under wet conditions. The species (chipmunk versus deer mouse) effect and species x condition interaction were also significant, but this appears to be the result of there being fewer caches available when chipmunks were the original cacher, and does not mean that the number of caches found varied with the species of cacher. The mean number of caches found in foraging sessions when chipmunks and deer mice were the original cachers (13.9 and 15.8, respectively) did not differ (one-way ANOVA on caches found per session F1,58 = 0.244, p =.623). The species x forager interaction was not significant, indicating that subjects had as much success when foraging for caches made by members of the other species as for caches made by their own species.
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Seeds recovered from caches by foragers had one of two fates: they were either recached or eaten. Except for knowledgeable chipmunks, most foragers did not recache many of the seeds that they found (Figure 2). Knowledgeable chipmunks recached 33.4 ± 8.8% of the seeds they recovered from caches, making 10.0 ± 3.0 caches, whereas knowledgeable deer mice recached 22.3 ± 9.4% of the seeds they recovered from caches, making 3.4 ± 1.8 caches (dry and wet trials combined; n = 10 for each species). As already noted, naive foragers under dry conditions found very few seeds and so they had few opportunities to handle them, but naive subjects under wet conditions found many seeds and recached 10.4 ± 4.3% of them (both species combined; chipmunks and mice behaved similarly). Naive chipmunks made 4.8 ± 2.2 caches and naive deer mice made 3.4 ± 1.1 caches (n = 10 trials). Subjects generally recached a smaller proportion of the seed they found under wet conditions. Most of the seeds that were not recached were either eaten or unaccounted for (missing seeds were probably eaten; the fragmenting of seed coats during feeding often makes them difficult to find) (Figure 3). Knowledgeable chipmunks ate 42.8 ± 4.6 seeds, whereas knowledgeable deer mice ate only 18.2 ± 5.5 seeds (dry and wet conditions combined). This difference probably reflects the species' different energetic requirements. Naive subjects ate many more seeds under wet conditions than under dry conditions, indicating that under the experimental conditions they used pilfered caches primarily as a food source.
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During a series of preliminary wet trials, we used only 175 l of water to moisten the soil of the enclosure. This amount of water proved inadequate to stimulate consistent cache recovery by naive subjects, so we doubled the water and repeated the trials. During the preliminary low-water trials, naive chipmunks found 3.8 ± 3.1 caches per trial (n = 5) and naive deer mice located 2.7 ± 2.1 caches per trial (n = 6). The numbers of caches located were a small percentage of the mean number located during high water trials (15.2% for chipmunks and 9.5% for deer mice), but considerably more than the mean number located by naive animals under dry conditions (19 times more for chipmunks and 13.5 times more for deer mice). The proportion of caches found during the low water and dry conditions were not significantly different (F1,27 = 1.789, p =.192).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONSTUDY AREAMETHODSRESULTSDISCUSSIONREFERENCES |
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When seeds or the soil are dry, the ability of naive rodents to find buried seeds is limited (Figure 1). Naive animals, which, of course, lack any spatial memory of cache sites, found very few caches despite the fact that they had 3 days during which to search the relatively small enclosures. It made no difference whether the forager was a yellow pine chipmunk or a deer mouse or whether they searched for seeds cached by their own species or those of the other species. Their limited success was apparently due to a combination of exploratory digging and weak olfactory signals emanating from cached seeds. These results are consistent with previous findings (Johnson and Jorgensen, 1981
; Vander Wall,
1993,
1995,
1998) that showed that
olfaction is much less effective under dry conditions. The seed and soil
moisture levels during dry trials were similar to those reported by Vander
Wall (1998) (i.e., <0.5%
soil water and 4.37 ± 1.46% Jeffrey pine seed water) for dry conditions
under which free-ranging rodents found relatively few artificial caches. The
much greater cache recovery success of yellow pine chipmunks and deer mice
that searched for their own caches under dry conditions was apparently the
result of detailed spatial memory of cache locations
(Jacobs, 1992;
Jacobs and Liman, 1991;
Vander Wall, 1991). The
difference between the foraging success of the cacher and naive subjects is a
measure of the importance of spatial memory when foraging for caches under dry
conditions.
The situation changed dramatically under wet conditions. When seeds are
moistened, they imbibe water and release organic molecules
(Duke et al., 1983;
Simon and Mills, 1983;
Simon and Raja Harun, 1972)
that contribute to relatively strong odors. Laboratory experiments have
demonstrated that foragers use these stronger odors to locate buried seeds
(Vander Wall, 1993,
1995). After we applied 350 l
of water to the enclosures, the foraging success of animals increased markedly
(Figure 1). These results
suggest that naive foragers benefited from a significant increase in their
ability to detect olfactory signals emanating from buried seeds under moist
conditions. The olfactory signal emanating from moist seeds is sufficiently
strong that it permits a naive forager to locate hidden seeds as effectively
as the hoarder.
We were surprised that the amount of water used in a series of 11 preliminary low water trials proved inadequate to stimulate consistent cache detection in naive rodents. Results from previous laboratory and field experiments had suggested that 175 l of water on a 100-m2 enclosure should have been sufficient to cause buried seeds to become apparent to olfactory-oriented foragers. Reasons for these unexpected results are probably related to the fact that under hot, dry conditions, most of the applied water evaporated from the enclosure within a few h. Natural rainfall events of the same magnitude are associated with clouds, cooler temperatures, and higher relative humidities, environmental conditions that reduce evaporation and provide time for the water to penetrate the soil. Consequently, real rainfall events may be more effective in causing buried seeds to release odor molecules than artificial waterings of the same magnitude.
Yellow pine chipmunks and deer mice occupy broadly overlapping home ranges where each individual shares foraging space with a dozen or more rodents that compete with it for seed resources. How individuals distribute seed caches within their home range is not known but is currently being explored (Crampton and Vander Wall, in preparation). But it is evident that these rodents forage in a complex environment where they not only make and deplete their own caches but potentially encounter caches made by other individuals of both their own and other species. Added to this, moisture conditions change over time, modifying the relative effectiveness of olfaction as a means of locating food. Based on the results presented here, we offer a hypothesis for how an individual seed-caching rodent that shares space with competitors might change its foraging tactics over time. Under dry conditions, rodents are likely to cache and recover seeds relying extensively on spatial memory to manage their array of scattered caches. The cacher apparently enjoys a degree of protection from other foragers that have no knowledge of the locations of its caches and that are relatively ineffective at pilfering its caches using olfaction, exploratory digging, or a combination of the two. On the other hand, the cacher is equally ineffective at pilfering the caches of other rodents. During dry periods, which can last for weeks or months at a time in arid and semi-arid environments, the most effective means of foraging would be to search for unstored seeds on the soil surface or directly from source plants and to cache and withdraw these seeds from a population of scattered caches, to which the cacher has nearly exclusive access.
When a drought is broken by rainfall, an individual's ability to find
hidden food changes and so might its foraging tactics. A rodent that was
depending largely on memory to manage caches now can use olfaction to locate
seeds buried by its competitors. Memory of cache sites would still be
important, but it would not necessarily confer an important foraging advantage
during the wet period. The rodent could and should dig up and move any cache
that it finds that it did not itself make, but it also is likely to experience
heavy pilferage of its own caches. As the soil dries out during the week or so
after a rain event, the relative importance of spatial memory of cache sites
increases while the importance of olfaction eventually decreases. As these
changes occur, the recaching of pilfered seeds serves to increase the share of
caches that an individual "controls." When the soil is finally
dry, pilferage of caches ebbs and spatial memory again becomes the predominate
means of locating and managing caches. This hypothesis suggests an explanation
for the recaching of seeds, a phenomenon that has been reported frequently
(Daly et al., 1992;
Jenkins et al., 1995;
Vander Wall and Joyner, 1998),
but whose cause is not well understood: an individual that pilfers the caches
of others during wet periods retains spatial memories or those cache sites and
gains the potential advantages that those memories will bring during future
dry periods. Conversely, any individual that does not participate in the cache
pilfering and recaching free-for-all that appears to ensue during wet periods
(this study; Vander Wall and Joyner,
1998) runs the risk of losing control of its share of the stored
seed reserve.
Behavioral ecologists have paid little attention to how the foraging modalities of rodents might change as the physical environment changes. Variation in foraging success is known to occur, but this variation is thought to be caused by factors such as changes in the abundance of food, differences in the foraging characteristics of species, and individual variation within a species. This study demonstrates that the ability of rodents to find buried seeds using olfaction varies with environmental conditions (i.e., the weather). Spatial memory does not, apparently, vary with environmental conditions, but animals may increase their reliance on spatial memory as olfaction becomes less effective in finding hidden seeds (and vise versa). The weather influences the foraging success of the hoarder and potential pilferers differently, and at least during dry conditions, the hoarder has a distinct advantage in recovering food that it has hidden.
It is uncertain whether these conclusions apply to desert-adapted
granivores like kangaroo rats (Dipodomys spp.) and pocket mice
(Perognathus and Chaetodipus spp.). The long evolutionary
history of these heteromyid rodents and seedbearing plants in arid
environments (Hafner, 1993;
Wahlert, 1993) suggests that
the rodents may have evolved greater levels of olfactory sensitivity to dry
seeds buried in dry soil than are found in yellow pine chipmunks and deer mice
that inhabit semi-arid or even mesic environments. Experiments that compare
the olfactory sensitivity of heteromyid and nonheteromyid rodents are under
way.
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ACKNOWLEDGEMENTS |
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This research was conducted with the careful assistance of Ted Thayer, Jenny Hodge, Jennifer Armstrong, Maurie Beck, Julie Roth, Jamie Joyner, Erin Vander Wall, and Mark Boon. Funding was provided by National Science Foundation grant DEB-9707098. We thank the Whittell Board of Control for granting permission to conduct this study in the George Whittell Forest and Wildlife Area.
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