Cached Seeds Research Articles

Mesquite ( Prosopis glandulosa) germination and survival in black-grama ( Bouteloua eriopoda) grassland: relations between microsite and heteromyid rodent ( Dipodomys spp.) impact

We conducted experiments to examine microsite and rodent influence on mesquite ( Prosopis glandulosa) germination and survival in Chihuahuan Desert black-grama ( Bouteloua eriopoda) grassland. Simulated caches with mesquite seeds were placed in undisturbed grassland, burned grassland patches with reduced grass cover, and on the periphery of kangaroo rat mounds. Rodent access to cached seeds was controlled by covering half of the caches with mesh domes. Kangaroo rats destroyed dung pats containing scarified mesquite seeds and excavated all seeds in simulated caches during a year in which no grasses or forbs set seed. Germination was the highest in unburned grassland, but over-winter survival was only observed across experiments in burned grassland microsites. A large proportion of seed caches were excavated (1.1–5.5% of the rodent-excluded caches and between 15.6% and 21.1% of the control caches). There were seasonal differences in numbers of emergent seedlings, with a peak in September 2002 following late summer rains. There were no significant differences in numbers of germinants in burned and unburned grassland microsites, but significantly fewer mesquite seeds germinated on banner-tail kangaroo rat mounds. Caches excavated by rodents yielded 46.6% of germinants. This study suggests that scarification of mesquite seeds is not necessary for germination and heteromyid rodents may be responsible for increased mesquite density in grasslands with some mature mesquite plants. The data also suggest that competition between mesquite seedlings and grasses in black-grama grassland is weak because of the physical characteristics of this system.

Life on the edge for limber pine: Seed dispersal within a peripheral population

:Interactions within populations at the periphery of a species’ range may depart from those in populations more centrally located. Throughout its core range, limber pine (Pinus flexilis, Pinaceae) depends on Clark’s nutcrackers (Nucifraga columbiana, Corvidae) for seed dispersal. Nutcrackers, however, rarely visit the Pawnee National Grassland peripheral population of limber pine on the eastern Colorado plains. Using live mammal trapping, fluorescent pigment tracking of disseminated seeds, and limber pine seed germination experiments in the field, we tested the hypothesis that limber pine seeds in this peripheral isolate are dispersed by nocturnal rodents. Live trapping and tracking indicated that deer mice (Peromyscus maniculatus, Muridae) and Ord’s kangaroo rats (Dipodomys ordii, Heteromyidae) are the likely seed dispersers in this population. Rodents cached seeds in surface caches on tree leaf litter, rock, or soil substrate or buried them under soil, tree litter, or plants. Seeds cached under soil and plants, as opposed to surface caches, accounted for the greater number of stored seeds. Numbers of seeds per cache for buried caches was significantly higher than numbers of seeds for surface caches. The largest caches on average were those buried under plants. Also, we found that some rodent caches contained one or more seeds up to 27 d after the caches were made. In experiments simulating observed cache types, we determined that most cache types, but especially buried caches, had some germination potential. Rodents disseminated seeds over shorter distances than do nutcrackers, possibly explaining previously observed genetic substructuring in the Pawnee population. Seed dispersal by rodents also precludes the metapopulation dynamics typical of limber pine in its core range, and may lead to the loss of peripheral populations over time.

Impacts of scatter-hoarding rodents on restoration of oil tea Camellia oleifera in a fragmented forest

Few studies have evaluated the role of seed dispersal by animals (especially scatter-hoarding rodents) in natural restoration in fragmented forests. In order to assess the potential benefits of oil tea from natural dispersal by seed-caching rodents, we tracked the individual seeds with coded tin tags of oil tea Camellia oleifera (Theaceae), an economically important evergreen shrubs, by establishing artificial seed sources (mimicking natural seed rain) in a secondary stand (i.e. Camellia-poor stand, where no oil tea shrubs grow) in a fragmented forest in the Dujiangyan Region of Sichuan Province, China. Our results indicate that the total survival of the released seeds was zero, which confirms the hypothesis that poor seeding regeneration may be caused by poor seed sources and subsequent high rodent predation in fragmented secondary forests. As small rodents repeatedly handled the tagged seeds, the proportions of seed consumption (i.e. eaten) and removal were constant in both Dispersal I and Dispersal II, but the caching proportion significantly decreased and the missing proportion significantly increased. Our results also indicate that seed-caching rodents are important in promoting natural regeneration of oil tea through scatter-hoarding seeds in soil: (1) seed removal was high (91.2%); (2) many removed seeds were found to be cached (48.2% for the relocated seeds); (3) dispersal distances of the cached seeds ranged from 0 to 38 m (mean, 7.9 m); (4) 81.3% of the caches (including primary and secondary caches) contained only one seed. Therefore, seed-caching rodents could have the potential to help restore natural populations of oil tea if we increase oil tea seed sources in the Camellia-poor stands.

Impacts of scatter-hoarding rodents on restoration of oil tea Camellia oleifera in a fragmented forest

Few studies have evaluated the role of seed dispersal by animals (especially scatter-hoarding rodents) in natural restoration in fragmented forests. In order to assess the potential benefits of oil tea from natural dispersal by seed-caching rodents, we tracked the individual seeds with coded tin tags of oil tea Camellia oleifera (Theaceae), an economically important evergreen shrubs, by establishing artificial seed sources (mimicking natural seed rain) in a secondary stand (i.e. Camellia-poor stand, where no oil tea shrubs grow) in a fragmented forest in the Dujiangyan Region of Sichuan Province, China. Our results indicate that the total survival of the released seeds was zero, which confirms the hypothesis that poor seeding regeneration may be caused by poor seed sources and subsequent high rodent predation in fragmented secondary forests. As small rodents repeatedly handled the tagged seeds, the proportions of seed consumption (i.e. eaten) and removal were constant in both Dispersal I and Dispersal II, but the caching proportion significantly decreased and the missing proportion significantly increased. Our results also indicate that seed-caching rodents are important in promoting natural regeneration of oil tea through scatter-hoarding seeds in soil: (1) seed removal was high (91.2%); (2) many removed seeds were found to be cached (48.2% for the relocated seeds); (3) dispersal distances of the cached seeds ranged from 0 to 38 m (mean, 7.9 m); (4) 81.3% of the caches (including primary and secondary caches) contained only one seed. Therefore, seed-caching rodents could have the potential to help restore natural populations of oil tea if we increase oil tea seed sources in the Camellia-poor stands.

SEED MASS AND MAST SEEDING ENHANCE DISPERSAL BY A NEOTROPICAL SCATTER‐HOARDING RODENT

Many tree species that depend on scatter‐hoarding animals for seed dispersal produce massive crops of large seeds at irregular intervals. Mast seeding and large seed size in these species have been explained as adaptations to increase animal dispersal and reduce predation. We studied how seed size and seed abundance simultaneously influenced seed dispersal and predation by scatter‐hoarding rodents in the large‐seeded rain forest tree Carapa procera (Meliaceae) in French Guiana. We individually tracked the fates of 3000 seeds, using remote video monitoring and thread‐marking. Seed size was manipulated by broadly varying intraspecific seed mass, whereas effects of seed abundance were examined by tracking seeds in three seed‐rich years and two seed‐poor years. The hypotheses, that seed mass and seed abundance both enhance dispersal success and that seed abundance reinforces the effect of seed mass, were supported by the results. Most seeds were removed by the scatter‐hoarding rodent red acouchy (Myoprocta acouchy) and subsequently were buried in scattered, single‐seeded caches up to distances >100 m. Seeds that were not removed failed to establish seedlings. Seed removal was slower, pre‐removal seed predation was greater, and seed dispersal was less far in seed‐rich years than in seed‐poor years, suggesting poorer dispersal under seed abundance. However, this was more than counterbalanced by a disproportionally greater survival of cached seeds in seed‐rich years. The per capita probability of seed survival and seedling establishment was at least 4½ times greater under seed abundance. Large seeds were removed faster, were more likely to be scatter‐hoarded, and were dispersed farther away than smaller ones, resulting in a higher probability of seedling establishment for larger seeds. Size discrimination was greater under seed abundance, albeit only during seed removal. Overall, large seeds shed in rich years had the highest probability of seedling establishment. Hence, both larger seed size and greater seed abundance stimulate rodents to act more as dispersers and less as predators of seeds. We conclude that scatter‐hoarding rodents can select for both large seed crops and large seeds, which may reinforce mast seeding.

Vertebrate seed dispersal of Marah macrocarpus (Cucurbitaceae) after fire in the Western Transverse Ranges of California

:The geophytic, perennial vine Marah macrocarpus resprouts vigorously from an underground tuber after chaparral fires and produces numerous large (1,400 mg), non-dormant seeds. Studies of the postfire response of M. macrocarpus were conducted at two sites. At the main study site, the number of stems per plant was highest in the first 2 y after fire but declined rapidly in the third year. Seed yield peaked in the first year and was estimated to be about 8,469 seeds·ha-1. The largest plants produced seeds, but despite prolific resprouting and growth, only 22% of the population set seed. Most seeds fell within 1.3 m of the centre of the parent plant. Over a 3-y period, I tracked the fate of magnet-bearing seeds. Rodents, primarily Dipodomys agilis and Peromyscus spp., dispersed and cached seeds in the burn area 13.6 ± 9.5 m (mean ± SD) from parent plants. In the first year, four seedlings (2%) established from 200 marked seeds; all seedlings originated from multi-seed rodent caches. In the second year, rodents consumed or larder hoarded all 300 experimental seeds, and in the third year, they cached 69 of 300 seeds at two locations. At another site after a different fire, western scrub jays (Aphelocoma californica) buried 16% of seeds in shallow, single-seed caches within 25 m of artificial sowings and transported the remaining 84% of the seeds beyond 60 m.

Effectiveness of six species of rodents as dispersers of singleleaf piñon pine (Pinus monophylla).

The effectiveness of six species of rodents as dispersers of singleleaf piñon pine (Pinus monophylla) was investigated in field enclosures. The rodent species were Panamint kangaroo rat (Dipodomys panamintinus), Great Basin pocket mouse (Perognathus parvus), pinyon mouse (Peromyscus truei), deer mouse (Peromyscus maniculatus), least chipmunk (Tamias minimus), and white-tailed antelope ground squirrel (Ammospermophilus leucurus). Five measures of seed harvesting and caching behavior were assessed and used to calculate the effect of species on the probability of effective dispersal (i.e., movement of seeds to potential establishment sites) of piñon pine seeds. These measures were proportion of seeds harvested, proportion of harvested seeds that were scatter hoarded, relative dispersion of scatter-hoarded seeds, seedling emergence and seedling establishment. To evaluate the effects of the last two variables, a 2-year study of seed germination and seedling establishment was conducted in the field. Seedling survival was 72-90% for seedlings emerging from 1-4 cm deep but was markedly lower from surface caches and for depths >4 cm. All seedlings in the open microsite perished before the end of the first year, whereas 38-97% of seedlings under shrubs survived. Cache size did not affect seedling survival over the 2 years, but large caches reduced the total number of potential establishment sites. All six species harvested over 90% of the piñon seeds presented and scatter hoarded from 31% (pocket mouse) to 66% (chipmunk) of the seeds they harvested. Piñon mice and deer mice distributed seeds to more cache sites (i.e., put fewer seeds in caches) than the other species and, consequently, created more potential establishment sites. All species scatter hoarded seeds at depths suitable for seedling emergence, but pocket mice and piñon mice cached under shrubs more than the other species, the microhabitat where seedlings had the greatest probability of establishing. Factors that had the greatest effect on the probability of effective dispersal were microhabitat selection and cache size.

Seed-cache exchange promotes coexistence and coupled consumer oscillations: a model of desert rodents as resource processors

Most models of resource competition assume that coexistence of consumers depends on tradeoffs in their abilities to exploit shared resources along dimensions of environmental heterogeneity generated by factors external to the consumers. However, consumers may create heterogeneity themselves by modifying resources that they do not immediately consume; such “resource processing” is predicted to allow coexistence if consumers vary in use of resources in primary vs. modified form. To explore whether external food storage (caching) represents a form of resource processing that contributes to observed patterns of species coexistence, we developed a biologically explicit simulation model of competition for a well-studied system, seed-eating desert heteromyid rodents. Here we present the model, compare competitive outcomes with and without inter-specific exchange of cached food, and describe population dynamics of coexisting competitors. The model predicts stable coexistence only when there is exchange of cached seeds via scavenging of caches left undefended by mortality or by pilferage of defended caches. Net interactions between coexisting consumers ranged from competition (10% of cases) to host–parasite (77%), commensalism (12%), and mutualism (1%). Population dynamics of coexisting consumers often showed strong periodicity and coupled synchronous or slightly lagged cycles, a possibility not previously anticipated for desert rodents occupying constant environments. Our model confirms that caching does represent a form of resource processing likely to play a significant role in the dynamics and diversity of communities of desert rodents and other caching animals.

Seed dispersal of Japanese stone pine by the Eurasian Nutcracker

Seed dispersal of Japanese stone pine Pinus pumila by the Eurasian Nutcracker Nucifraga caryocatactes was studied at Mt. Apoi in Hokkaido, northern Japan. The seed foraging and caching behavior of diurnal birds and mammals was observed, and the relative importance of each species for pine seed dispersal was examined. All mature cones disappeared from the pine shrubs by mid-October each year regardless of the cone crop size. Eurasian Nutcrackers, Varied Tit Parus varius, Eurasian Nuthatch Sitta europaea, Red Squirrel Sciurus vulgaris, and Siberian Chipmunk Tamias sibiricus were all potential seed dispersal agents, however, observations revealed that nutcrackers carried 96% of all seeds transported from the pine trees. The nutcracker carried 142 seeds on average (max. 209) in one trip. Nutcrackers mainly carried pine seeds into their mixed coniferous forest, breeding habitat, where stone pines cannot normally become established and cached them in the soil there. Nutcracker caches averaged 12 seeds with a maximum of 51 seeds. Pine seedlings were found growing in clusters (in groups of two or more trees). The number of seedlings per cluster closely resembled the number of seeds in nutcracker caches. Results suggest that most stone pine seedlings originated from nutcracker caches. Eurasian Nutcrackers thus play an important role in the regeneration of Japanese stone pine despite their small number of caches.

MASTING IN ANIMAL-DISPERSED PINES FACILITATES SEED DISPERSAL

Masting increases the efficiency of wind pollination and satiates seed predators, but there is little evidence that masting facilitates seed dispersal by animals. Masting in pines was studied over three field seasons by simulating seed crops in synchrony (mast years, autumn 1998 and 1999) or out of synchrony (non-mast year, autumn 2000) with the local population. Rodents removed simulated wind-dispersed Jeffrey pine (Pinus jeffreyi) and sugar pine (Pinus lambertiana) seeds significantly more rapidly in mast years than in the non-mast year. Radioisotopes were used to follow the fates of individual pine seeds taken from three source trees. Rodents cached nearly all experimental seeds in mast and non-mast years, making 562 caches in 1998, 510 caches in 1999, and 1034 caches in 2000. Mean dispersal distances of seeds in primary caches were 5.9 and 4.6 m (28.6% and 22.1%) farther in the two mast years than in the non-mast year. Rodents often excavated cached seeds and moved them to new sites (secondary caching). During mast years, some seeds were found in as many as three cache sites, but during the non-mast year, the level of secondary caching approximately tripled, with some seeds being found in five or six separate cache sites. Large seed crops were associated with reduced secondary caching of seeds, reduced seed consumption, and greater seed survival in the following spring. Animal-mediated seed dispersal may be a selective force, along with predator satiation and increased efficiency of wind pollination, driving temporal variation in seed production in some large-seeded pines.

Interspecific competition in tree squirrels: do introduced grey squirrels ( Sciurus carolinensis ) deplete tree seeds hoarded by red squirrels ( S. vulgaris )?

Red squirrels (Sciurus vulgaris) and introduced eastern grey squirrels (S. carolinensis) scatterhoard seeds of broadleaf trees. Scatterhoarded seeds are an essential resource in spring and their consumption increases red-squirrel fitness. We examined whether grey squirrels partly deplete the high-energy food resources cached by red squirrels, reducing their consumption, in two ways: (1) at the population level, comparing energy intake of feeding on cached seeds between a study site with red and grey squirrels and one with only red squirrels present; and (2) at the individual level, in the study site where species co-exist, relating hoard recovery of red squirrels to the amount of core-area overlap with grey squirrels. There were no significant site differences in the mean daily energy intake of red squirrels feeding on seeds recovered from caches. However, in the red-grey site, during spring, red squirrels that had a high percentage of their home-range core area overlapped by grey squirrels had a lower daily energy intake than low-overlap red squirrels. Body mass of red squirrels in spring was negatively correlated with the percentage of interspecific core-area overlap, but not with core-area overlap with other red squirrels. Our data suggest that interspecific competition for scatterhoarded seeds, with grey squirrels pilfering red squirrels' food caches, caused a reduced energy intake in red squirrels with a high degree of interspecific core-area overlap, and reduced body mass in spring. Therefore, cache pilfering is likely to reduce reproductive output in red squirrels, and thus play a role in the replacement of red by grey squirrels.

Food caching and differential cache pilferage: a field study of coexistence of sympatric kangaroo rats and pocket mice.

Ecologists studying sympatric heteromyid rodents have sought evidence for species differences in primary foraging abilities and preferences and/or behavioural responses to predation risk in order to explain coexistence. The present field study was conducted to test the hypothesis that another factor may be involved, namely differences in caching patterns, which may result in differences in vulnerability to pilferage. We examined differences between kangaroo rats (Dipodomys merriami) and pocket mice (Chaetodipus spp.) in foraging, caching and pilferage behaviour. Specifically, we examined interactions at food patches, differential food caching patterns, and differential vulnerability to cache pilferage. Observations conducted at artificial seed patches showed that kangaroo rats dominated access to the patches by arriving and foraging first and by chasing pocket mice away. Individually provisioned pocket mice stored most seeds in underground burrows (larder hoarding), whereas kangaroo rats predominantly cached seeds in small, spatially dispersed caches in shallow pits in the surface of the sand (scatter hoarding). Pocket mice pilfered from each other as well as from the kangaroo rats, but the kangaroo rats rarely pilfered, and the only instance was from another kangaroo rat. Kangaroo rats and pocket mice were both vulnerable to cache pilferage. The results suggest that coexistence of kangaroo rats and pocket mice may be facilitated by a trade-off between primary harvest ability and the ability to exploit a resource that has been processed by another species, namely pilferage ability.

Clark’s Nutcrackers Nucifraga columbiana Remember the Size of Their Cached Seeds

Clark’s nutcrackers (Nucifraga columbiana) hide thousands of seeds in subterranean caches that they later recover using spatial information about cache location. In two experiments, we tested whether nutcrackers also remember another type of information regarding their caches – the size of the seeds in each cache. We videotaped birds during cache recovery and then measured their bill gape during probing behaviour as captured on the videotape. In experiment one, six birds each experienced two treatments: one that allowed them to cache and then recover large seeds, and the other, an identical treatment using small seeds. During this experiment, all six birds used a wider gape when attempting to recover seeds during the large‐seed treatment than during the small‐seed treatment, and gape width was significantly correlated with seed size. During experiment two, we presented birds with both large and small seeds within the same caching session. We also increased the retention interval between caching and recovery. These modifications increased the difficulty of the task. Six of the seven birds used a wider gape during seed recovery when digging for caches that contained large seeds than they did when searching for small seeds. The ability to remember the size of seeds placed within caches may serve to increase the likelihood of speedy and successful recovery. It also allows the birds another level of organization of their food supply. These are the first experiments to suggest that Clark’s nutcrackers remember more about their caches than location alone.

Seed scatterhoarding by white-tailed rats: consequences for seedling recruitment by an Australian rain forest tree

The role of white-tailed rats (Uromys caudimaculatus) as dispersers of seeds of the Australian tropical rain forest tree Beilschmiedia bancroftii, (Lauraceae) was investigated by following the fates of seeds and seedlings over 2 y. Fruits of this tree are too large to be consumed by any avian frugivore except the southern cassowary (Casuarius casuarius), and the only other native mammal capable of dispersing the seeds is the musky rat kangaroo (Hypsiprimnodon moschatus). However, neither of these species has been documented to disperse the seeds of this tree. During a mast year, white-tailed rats cached seeds an average of 13 m from parent trees in a variety of microsites. Although none of the 61 cached seeds followed in this study survived to germination, comparison of seed, cache and seedling distributions suggested that most seedlings arose from rat-cached seeds. White-tailed rats cached seeds in both mast and non-mast years, but the time seeds remained on the forest floor and in caches was significantly shorter in non-mast years, suggesting that synchronous seed production increases the probability that some caches survive to germination. Because white-tailed rats are the most common and widespread native mammal capable of dispersing large-seeds, this study suggests that they may play an important role in the seed and seedling dynamics of large-seeded tree species in Australian tropical rain forests.

CACHE MANAGEMENT BY SMALL MAMMALS: EXPERIMENTAL EVIDENCE FOR THE SIGNIFICANCE OF ACORN-EMBRYO EXCISION

We conducted 2 field experiments to assess relative importance of acorn-embryo excision in the caching decisions of small mammals. In the 1st, we selectively provisioned small mammals with metal-tagged acorns of red oak (Quercus rubra) and white oak (Q. alba) at 40 point locations in 8 sites in an oak forest in northeastern Pennsylvania. We then followed the fate of cached seeds by relocating acorns with metal detectors soon after they were cached and again in spring after seeds began to germinate. At least 1 species of small mammal excised embryos of .70% of the cached acorns of white oak and ,4% of those of red oak. Animals also were observed to revisit caches in spring and excise embryos of germinating acorns. More excised acorns of white oak were found intact in spring than those of red oak, indicating that the behavior is important for long-term storage of these seeds. In a 2nd experiment, we presented free-ranging Mexican gray squirrels (Sciurus aureogaster) with pairs of acorns of 5 native white oak and 5 native red oak species and recorded caching events and whether or not cached seeds had their embryos removed. Squirrels cached significantly more acorns of white oak species, frequently excised embryos of these seeds, and only excised embryos of red oaks when they were germinating. These results support our previous hypothesis that the behavior of embryo excision is geographically widespread and has important implications for cache-management strategies of some diurnal tree squirrels and their effect on dispersal of oaks.

Animal navigation: birds as geometers?

New experiments on a bird species able to remember the sites of thousands of cached seeds have revealed how a site can be specified by combining distance information from several landmarks.

Patterns of movement and orientation during caching and recovery by Clark’s nutcrackers, Nucifraga columbiana

Clark’s nutcrackers regularly store large numbers of pine seeds and remember the locations of the cached seeds. Although they are very accurate, they do make some errors during recovery. In an attempt to determine whether any behaviours during caching predicted the occurrence of errors during recovery, we videotaped Clark’s nutcrackers while they cached and recovered seeds under laboratory conditions. We used the videotapes to develop complete, quantitative descriptions of caching and recovery behaviour, with an emphasis on body orientation and directions of movement. During caching, the birds showed the greatest change in their orientation and direction following cache creation. During cache recovery, in contrast, body orientation changed most following successful recovery of a seed. When orientation while making a cache was compared with orientation when recovering the same cache, orientations were similar more often than would be expected by chance. However, this consistency of direction was not related to the accuracy of cache recovery, indicating that such consistency is not necessary for accurate cache recovery. The location in which the birds chose to place their caches was the only variable that predicted the location of probes during recovery.

Seasonal variation in the effect of cache pilferage on cache and body mass regulation in Carolina chickadees: what are the trade-offs?

Current dynamic optimization models predict that animals should respond to cache pilferage by decreasing the probability of caching food and by increasing internal fat storage to compensate for a reduction in cache size. We tested these predictions under laboratory conditions with variable food access (four 15-min intervals/day). Carolina chickadees (Poecile carolinensis) were subjected to two environments: under pilferage conditions, one-quarter of their cached seeds were stolen every 0.5 h, and under no-pilferage conditions, seeds were left in place. Half the birds started with pilferage conditions and were then switched to the no-pilferage condition; the other half started with no pilferage and were then switched to pilferage conditions. The experiment was conducted over the course of a year to test for seasonal variation in the response to seed pilferage. The birds responded to seed pilferage by taking more seeds from a feeder, suggesting that they monitored cache availability. Alternatively, the birds may have taken additional seeds from the feeder in response to increased hunger caused by a loss of cached food. Contrary to our prediction, birds cached a higher percentage of seeds from the feeder when cached seeds were pilfered than when caches were left in place. Treatment order also affected caching behavior for all but the summer birds: chickadees initially subjected to pilferage stored a higher proportion of seeds than those initially subjected to no pilferage. Caching percentages in the summer were unaffected by cache pilferage. Caching rates (number cached/day) also followed the same trends: rates were higher when seeds were pilfered than when seeds were not pilfered, and there was a treatment-order effect for all but the summer birds. Variation in body mass also failed to match predicted trends. All birds exhibited a monotonic increase in mass as the experiments proceeded, irrespective of treatment order. Controlling for this monotonic increase in mass, an analysis of residual variation in body mass indicated that birds gained less weight when seeds were pilfered than when seeds were left in place. Finally, birds tested in the fall and spring were heavier than those tested in the summer. These results fail to support the relationship between cache maintenance and body mass regulation predicted by current models of energy regulation. We discuss the applicability of three hypotheses for the observed trends.

Recaching of Jeffrey pine (Pinus jeffreyi) seeds by yellow pine chipmunks (Tamias amoenus): potential effects on plant reproductive success

Animals that scatter-hoard seeds frequently dig up and recache them at new locations. The effect of the recaching of seeds on plant reproductive success was studied in the Sierra Nevada of western Nevada. The fate of 1000 individually marked Jeffrey pine (Pinus jeffreyi) seeds initially placed in 100 primary caches in a 10 x 10 array was monitored during autumn 1995 and spring 1996. Yellow pine chipmunks (Tamias amoenus) quickly removed nearly all of the seeds and recached many of them in 377 secondary caches containing 727 seeds. Later, rodents dug up most of these caches and transferred them to 213 tertiary caches (283 seeds), 75 quaternary caches (92 seeds), and 13 quintic (fifth order) caches (13 seeds). Overall, rodents ate 15.3% of the seeds they took from primary through quintic caches, and an additional 71.1% of the seeds disappeared, probably to underground runways and larders. During our spring survey of the study site, 133 seeds (13.6%) from 84 caches had germinated or were about to germinate. As rodents moved seeds from cache site to cache site, several changes occurred that potentially influenced the distribution and survival of Jeffrey pine seedlings. First, the number of seeds per cache decreased. Second, cached seeds were gradually moved farther from the source area. Third, the dispersal distance between successive cache sites decreased. Fourth, the distribution of cached seeds became more even. Lastly, more seeds were cached beneath shrubs, which serve as nurse plants for Jeffrey pine seedlings. Consequently, the movement of seeds between cache sites by chipmunks may increase the probability that Jeffrey pine seedlings will establish from rodent caches.

FORAGING SUCCESS OF GRANIVOROUS RODENTS: EFFECTS OF VARIATION IN SEED AND SOIL WATER ON OLFACTION

The ability of yellow pine chipmunks (Tamias amoenus), deer mice (Peromyscus maniculatus), and other rodents to detect buried Jeffrey pine (Pinus jeffreyi) and antelope bitterbrush (Purshia tridentata) seeds using olfaction was investigated during dry and moist conditions at the Whittell Forest and Wildlife Area in Little Valley, Washoe County, Nevada, USA. Removal of cached seeds was monitored on two Jeffrey pine foraging grids (each with 100 caches of five seeds buried 5 mm deep) and on two bitterbrush foraging grids (each with 100 caches of 15 seeds buried at the same depth). Under dry conditions, soil typically contained <0.5% water, whereas Jeffrey pine and bitterbrush seeds contained 4.37 ± 1.46% water (mean ± 1 sd) and 7.45 ± 1.83% water, respectively. Under these conditions, rodents found a mean of 0.33% of available Jeffrey pine seed caches each day and 0.14% of available bitterbrush seed caches each day. After rains, soil moisture increased to as much as 8.5%, and Jeffrey pine and bitterbrush seeds contained 17.50 ± 12.04% water and 28.04 ± 16.35% water, respectively. When seeds and soil were moistened, the rate at which rodents removed caches increased dramatically; 8.8% of available Jeffrey pine seed caches and 2.1% of available bitterbrush seed caches were removed each day. Similar results were obtained when other foraging grids were artificially watered during a summer drought. Seeds are very hygroscopic, rapidly absorbing water whenever the environment around them becomes moistened. This moisture appears to trigger the release of odorant molecules from the seeds, which rodents can readily detect. Under dry conditions, release of odorant molecules is apparently reduced to the point that some rodents have difficulty detecting buried seeds. These results may have important implications for rodent–seed interactions, but more generally, they suggest that, for terrestrial animals inhabiting arid environments, variation in environmental water level may modulate olfactory sensitivity in many other contexts where olfaction is important, such as assessing food patch quality, social behavior, predator–prey relations, and interspecific competition.