Scatter Hoarding Research Articles

Conspecific pilferage but not presence affects Merriam's kangaroo rat cache strategy

We investigated the effects of pilferage on caching behavior in the Merriam’s kangaroo rat by manipulating two factors associated with pilferage: the presence of a conspecific, and the opportunity for pilferage. In one experiment we assessed animals in either ‘‘Stealer’’ or ‘‘Victim’’ roles and measured changes in caching, space use, and behavior after caches were pilfered. Victims shifted from a majority scatter-hoarding to a majority larder-hoarding strategy after their caches were pilfered by the Stealer. In Experiment 2, we measured changes after exposure to a conspecific when there was no pilferage, with or without prior exposure to pilferage from Experiment 1. Merriam’s kangaroo rats were vigilant when a conspecific was present, but did not change cache strategy. Prior exposure did not have any major effect on caching or behavior. Food storage is an economic decision that is often made by a solitary forager. Our results suggest that social competition nonetheless influences such economic decisions, even in a nonsocial forager. Key words: anxiety, decision processes, Dipodomys, kangaroo rat, pilferage, scatter hoarding. [Behav Ecol 12:517–523 (2001)]

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.

Ignorant seed predators and factors affecting the seed survival of a tropical palm

In addition to acting as seed predators, some terrestrial mammals bury seeds via scatter hoarding. This study system used two permanent plots in examining the interaction between small rodents and the seeds of the palm Astrocaryum mexicanum. We tested how experimental burial, and fruiting status of the parent, distance to the parent, seed size, and microsite characteristics affect the survival of these seeds. Up to 34% of the buried seeds that were exposed only to ignorant rodent foragers (individuals not responsible for burial) survived. In comparison, less than 1% of seeds buried by scatter hoarding rodents survived in previous studies, a percentage that is comparable to the low survival of unburied seeds in this study (<2%). Although unburied seeds had very low survival, increasing distance and/or seed density positively affected survival of unburied seeds. Distance to parent had no effect on buried seed survival. Buried seed survival was most strongly and significantly determined by the fruiting status of the trees under which they occurred. Seeds experienced significantly greater predation if buried under “parent” trees that fruited during the experiment. Buried seed survival was also negatively affected by germination, as germination may signal the presence of a seed to foraging rodents. There was some indication of a positive effect of tree density on seed survival between the two plots, whereas differences in rodent abundance appear to have no effect on seed survival. Seed size and microsite characteristics had no significant effect on buried seed survival, likely due to the greater proportional effects of other factors and the longevity of A. mexicanum seeds. The results of this study were used to generate a hypothetical causal network showing how comparatively low recovery of buried seeds by ignorant foragers – combined with processes determining the removal of scatter hoarding foragers from their scattered seed caches – may affect seedling recruitment in A. mexicanum.

Predation and dispersal of large and small seeds of a tropical palm

Seed size may vary greatly among individuals within plant species. What effects the extremes of this variation have for seeds taken by small mammals are poorly understood. Not all seeds removed by small mammals are necessarily eaten. Small rodents are common seed predators, but they may disperse a significant proportion of seeds by scatter hoarding them via burial. Size‐dependent predation and dispersal of seeds has not been directly tested within a plant species for tropical rodents. This study tested whether or not large and small nuts of Astrocaryum mexicanum (Palmae) differed in their fates due to handling by the spiny pocket mouse Heteromys desmarestianus (Heteromyidae). Exclosures were used to give small rodents exclusive access to A. mexicanum nuts. H. desmarestianus preferentially consumed large over small A. mexicanum nuts, but cached (in burrows and by scatter hoarding) similar proportions of these nuts by size. Small nuts tended to be buried farther away from exclosures than large nuts. Although sample sizes of buried nuts were small, the rodents retrieved all buried large nuts, but 30% of the small nuts remained buried long enough to germinate. I also examined predispersal predation by insects and found that insects appear to have no size preference for A. mexicanum nuts, but insect predation appears to hinder nut development. Thus, nuts attacked by insects develop to be significantly smaller, with a low proportion of undamaged endosperm, than uninfested nuts. It is hypothesized that the preferential predation of large A. mexicanum nuts by H. desmarestianus is a response by these rodents to insect predation.

Placement, Retrieval, and Memory of Caches by Thirteen‐Lined Ground Squirrels

Mechanisms governing placement and retrieval of scatter hoards were investigated in 13‐lined ground squirrels (Spermophilus tridecemlineatus). The squirrels used several placement tactics, including relatively deep burial of seeds, avoidance of prominent objects, and cryptic placement, which would reduce chance discovery by competitors but that might also increase the difficulty of retrieval for the hoarder. Nevertheless, scatter hoards were unfailingly retrieved within a day or two after placement, despite experimental elimination or displacement of local sensory cues emanating from the sites. Artificial caches placed close to true caches were not discovered, indicating that recovery attempts were quite precise. These results imply that 13‐lined ground squirrels rely heavily on spatial memory for retrieval and are the first experimental demonstration of the importance of memory for cache recovery in a natural population of mammals.

Differences in Food Hoarding among Six Species of Heteromyid Rodents

Storing food is an important adaptation of heteromyid rodents to desert environments of southwestern North America. These rodents store seeds in burrows (larder hoarding) and in small, widely scattered caches buried at shallow depths in soil (scatter hoarding). Laboratory trials were done with five species from one site and two species from two other sites to test the hypothesis that larger species would larder hoard relatively more seeds than smaller species. Subjects were tested in arenas consisting of a sand-filled box for scatter hoarding and a nest box for larder hoarding. Chisel-toothed and Merriam's kangaroo rats ( Dipodomys microps and D. merriami ) from one site larder hoarded more than sympatric dark kangaroo mice ( Microdipodops megacephalus ) and long-tailed and little pocket mice ( Chaetodipus formosus and Perognathus longimembris ). Chisel-toothed kangaroo rats larder hoarded more than Merriam's kangaroo rats and little pocket mice larder hoarded more than dark kangaroo mice. These results were consistent with reported information on the relative aggressive behaviors of the species. For subjects from the other two sites, Ord's kangaroo rats ( D. ordii ) larder hoarded more than the smaller Merriam's kangaroo rats. Pocket mice made larger scatter hoards than kangaroo rats and placed these scatter hoards closer to edges and corners of arenas than kangaroo rats. Differences among species in food-hoarding behavior should be considered in future studies of the community ecology of desert rodents.

Food Availability and Food Hoarding Behaviour by Red and Arctic Foxes

Both red and arctic foxes cache food. The present and previous studies have documented scatter hoarding (hiding single or small numbers of prey at dispersed sites) and larder hoarding (hiding many prey at or near den sites) by both species. Red foxes resident on an island with a large seabird community in Newfoundland, where food is superabundant in summer and scarce during winter, made large larder hoards over an eight-year period. Scat analyses revealed that hoarded prey were heavily utilized during winter. An arctic fox encountered on an offshore seabird island hoarded seabirds in larders. For both species, larder hoarding is associated with a superabundance of prey and appears to represent a flexible response to environmental conditions.

The Effects of Seed Value on the Caching Behavior of Yellow Pine Chipmunks

Yellow pine chipmunks scatter-hoard a variety of seed types, and these seeds vary considerably in their inherent value to the chipmunks. Models of optimal scatter hoarding behavior predict that animals should space items of greater value farther apart than items of lesser value to discourage theft from cache robbers. This prediction was tested by comparing how yellow pine chipmunks Tamias amoenus distribute Jeffrey pine Pinus jeffreyi and antelope bitterbrush Purshia tridentata seeds to cache sites. Chipmunks prefer Jeffrey pine seeds over bitterbrush seeds ; the pine seeds provide energy 3.8 times faster and contain a higher proportion of fat than bitterbrush seeds. The prediction of the model was upheld on several counts : chipmunks carry Jeffrey pine seeds farther from source plants to cache sites, they space Jeffrey pine seed caches farther apart, and they place fewer Jeffrey pine seeds than bitterbrush seeds in individual caches. Yellow pine chipmunks appear to be sensitive to the qualities of the food items they store and modify their caching behavior to reduce loss of more valuable food items.

Cache Pilfering and Its Prevention in Pairs of Black-Capped Chickadees

Black-capped Chickadees Parus atricapillus scatter hoard food. Because they live in social groups during the winter, flock members may observe caching by others and take their caches. We present two laboratory experiments that investigate the possibility of observational learning of cache location and related behaviours. In the first experiment, 15 chickadees were observed storing food in an indoor aviary, either individually or in pairs. Five percent of caches were taken within a few minutes by birds that observed the occurrence of caching. These cache thefts may have been assisted by observation. Dominant birds made more caches and subordinate birds were more likely to take another bird's caches. When cache recovery was tested several hours later, birds recovered their own caches more accurately than those of other birds but recovery of other birds' caches was not improved by the previous observation of caching. A second experiment with 6 additional chickadees examined whether birds attempt to conceal their caching activity. In this experiment 9% of caches were taken by birds that had the opportunity to observe caching. When birds carried seeds, they flew away from their companion more than they did when they were flying without a seed. Overall, these results are consistent with those of previous laboratory studies and suggest that conspecific cache theft may be relatively rare in flocks of food-storing chickadees.

Sequential Patterns of Scatter Hoarding by Yellow Pine Chipmunks (Tamias amoenus)

Sequential Patterns of Scatter Hoarding by Yellow Pine Chipmunks (Tamias amoenus)

Seed Fate Pathways of Antelope Bitterbrush: Dispersal by Seed-Caching Yellow Pine Chipmunks

Seed fate pathways were investigated for antelope bitterbrush (Purshia tri- dentata), a common and economically important shrub, from fall 1988 to fall 1992 at the Whittell Forest and Wildlife Area in Little Valley, Washoe County, on the east slope of the Sierra Nevada in western Nevada. Rodents harvested bitterbrush seeds from late July to September, taking them either directly off the fruiting twigs or picking up fallen fruits from the ground surface and carrying from 63 to 80% of the seeds away intact from shrubs. Radioactively labelled bitterbrush seeds placed under four source were removed and scatter hoarded primarily by yellow pine chipmunks (Tamias amoenus). Caches con- tained a mean of 4-1 1 seeds and were located within 25 m from the source shrubs. Cache depth and microhabitat selection were generally favorable for bitterbrush seedling estab- lishment. Chipmunks and other rodents found many of the seed caches and either ate the seeds (53%) or recached them elsewhere. Chipmunks probably also carried some intact seeds to larders in hibernacula to use as a winter food source. In spring, >99% of emerging seedlings occurred in clumps containing from 2 to 104 individuals. An estimated 5.3 and 8.5% of those seeds cached by rodents and 1.1 and 2.4% of the seeds produced by in 1990 and 1991 produced emergent seedlings the following spring. Seedling mortality rates during their 1st yr ranged from 78 to 92%. Grazing by rodents was the most important source of mortality during early spring, and most seedling deaths were caused by desiccation during summer. Despite considerable attrition of individuals within clumps, most mature shrubs consisted of two or more individuals. The seed fate pathway that resulted in the highest probability of seedling establishment at this study site was harvest, dispersal, and scatter hoarding by chipmunks, and subsequent neglect of those caches. Retrieval of cached seeds by chipmunks followed by redistribution of those seeds to neglected secondary caches also resulted in many seedlings. Modes of seed dispersal based solely on abiotic agents appear to be insufficient to account for sig- nificant recruitment of bitterbrush seedlings.

The placement, recovery, and loss of scatter hoards by eastern chipmunks,Tamias striatus

The placement, recovery, and loss of scatter hoards by eastern chipmunks,Tamias striatus

Low Rate of Loss of Willow Tit Chaches May Increase Adaptiveness of Long-Term Hoarding

Low Rate of Loss of Willow Tit Chaches May Increase Adaptiveness of Long-Term Hoarding

A Model of Caching Depth: Implications for Scatter Hoarders and Plant Dispersal

The ability of foragers to detect stored seeds and nuts is known to decrease with increasing depth at which scatter hoarders bury these food items. Depth of burial of seeds and nuts is known to influence the ability of seedlings to emerge and establish if seeds are not discovered by foragers. These two patterns are combined in a model of caching depth, and implications of the model for food hoarders and plant propagules are explored. Optimal caching depth for hoarders is the depth where the probable recoverable energy in a cache discounted by the energy to make and retrieve a cache is greatest. Optimal burial depth for a propagule is where the probability of it escaping detection by a forager times the probability that it will germinate, emerge, and survive in the absence of predation is maximized. Certain aspects of the cache depth model were tested for yellow pine chipmunks caching the seed of antelope bitterbrush in field and laboratory experiments. The depths yielding the greatest probability of establishment of bitterbrush seedlings was 10-30 mm, with seeds buried 20 mm deep having the greatest success. Yellow pine chipmunks, the primary dispersal agent of bitterbrush seeds in this Sierra Nevada study area, cached most seeds between 5 and 20 mm deep, in the upper portion of the range yielding the greatest bitterbrush establishment. The partial agreement between chipmunk caching behavior and bitterbrush requirements may be largely fortuitous, but certain evolutionary adjustments by the plants may result in a closer match between seedling requirements and the behavior of seed-caching animals.

Food Storage in Gray Jays: Source Type and Cache Dispersion

AbstractAnimals that store food items in scattered sites must decide how to distribute their caches in space. Our theoretical approach is based on the assumption that such animals disperse their caches in a manner that maximizes the long‐term rate of storage of recoverable (surviving) food items in the habitat. We investigate the cache‐spacing behavior of theoretical scatter hoarders that encounter food sources differing in the energetic content of the items they contain. We then describe a field experiment in which gray jays (Perisoreus canadensis) were presented with both small‐ and large‐item food sources. The jays compensated for source type by spacing larger‐item caches more widely, a compensation that would tend to yield a high rate of storage of recoverable food energy over the long term and throughout the territory. Previous models do not adequately account for the observed patterns of cache dispersion.

Food Habits of Arctic Foxes (<i>Alopex Lagopus</i>) on the Western Coast of Svalbard

Food habits of arctic foxes (Alopex lagopus) on the western coast of Svalbard were studied in the years 1986-89. Faeces (n=1018) were collected mostly in summer, and food remains were recorded both at dens and elsewhere in the region. The foxes were opportunistic in their hunting and feeding habits, utilizing a wide variety of available food items. Alcids (mainly little auks and Brunnich's guillemot), gulls (mainly kittiwakes), and fulmars were the major foods in summer. The consumption of alcids by fox families was correlated with availability near the den. In winter, fulmars and, in one region, seals were important foods. Some regional differences in food consumption were found. A change in diet was observed when a litter of pups moved from one den to another (2 of 3 cases). Differences in food habits between years also were found at the same den (4 of 5 cases). Foxes frequently cached food by scatter hoarding, placing only a single item in each cache.Key words: arctic fox, Alopex lugopus, food habits, prey species, food caching, Svalbard

Balanites wilsoniana: elephant dependent dispersal?

ABSTRACTThis study demonstrates that forest elephants (Loxodonta africana) play a key role as seed dispersal agents for the upper canopy forest tree, Balanites wilsoniana. Seed that passed through elephants had a much greater probability of germinating (50.7%) than seeds from fruits eollected directly from the tree (3%). Of 515 seeds that were collected from under parent trees, 89.1% were rotten at the time of collection, 2.6% had been eaten by forest pigs, 2.8% had been killed by squirrels, and 0.7% had been killed by insect damage. The remaining 4.8% of the seeds were still alive. In 150 hours of focal observations on B. wilsoniana trees, two fruits were seen to be destroyed by squirrels, no fruits were seen to be consumed by potential dispersers, and no evidence of scatter hoarding was found. Experimental seedling stations placed out under parent trees and away from parent trees indicated no significant effect of dispersal on seedling survival. Although elephants are not essential for the successful germination of B. wilsoniana seeds, they do greatly increase probability of germination and play a vital role in their dispersal, suggesting that this is a highly interdependent plant-animal seed dispersal system.

Spatial patterns of food storage by Merriam's kangaroo rats

We investigated scatter-hoarding behavior by Merriam's kangaroo rats (Dipodomys merriami) in a complex arena in the laboratory. Animals cached seeds in four sand-filled boxes connected by runways, with an artificial burrow at one end of the system and a food source at the other. Animals initially cached more seeds in boxes close to the food source, but amounts cached became more evenly distributed among boxes as sessions progressed. The results were consistent with the hypothesis that scatter hoarding by kangaroo rats may represent a compromise between rapidly sequestering seeds, hence making them unavailable to nondigging competitors, and spacing-out caches, making them less vulnerable to pilferage by other rodents.

Scatter hoarding by kangaroo rats (Dipodomys merriami) and pilferage from their caches

Scatter hoarding by kangaroo rats <i>(Dipodomys merriami)</i> and pilferage from their caches

Food Hoarding Behavior in the Short-tailed Shrew Blarina brevicauda

Short-tailed shrews Blarina brevicauda cached most (86.6%) prey captured. Few were consumed immediately (9.4%) or left where killed (3.9%). Shrews most often ate the first prey captured but normally cached subsequent prey. The smallest prey offered, mealworms, was the only type of prey eaten more often than cached. Shrews returned to and consumed cached prey, including those cached by conspecifics. INTRODUCTION Storage of food for later consumption, commonly called caching or hoarding, is done by a wide variety of mammals and birds (Ewer, 1969; Lack, 1954). Two main categories of hoarding are distinguished: (1) larder hoarding is storage of food in a concentrated area of an animal's home range (Stapanian and Smith, 1978); (2) scatter hoarding is burying each object separately at dispersed sites in an animal's home range (Morris, 1962). Larder hoarding may have originated from the need of the animal to find a safe place to consume prey (Bindra, 1948; Morris, 1962) and is common in animals with protected burrows (Ewer, 1969). The tendency of laboratory rats to carry food to the nest to eat is increased by making the feeding place more exposed and decreased by making it more protected (Bindra, 1948). Scatter hoarding appears to have evolved in connection with an increase in mobility within the home range (Morris, 1962) or where loss of cached food to conspecifics or other species is prevalent (Shaffer, 1980). In scatter hoarding, individuals have food buried all around them and are less dependent on one large cache which may be endangered in some way. Size of food also may be an important factor initiating hoarding behavior; the larger a piece of food, the greater the chance there will be something left over after the animal has become satiated (Ewer, 1969). The purpose of this study was to examine experimentally the food caching behavior of the short-tailed shrew Blarina brevicauda. Blarina is a nonhibernating, nonmigratory insectivore which feeds as a carnivore (Brooks, 1908). Responses to prey of different size and type and to prey cached by conspecifics were tested. METHODS AND MATERIALS Ten short-tailed shrews were collected on the E. N. Huyck Preserve, Rensselaerville, Albany Co., New York. Nine were collected fromJune to August 1977 and one inJuly 1976. Shrews were housed individually in 10-gal. aquaria (507 mm x 260 mm x 320 mm) with commercial canned dog food and water available ad lib. Amphibian prey, Desmognathus ochrophaeus and Rana clamitans, were collected in Albany and Ulster counties, New York; the crickets and mealworms were obtained comercially. Test chambers were constructed of wood. The chambers used in Sets A-D and G-H were 1 x 1 x 0.23 m and painted with white enamel. Set refers to a series of identical experiments or trials conducted with individual shrews. A 30-mm horizontal overhang of aluminum flashing was attached to the rim of the chambers to prevent escape of shrews and prey. Chambers used in Sets E and F were 2.28 m2, 1.2 x 1.8 x .4 m and varnished. A 73-mm horizontal overhang of aluminum flashing was attached to these chambers. These chambers could be partitioned in half by a piece of wood. 1 Present address: Philadelphia Zoological Garden, Philadelphia, Pennsylvania 19104