Scatter-hoarding Animals Research Articles

Scatter hoarding by the Central American agouti: a test of optimal cache spacing theory

Optimal cache spacing theory predicts that scatter-hoarding animals store food at a density that balances the gains of reducing cache robbery against the costs of spacing out caches further. We tested the key prediction that cache robbery and cache spacing increase with the economic value of food: the ratio of food to consumer abundance. We quantified cache pilferage and cache spacing by the Central American agouti, Dasyprocta punctata, in the tropical forest of Barro Colorado Island, Panama, across 10 1 ha plots that encompassed a more than100-fold range in the availability of Astrocaryum palm seeds, the agouti's principal food. We found that caches were pilfered at higher rates in plots with lower seed availability, and that agoutis cached seeds further away and into lower densities where seed availability was lower. Food scarcity apparently increased the pressure of food competitors on caches, stimulating agoutis to put more effort into caching seeds to create lower cache densities, fully consistent with theory. We conclude that the optimal cache density depends not only on the nutritional value of food but also on the economic value, which may vary in space as well as time.

Mechanisms of Cache Decision Making in Fox Squirrels (Sciurus Niger)

The cache decisions of scatter-hoarding animals are influenced by a number of factors, including satiety, food quality, number of competitors, and the risk of predation and pilferage. However, it is unknown how animals assess these variables. We investigated this process experimentally in free-ranging fox squirrels (Sciurus niger) by measuring the effects of nut characteristics and social context on nut-handling behavior and subsequent cache decisions. We found that a behavior involved in nut handling, the head flick, was correlated with nut quality, shell presence, the decision to cache rather than eat the nut, and the time and energy spent caching. In contrast, a 2nd nut-handling behavior, the paw maneuver, was correlated with the social context but not the cache decision, and may instead reflect a response to social competition. Our results suggest that fox squirrels assess nut quality using overt, observable nut-handling behaviors. The experimental study of these behaviors can help us understand how animals use information about food and the social context to make adaptive food-storing decisions.

High Relative Humidity Increases Pilfering Success of Yellow Pine Chipmunks

Scatter-hoarding animals store food items to be used later when food is scarce. However, other individuals can pilfer food stores because caches are not usually defended. We tested how associative learning contributes to foraging success of pilferers searching for scatter-hoarded food. We conducted a field-based, seed-removal experiment to test 2 hypotheses. First, yellow pine chipmunks (Tamias amoenus) will learn to associate buried food with recurring objects faster than they will learn to associate food with singular (distinctive) objects. Second, they will learn to associate buried food with man-made objects faster than they will learn to associate food with natural objects. Rodents clearly learned to associate objects with buried food regardless of distinctiveness or origin. The observed pattern of seed removal suggested that high relative humidity (RH) events (storm systems) increased seed odor, facilitating olfaction by rodents, and increasing the rate of seed removal. We tested this hypothesis in a laboratory experiment using 8 wild-caught yellow pine chipmunks and 5 levels of RH (17%, ,27%, ,50%, ,75% ,o r,95%). Foraging success at 17–75% RH was not different from random, but at 95% RH seed recovery was significantly higher than random. High RH facilitates discovery of buried seeds, and with higher foraging success, associative learning of cache markers may be easier. Thus, cache pilfering may be facilitated by high humidity.

Effects of competition and food availability on travel time in scatter-hoarding gray squirrels (Sciurus carolinensis)

Scatter-hoarding animals transport food away from its source before storing it to maximize the likelihood that the cache will survive for use at a later time. However, the more time animals spend transporting and storing the food the less time they have for food collection. Previous studies suggest that the time animals spend on caching depends to some extent on the availability of food and on the presence of conspecifics at the food source. We tested whether a scatter-hoarding mammal responds flexibly to conspecifics as competitors according to the availability of food or if they respond more directly to them as potential pilferers. We used multiple linear regression to model the time eastern gray squirrels (Sciurus carolinensis) took to return to nut patches of varying quality after collecting a nut for caching when they were alone and when conspecifics were present. The results provide the first evidence that scatter hoarders respond to conspecifics at a food source primarily as competitors rather than as potential pilferers; the squirrels increased the rate at which they returned to the nut patch when conspecifics were present rather than transport the nuts farther away to minimize the risk of pilferage. In contrast, and in line with previous studies, the squirrels responded to a reduction in the availability of food by increasing the distance at which they stored the food from the source.

ON THE RELATIVE CONTRIBUTIONS OF WIND VS. ANIMALS TO SEED DISPERSAL OF FOUR SIERRA NEVADA PINES

Selective pressures that influence the form of seed dispersal syndromes are poorly understood. Morphology of plant propagules is often used to infer the means of dispersal, but morphology can be misleading. Several species of pines, for example, have winged seeds adapted for wind dispersal but owe much of their establishment to scatter-hoarding animals. Here the relative importance of wind vs. animal dispersal is assessed for four species of pines of the eastern Sierra Nevada that have winged seeds but differed in seed size: lodgepole pine (Pinus contorta murrayana, 8 mg); ponderosa pine (Pinus ponderosa ponderosa, 56 mg); Jeffrey pine (Pinus jeffreyi, 160 mg); and sugar pine (Pinus lambertiana, 231 mg). Pre-dispersal seed mortality eliminated much of the ponderosa pine seed crop (66%), but had much less effect on Jeffrey pine (32% of seeds destroyed), lodgepole pine (29%), and sugar pine (7%). When cones opened most filled seeds were dispersed by wind. Animals removed > 99% of wind-dispersed Jeffrey and sugar pine seeds from the ground within 60 days, but animals gathered only 93% of lodgepole pine seeds and 38% of ponderosa pine seeds during the same period. Animals gathered and scatter hoarded radioactively labeled ponderosa, Jeffrey, and sugar pine seeds, making a total of 2103 caches over three years of study. Only three lodgepole pine caches were found. Caches typically contained 1-4 seeds buried 5-20 mm deep, depths suitable for seedling emergence. Although Jeffrey and sugar pine seeds are initially wind dispersed, nearly all seedlings arise from animal caches. Lodgepole pine is almost exclusively wind dispersed, with animals acting as seed predators. Animals treated ponderosa pine in an intermediate fashion. Two-phased dispersal of large, winged pine seeds appears adaptive; initial wind dispersal helps to minimize pre-dispersal seed mortality whereas scatter hoarding by animals places seeds in sites with a higher probability of seedling establishment.

Effects of mast seeding and rodent abundance on seed predation and dispersal by rodents in Prunus armeniaca (Rosaceae)

Mast seeding is a very common phenomenon in plants that are dispersed by seed scatter-hoarding animals. This reproductive strategy has been explained as adaptation to satiate predators, and predator abundance affect the seed availability and the extent of satiation. We studied how mast seeding and rodent abundance co-influenced seed predation and dispersal by rodents in Prunus armeniaca (Rosaceae) in the Dongling mountain of Beijing, China. We tracked the individual seeds marked with coded tin tags. The effects of seed crop size and rodent density were examined in May, August and October of a mast year (2000) and a non-mast year (2001). The seed removal rate was slower in mast seeding year than that in non-mast seeding year. There were more scatter hoarding and less larder hoarding with farther seed dispersal distance in the mast seeding year than that in the non-mast seeding year. The seed removal rates were positively correlated to the rodent abundance when data of six seasons of 2 years were pooled. There was high proportion of buried seeds in spring for both years. These results indicate that mast seeding and/or low rodent density increases scatter hoarding and dispersal distance, and then benefit seeding regeneration under effect of predator satiation; photoperiod seems not be very important in determining scatter hoarding activities of rodent in spring in the study region.

Ecology and Conservation of the Critically Endangered Tree SpeciesGymnocladus assamicusin Arunachal Pradesh, India

Gymnocladus assamicusis a critically endangered leguminous tree species endemic to Northeast India. Mature pods of the trees yield soap material and are collected by local people for domestic purposes and religious activities.G. assamicusgrows on hill slopes and along banks of streams. Male and hermaphrodite flowers are borne by separate individual trees. Altogether 28 mature trees were documented from nine populations. Of these, very few regenerating trees were found. This species regenerates only through seeds. The major constraints to natural regeneration are overharvesting of mature fruits, habitat destruction, grazing, predation of seeds by scatter-hoarding animals, poor percentage of seed germination due to their hard-waxy seed coats, and the lack of seed dispersal. Effective conservation initiatives should emphasize sustainable harvesting of mature pods, awareness among local people, and preservation of surviving individuals of the species. Nonetheless, reintroduction of the species to suitable ecological habitats is also recommended.

SEED REMOVAL, SEED PREDATION, AND SECONDARY DISPERSAL

Many studies of postdispersal seed fate use seed removal as an index of seed predation. However, following primary seed dispersal, some seeds are transported intact by ants, dung beetles, scatter-hoarding animals, or abiotic processes to new microsites (secondary dispersal) where germination is possible. Despite a growing realization that secondary seed dispersal can play an important role in plant recruitment, many researchers continue to use seed removal as a proxy for seed predation and are focused too intently on only the initial step of seed fate. We describe, using examples from the recent literature, how the results of some seed removal studies may have been misinterpreted, present plausible, alternative explanations for the fate of seeds in those studies, and discuss the importance of detailed studies of seed fates. Following the fates of seeds can be difficult, but such studies contribute much more to our understanding of seed dynamics and plant fitness.