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 shouldrespond to cache pilferage by decreasing the probability ofcaching food and by increasing internal fat storage to compensatefor a reduction in cache size. We tested these predictions underlaboratory conditions with variable food access (four 15-minintervals/day). Carolina chickadees (Poecile carolinensis) weresubjected to two environments: under pilferage conditions, one-quarterof their cached seeds were stolen every 0.5 h, and under no-pilferageconditions, seeds were left in place. Half the birds startedwith pilferage conditions and were then switched to the no-pilferagecondition; the other half started with no pilferage and werethen switched to pilferage conditions. The experiment was conductedover the course of a year to test for seasonal variation inthe response to seed pilferage. The birds responded to seedpilferage by taking more seeds from a feeder, suggesting thatthey monitored cache availability. Alternatively, the birdsmay have taken additional seeds from the feeder in responseto increased hunger caused by a loss of cached food. Contraryto our prediction, birds cached a higher percentage of seedsfrom the feeder when cached seeds were pilfered than when cacheswere left in place. Treatment order also affected caching behaviorfor all but the summer birds: chickadees initially subjectedto pilferage stored a higher proportion of seeds than thoseinitially subjected to no pilferage. Caching percentages inthe summer were unaffected by cache pilferage. Caching rates(number cached/day) also followed the same trends: rates werehigher when seeds were pilfered than when seeds were not pilfered,and there was a treatment-order effect for all but the summerbirds. Variation in body mass also failed to match predictedtrends. All birds exhibited a monotonic increase in mass asthe experiments proceeded, irrespective of treatment order.Controlling for this monotonic increase in mass, an analysisof residual variation in body mass indicated that birds gainedless weight when seeds were pilfered than when seeds were leftin place. Finally, birds tested in the fall and spring wereheavier than those tested in the summer. These results failto support the relationship between cache maintenance and bodymass regulation predicted by current models of energy regulation.We discuss the applicability of three hypotheses for the observedtrends.     

森林鼠类对秦岭南坡3种壳斗科植物种子扩散的影响

种子作为森林鼠类的主要食物来源,它们的扩散和更新很大程度上依赖于鼠类的传播。在鼠类扩散种子的过程中,种子特征和食物相对丰富度是影响鼠类对种子进行何种选择策略的重要因素。2011—2012年的8—12月,采用塑料片标记法在秦岭南坡的佛坪国家级自然保护区内调查了森林鼠类对同域分布的3种壳斗科植物(锐齿槲栎Quercus aliena var.acuteserrata、栓皮栎Q.variabilis和短柄枹栎Q.serrata var.brevipetiolata)种子的扩散差异。结果表明:(1)鼠类倾向于贮藏营养价值较大的栓皮栎种子,并且其贮藏距离也最远(2011:1.52 m,2012:4.03 m),3种种子在食物相对丰富度较低年份被贮藏的距离均较远。(2)在食物相对丰富度较高的年份(2011),种子的消耗速率较慢,在种子释放10 d后种子释放点仍有67.33%的种子,贮藏量较高,至实验结束仍有29.67%的种子被贮藏。在食物相对丰富度较低的年份(2012),种子消耗速率较快,在种子释放后10 d内所有种子均被取食或搬离种子释放点,贮藏量较低,至实验结束仅有3.83%的种子仍被贮藏。(3)虽然栓皮栎种子的贮藏量最大,被贮藏后的存留量也最大,但其在实验地的分布却较小,说明种子扩散仅是植物分布与存活的第一步。以上结果表明,鼠类倾向于贮藏营养价值高的种子。在食物相对丰富度较高的年份会更多的贮藏种子,但种子被贮藏的距离较近,在食物相对丰富度较低的年份会更多的取食种子。     

Invasive cutleaf coneflower seeds cached in nest boxes: possibility of dispersal by a native rodent

Invasive species present serious problems for ecosystems and economies. One such species, the invasive cutleaf coneflower (Rudbeckia laciniata L.), has several modes of seed dispersal, namely autochory, anemochory, epizoochory, myrmecochory and anthropochory. Because we found caches of this invasive plant's seeds in two nest boxes, suggesting the possibility of synzoochory, we report here the details of the caches. In one of the boxes, many of the seeds were cached in autumn and eaten during the winter. Automatic sensor cameras directed at the boxes revealed that three rodent species and two avian species visited the boxes. The characteristics of the caches and the nest‐box visitors suggested that the small Japanese field mouse (Apodemus argenteus Temm.) was the animal most likely to have cached the seeds in the boxes.     

Effects of Food Availability on Space and Refuge Use by a Neotropical Scatterhoarding Rodent

Animals that rely on refuges for safety can theoretically increase their foraging area without simultaneously increasing predation risk and travel costs by using more refuges. The key prediction of this theory, a negative correlation between food abundance, home range size and the number of refuges used, has never been empirically tested. We determined how home range size and refuge use by the Central American agouti (Dasyprocta punctata) varied across a gradient of abundance of the agoutis' principal food source: seeds and fruits of the palm Astrocaryum standleyanum. We used both manual and automated radio telemetry to measure space use of 11 agoutis during 2 mo of the Astrocaryum fruiting season, and of another set of 10 agoutis during 6 mo in which the animals largely relied on cached Astrocaryum seeds. We found that agoutis living in areas of lower food density had larger home ranges, and that all individuals used multiple refuges. The number of refuges, however, was not correlated with home range size. Consequently, agoutis that had larger home ranges roamed farther from their refuges. These results suggest that agoutis increase their home range size in response to food scarcity at the cost of their safety.     

Causes and consequences of pre‐laying weight gain in a food‐caching bird that breeds in late winter

Animals that reside at high latitudes and altitudes year‐round often use cached food to survive over the winter months, but a few species also rely on stored food to sustain them during the breeding season when the nutritional requirements of females are higher than normal. Gray jays Perisoreus canadensis rely on perishable cached food during the winter and females begin breeding in late winter when fresh food is rarely available. To examine pre‐laying patterns of weight gain, as well as the causes and consequences of weight gain among individuals, we weighed females regularly throughout the pre‐laying period. Females began increasing their weight approximately nine days prior to their first egg date, and on average increased their body weight by 25%, which is on par with other bird species that rely on non‐cached food. Final pre‐laying weight was positively influenced by the percent of conifers on territories, providing some support for previous results showing that coniferous trees are better able to preserve cached food. We also found that both final pre‐laying weight and the rate of weight gain were positively related to female age, supporting the hypothesis that female caching ability improves with age. With increasing final weight, females tended to lay larger clutches and hatched more nestlings, despite the fact that final weight was not influenced by weight at the beginning of the weighing period. Our results confirm that gray jays are able to reach breeding condition while relying primarily on food stored before winter, and suggest a novel mechanism by which habitat‐mediated carry‐over effects and female age may influence reproductive performance in a food‐caching animal.     

Substrate type affects caching and pilferage of pine seeds by chipmunks

The abiotic environment often influences the ways in which animalsinteract. By affecting the cues associated with buried seeds,the type of substrate used by seed-caching rodents may alterthe relative probabilities of cache pilferage and cache retrieval.We predicted that, after a wildfire, the presence of ash wouldimpair rodents' ability to smell pine seeds on the forest floor.In a laboratory experiment, we compared the foraging success,caching frequency, and cache recovery of chipmunks (six Tamiasamoenus and six T. quadrimaculatus) in ash versus sand substrates.Initial results supported our hypothesis: chipmunks found only2.3% of 108 caches of Jeffrey pine (Pinus jeffreyi) seeds thatwe buried in ash but found 98% of caches in sand. However, chipmunksmade as many or more of their own caches in ash compared withsand (48% for T. amoenus, 73% for T. quadrimaculatus.) Whenforaging for seeds cached in ash by themselves and by otherindividuals, they found significantly higher proportions oftheir own caches (62%) than of caches made by others (25%).However, when foraging in sand, they found high proportionsboth of their own caches and those of others (86 versus 81%).These results suggest that olfaction is less effective in ashthan in sand, that spatial memory enables chipmunks to recovertheir own caches in ash, and that caching in ash may allow animalsto avoid pilferage of stored food. As chipmunks are importantdispersers of seeds, changes in their foraging patterns or competitiveinteractions after fire could significantly affect pine regeneration.     

Pre- and postdispersal seed predation by rodents: balance of food and safety

Seed presentation and availability for seed predators changeduring every plant reproductive cycle. We know very little abouthow those changes impinge on both the ability of seed predatorsto impact plant populations and the foraging costs associatedwith seed consumption. Therefore, we conducted several fieldexperiments to evaluate whether wood mice Apodemus sylvaticusbalance food and safety while foraging on Helleborus foetidusseeds during both the pre- and early postdispersal phases ofthe plant reproductive cycle. Both food and safety were keydeterminants of mouse foraging on H. foetidus seeds, thoughtheir roles were not consistent along the plant reproductivecycle. Thus, augmenting ambient food reduced fruit removal bymice during the predispersal phase. During the postdispersalphase, seeds in sheltered microsites experienced higher removalrates than those located in nonsheltered microsites; however,no effect of food augmentation was detected. This apparent reversedrole of food and safety on decision making by mice seemed closelylinked to both the dramatic changes in accessibility and presentationof H. foetidus seeds and the coupled changing foraging costsfaced by mice at different phases of the plant reproductivecycle. For instance, because the cost of foraging for predispersalseeds was higher than for postdispersal seeds, the effect offood augmentation on foraging by wood mice was greater duringthe predispersal phase. Thus, our study illustrates the needof considering differences between pre- and postdispersal seedpredation in the study of granivore rodents and their impacton plant populations.     

Regulation of cache stores and body mass in Carolina chickadees (Panes carolinensis)

Here I report the results of laboratory experiments on cachingbehavior of Carolina chickadees (Parus carolinensis) designedto test the following predictions from a recent dynamic optimizationmodel: under limited resources, small birds, when fat, shouldcache food instead of eating it, but they should eat insteadof caching when lean; when resources are abundant, birds shouldcache less when fat and more when lean. In addition, when resourcesare abundant the amount of time spent in foraging-related activityshould decrease with an increase in body mass. Chickadees weretested for 2 weeks in either a poor-quality environment (wherethey were given four 5-min periods per day of access to a feederfilled with sunflower seeds) or a richer environment (wherethey were given four 10-min access periods); they were thenswitched to the alternative environment for an additional 2weeks. The entire experiment lasted from October through June.Within-individual comparisons showed that birds in both thepoor and rich environments exhibited the predicted correlationsbetween the probability that a seed was cached and body massmeasured at dawn. The number of seeds already stored had a weakand variable effect on caching decisions. Time budgets changedas predicted; birds spent less time with food-related behaviorswhen their energy stores were high compared to when they werelow, and overall spent a higher fraction of their time foragingin the poor-quality environment compared to the rich environment.Two other variables also affected caching behavior: experienceand season. Individuals were more likely to cache in the poorenvironment when they had been tested in the rich environmentthe previous 2 weeks, as compared with birds started in thepoor environment. However, this was true only for tests donein the winter. In late spring, all birds stopped caching orreduced caching rates when faced with limited food availability,irrespective of previous experience. The birds harvested seedsfaster when they cached instead of eating seeds; as a result,long-term weight regulation in the poor environment was affectedby caching decisions. Birds tested in late spring lost weightin the poor environment, whereas those tested in the wintermaintained a stable weight when switched from the rich to thepoor environment. Thus, annual endogenous cues directly affectcaching decisions and indirectly influence the long-term regulationof body mass     

Re-caching of acorns by rodents: Cache management in eastern deciduous forests of North America

Scatter-hoarding rodents such as tree squirrels selectively cache seeds for subsequent use in widely-spaced caches placed below the ground surface. This behavior has important implications for seed dispersal, seedling establishment, and tree regeneration. Hoarders manage these caches by recovering and eating some seeds, and moving and re-caching others. This process of re-caching, however, is poorly understood. Here, we use radio-telemetry to evaluate re-caching behavior for the management of acorn caches by rodents in eastern deciduous forests. We also test the hypothesis that as seeds are re-cached, the distance from the source increases. Radio transmitters were implanted in Northern red oak (Quercus rubra) acorns and presented to rodents in a natural setting over 3 seasons. We used radio-telemetry to track and document evidence of recovery and re-caching. We tracked a total of 102 acorns. Of the 39 radio-tagged acorns initially cached, 19 (49%) were cached on two or more occasions; one acorn was cached four times. The hypothesis that rodents move seeds to progressively greater distances from the source is not well-supported, suggesting that acorns are being moved within an individual's home range. Given the species of rodents in the study area, gray squirrels (Sciurus carolinensis) are the most likely to be responsible for the caching and re-caching events. Gray squirrels appear to engage in extensive re-caching during periods of long-term food storage, which has important implications for understanding how caching behavior influences acorn dispersal and oak regeneration.     

Directed seed dispersal towards areas with low conspecific tree density by a scatter‐hoarding rodent

Scatter‐hoarding animals spread out cached seeds to reduce density‐dependent theft of their food reserves. This behaviour could lead to directed dispersal into areas with lower densities of conspecific trees, where seed and seedling survival are higher, and could profoundly affect the spatial structure of plant communities. We tested this hypothesis with Central American agoutis and Astrocaryum standleyanum palm seeds on Barro Colorado Island, Panama. We radio‐tracked seeds as they were cached and re‐cached by agoutis, calculated the density of adult Astrocaryum trees surrounding each cache, and tested whether the observed number of trees around seed caches declined more than expected under random dispersal. Seedling establishment success was negatively dependent on seed density, and agoutis carried seeds towards locations with lower conspecific tree densities, thus facilitating the escape of seeds from natural enemies. This behaviour may be a widespread mechanism leading to highly effective seed dispersal by scatter‐hoarding animals.     

Clark’s Nutcracker Breeding Season Space Use and Foraging Behavior

Considering the entire life history of a species is fundamental to developing effective conservation strategies. Decreasing populations of five-needle white pines may be leading to the decline of Clark’s nutcrackers (Nucifraga columbiana). These birds are important seed dispersers for at least ten conifer species in the western U.S., including whitebark pine (Pinus albicaulis), an obligate mutualist of Clark’s nutcrackers. For effective conservation of both Clark’s nutcrackers and whitebark pine, it is essential to ensure stability of Clark’s nutcracker populations. My objectives were to examine Clark’s nutcracker breeding season home range size, territoriality, habitat selection, and foraging behavior in the southern Greater Yellowstone Ecosystem, a region where whitebark pine is declining. I radio-tracked Clark’s nutcrackers in 2011, a population-wide nonbreeding year following a low whitebark pine cone crop, and 2012, a breeding year following a high cone crop. Results suggest Douglas-fir (Pseudotsuga menziesii) communities are important habitat for Clark’s nutcrackers because they selected it for home ranges. In contrast, they did not select whitebark pine habitat. However, Clark’s nutcrackers did adjust their use of whitebark pine habitat between years, suggesting that, in some springs, whitebark pine habitat may be used more than previously expected. Newly extracted Douglas-fir seeds were an important food source both years. On the other hand, cached seeds made up a relatively lower proportion of the diet in 2011, suggesting cached seeds are not a reliable spring food source. Land managers focus on restoring whitebark pine habitat with the assumption that Clark’s nutcrackers will be available to continue seed dispersal. In the Greater Yellowstone Ecosystem, Clark’s nutcracker populations may be more likely to be retained year-round when whitebark pine restoration efforts are located adjacent to Douglas-fir habitat. By extrapolation, whitebark pine restoration efforts in other regions may consider prioritizing restoration of whitebark pine stands near alternative seed sources.     

Post-dispersal predation and scatterhoarding of Dipteryx panamensis (Papilionaceae) seeds by rodents in Panama

In tropical rain forests of Central America, the canopy tree Dipteryx panamensis (Papilionaceae) fruits when overall fruit biomass is low for mammals. Flying and arboreal consumers feed on D. panamensis and drop seeds under the parent or disperse them farther away. Seeds on the ground attract many vertebrate seed-eaters, some of them potential secondary seed dispersers. The fate of seeds artificially distributed to simulate bat dispersal was studied in relation to fruitfall periodicity and the visiting frequency of diurnal rodents at Barro Colorado Island (BCI), Panama. The frequency of visits by agoutis is very high at the beginning of fruitfall, but in the area close (<50 m) to fruiting trees (Dipteryx-rich area) it declines throughout fruiting, whereas it remains unchanged farther (>50 m) away (Dipteryx-poor and Gustavia-rich area). Squirrels were usually observed in the Dipteryx-rich area. Along with intense post-dispersal seed predation by rodents in the Dipteryx-rich area, a significant proportion of seeds were cached by rodents in the Dipteryx-poor area. Post-dispersal seed predation rate was inversely related to hoarding rate. A significantly greater proportion of seeds was cached in March, especially more than 100 m from the nearest fruiting tree. This correlates with the mid-fruiting period, i.e. during the height of D. panamensis fruiting, when rodents seem to be temporarily satiated with the food supply at parent trees. Hoarding remained high toward April, i.e. late in the fruiting season of D. panamensis. Low survival of scatterhoarded seeds suggests that the alternative food supply over the animal's home-ranges in May–June 1990 was too low to promote survival of cached seeds. Seedlings are assumed to establish in the less-used area of the rodents' home-range when overall food supply is sufficient to satiate post-dispersal predators.     

同种竞争压力对小泡巨鼠贮藏油茶种子行为的作用分析

2002年11～12月,在四川省都江堰地区的亚热带常绿阔叶林内利用人工修建的半自然状态围栏进行实验,研究了小泡巨鼠在有同种竞争存在条件下对油茶种子的埋藏行为。结果表明,小泡巨鼠在有竞争存在条件下,显增加了埋藏油茶种子的量。这一结果支持了“竞争的存在刺激鼠类埋藏”的假说。同时,研究结果表明,小泡巨鼠在有竞争存在条件下,显增加了对埋藏种子的搬运距离,每个贮藏点埋藏种子的数量也有所增加,同时埋藏的生境更多地偏向于遮蔽较好的微生境(草丛底层、灌丛下层)中。这些行为策略有可能有利于种子被埋藏植物的种群扩散。在讨论中,我们还通过比较鸟类和兽类在感觉器官上的差别,分析它们在有竞争存在条件下所采取的不同贮食策略。     

THE OLFACTORY-GUSTATORY BASIS OF FOOD PREFERENCE IN THE HERBIVOROUS PROSOBRANCH, LITTORINA LITTOREA (LINNAEUS)

Studies of food preference often measure the rate of consumptionof prey items by a predator; this confounds the factors determiningattractiveness and edibility, ignoring the fact that preferencesare probably resolved at the pre-ingestive stage of a consumer'sfeeding behaviour. In this paper, a novel technique has beendeveloped to measure the importance of chemical stimuli to feed,perceived by olfactory and gustatory means, as determinantsof the subsequent feeding behaviour of the generalist intertidalherbivore,Littorina littorea (L.). Cell-free extracts of a rangeof algae were presented simultaneously, impregnated in artificialcellulose-based substrates. Subsequent ranking of feeding preferencesreveals hierarchies which correspond with those establishedby other workers, using intact algae to feed L. littorea. Extractsof ephemeral early successional species (e.g. Ulva lactuca,Porphyra umbilicalis) generally provoke a more positive feedingresponse than perennial late successional species (e.g. Fucusvesiculosus, Asco-phyllum nodosum). Animals fed a preferredalga (Ulva lactuca) prior to preference trials displayed a reductionin the strength of preference relative to starved animals. Therewas no evidence of ‘switching’ of preference rankingsdue to satiation, however. These findings confirm the importanceof olfactory-and gustatory-mediated feeding behaviour as theprimary determinant of food preference in L. littorea. Theyare consistent with the hypothesis that the factors which determinethe attractiveness of a food type, such as chemical cues, arethe criteria by which consumers predict the value of that food.     

Social influences on food caching in willow tits: a field experiment

We studied the food hoarding behavior of willow tits (Parusmontanus), a scatter-hoarding passerine wintering in dominance-structuredflocks. We examined social influences on microhabitat selectionand spatial cache distribution at temporary feeders. Dominantadult males stored food closer to die feeder and at a greaterrate than did subordinates. When alone, the birds stored foodcloser to the feeder than when accompanied by conspecifics.Conifers were preferred over deciduous trees as cache trees.The subordinates cached more in die outer parts of branchesthan dominants. There were no significant differences in dierelative or absolute heights of die caches, nor in the verticalor horizontal hoarding niche breadths between dominants andsubordinates. We experimentally removed die dominants from dieflock for 90 min and recorded the behavior of die remainingsubordinates immediately after die removal. The removal resultedin a decrease in die hoarding distance of die remaining birds,indicating that die presence of dominants directly affecteddie behavior of subordinates and suggesting that kleptopar-asitismby dominants may be prevented by rarhing farther away. Withdie dominants removed, die subordinates cached at a greaterrate than before die removal. The decrease in die hoarding distanceand increase in die hoarding rate were die only significanteffects of die experiment, perhaps suggesting that, during ashort absence of dominants, die subordinates do not benefitfrom changing dieir caching microhabitat They might be excludedfrom those new, possibly safer, microhabitats after die dominantbird rejoins die flock.     

Vertical migration and feeding of Euphausia lucens (Euphausiacea) in the Southern Benguela

Diel migration and feeding were examined for populations ofEuphausia lucens living in the near- and offshore waters ofthe southern Benguela. Euphausiids at both stations displayednocturnal diel vertical migration patterns. Animals inshoreascended in a slow-fast-slow manner which seems to be relatedto differential food concentrations throughout the water columnPronounced quantitative and qualitative changes in the dietof E.lucens were observed offshore but not inshore. These differencesare discussed in relation to different ambient food environments     

Black bears Ursus americanus are effective seed dispersers,with a little help from their friends

Black bears Ursus americanus are generally considered effective seed dispersal agents for fleshy‐fruited plants because they can consume hundreds of fruits at once and have large home ranges. Although seedlings can emerge from faecal piles, establishment of such seedlings seems to be infrequent. Removal of seeds from faeces by rodents is often considered seed predation. We show that removal of seeds from bear faeces by seed‐caching rodents in the Sierra Nevada, USA, represents a second phase of seed dispersal that benefits some fleshy‐fruited plants. Using Trail Master infrared cameras to photograph animals and scandium‐46, a gamma‐emitting radionuclide, to track seeds, we determined that deer mice Peromyscus maniculatus removed seeds from bear faeces and cached them in soil. Caches typically contained 1–3 seeds buried 5–10 mm deep. These seeds escaped several sources of mortality by being moved to relatively safe locations, but deer mice also eventually eat many of the cached seeds. A field germination study confirmed that seed burial increased seedling emergence. Rodents removed seeds in bear faeces more quickly than those in bird faeces in one year, but seeds in bird faeces were removed faster in another year. Results varied across two years, probably because of availability of alternative food sources or changes in deer mice population sizes. The two‐phase seed dispersal syndrome described here may be important in understanding seed dispersal by carnivores and large ungulates that produce large faecal deposits containing many relatively large seeds.     

食物源距离对中华姬鼠贮藏策略的影响

在围栏条件下, 检验食物源距离对中华姬鼠贮藏策略的影响,并检验根据快速隔离假说和最优贮藏空间分布模型做出的两个预测：（1）3个释放点无论距离远近,种子均先被大量分散贮藏在各个释放点周围;（2）随着释放点远离巢穴,种子被集中贮藏的比例下降。结果发现: 分散贮藏是中华姬鼠的主要贮藏方式,3个释放点种子被分散贮藏的数量无显著差异,这个结果支持预测（1）;3个种子释放点（1m,5m,13m）被集中贮藏的比例分别为(1m: 6.25%; 5m: 1.88%; 13m: 1.25%),说明被集中贮藏的种子数量随种子释放点与巢穴距离的增大而逐渐减少,这个结果支持预测（2）。本研究表明中华姬鼠应对捕食风险升高的策略是降低取食量和贮藏量。     

Selfish long-term benefits of hoarding in the Siberian jay

We followed the consumption of cached food in Siberian jay flocksduring winter. Siberian jay flocks contain kin associationscomposed of parents in company with mature offspring. Further,flocks often contain immigrants not closely related to the groupmembers. We studied the extent to which Siberian jay offspringkept caches to themselves or whether they showed a kin biasand shared cached food mutualistically with relatives (parents/siblings)thus making an inclusive fitness gain. We recorded the retrievalof cached food using radio-ptilochronology, a technique thatdetects the consumption of a radioactively labeled food itemin the growth bar laid down in a growing feather the day ofconsumption. Food caching entailed a selfish benefit to thehoarder. The hoarding bird showed a substantial "recovery advantage"and retrieved its own cached food seven times as often as anyof its flock mates. There was no evidence for mutualistic sharingof caches among relatives. This selfish benefit persisted throughoutthe 7-week period for which we could follow the consumptionof labeled food.     

Effects of seed size of wind-dispersed pines (Pinus) on secondary seed dispersal and the caching behavior of rodents

Seeds and nuts dispersed by scatter-hoarding animals are relatively large compared to propagules dispersed by other means. Possible selective forces in the evolution of large seed size include the selectivity of foraging animals and the ways that food-storing animals treat seeds and nuts after harvest. Treatment by rodents, primarily yellow pine chipmunks ( Tamias amoenus ), of four species of pine seeds that vary in size was studied in the Carson Range of western Nevada. The pines, lodgepole pine ( Pinus contorta , 8.7 mg), ponderosa pine ( P. ponderosa , 55 mg), Jeffrey pine ( P. jeffreyi , 157 mg), and sugar pine ( P. lambertiana , 213 mg), produce winged seeds that are initially wind-dispersed but are gathered by rodents and cached in the soil. Radioactive scandium-46 was used to follow the fates of seeds of all fours species placed around three source trees during autumn 1998 to 2000. Rodents gathered the seeds of all four species, but they took fewer of the lodgepole pine seeds and only six lodgepole seed caches (n=2106 total caches) were found during the three years. Among the other three species, number of seeds per cache decreased with increasing seed mass. However, the product of number of seeds per cache and seed mass was similar for all species. Sugar pine seeds were cached slightly deeper than ponderosa and Jeffrey pine seeds. For the species examined, seed size appeared to have had little effect on several other attributes, including mean dispersal distance, substrate choice, and microhabitat choice. Large size decreases wind dispersibility of pine seeds, but secondary dispersal by scatter-hoarding rodents compensates for poor wind dispersal so that total dispersibility of large-seeded pines is not compromised.