Search for lunar periodicity in the bannertail kangaroo rat (Dipodomys spectabilis)

Abstract

The endogenous activity cycle of the nocturnal bannertail kangaroo rat was investigated. Although bannertail activity is a function of the lunar day as well as the solar day, all ten subjects exhibited free‐running activity periods of solar‐day length; there was no evidence of an endogenous lunar‐day cycle. Animals were provided with a burrow system and a small pseudo‐desert, a laboratory facility in which animal activity data closely resembled measurements taken in the field. Several analytical techniques for quantifying the data were utilized, and one, the mean interval of activity, is recommended to other investigators.     

Effects of food value, predation risk, and pilferage on the caching decisions of Dipodomys merriami

Animals that scatter cache their food face a trade-off betweenthe benefits of protecting caches from pilferers and the costsassociated with caching. Placing food into a large number ofwidely spaced caches helps to protect it from pilferage butalso involves costs such as greater exposure to predators. Ipredicted that animals would disperse food into a larger numberof more widely spaced caches when caching (1) a preferred foodversus a less preferred food and (2) under conditions of lowpredation risk versus high predation risk. To test these predictions,I examined the scatter-caching decisions of Merriam's kangaroorats (Dipodomys merriami). D. merriami distributed caches inclumped patterns, regardless of food preference, but they showeda tendency to invest more in a preferred food by distributingcaches more widely. Under the relative safety of the new moon,they did not disperse caches more widely, rather they partitionedthe same amount of food into a larger number of caches thanthey did under the full moon, when predation risk is higher.To examine whether their cache spacing decisions had a significantimpact on the success of cache pilferers, I measured discoveryby pilferers of artificial caches of two food types at differentcaching distances. Results indicate that the cache spacing behaviorof D. merriami functions to protect caches from pilferers, becauseincreased spacing of artificial caches decreased the probabilityof pilferage for both types of food.     

The influence of environmental conditions on cache recovery and cache pilferage by yellow pine chipmunks (Tamias amoenus) and deer mice (Peromyscus maniculatus)

I conducted a field experiment in 10 x 10 m enclosures to explorehow seed and soil moisture levels influence the ability ofknowledgeable and naive rodents to find natural caches of Jeffreypine (Pinus jeffreyi) seeds. Subjects were yellow pine chipmunks(Tamias amoenus) and deer mice (Peromyscus maniculatus) searchingfor caches that they had made, caches made by other individualsof the same species, or caches made by individuals of the otherspecies. Subjects that made caches (knowledgeable subjects)relied on spatial memory to find many of their own caches duringrecovery sessions, and their ability to locate caches was notaffected by water content of seeds or soil. Naive subjectsfound few caches under dry conditions, but under wet conditions,they located as many caches as did the rodents that made them.Naive subjects apparently relied on olfaction to find caches,a sensory modality that works more effectively under moist conditions.Subjects had as much success foraging for caches made by membersof their own species as for caches made by the other species.I present a hypothesis that predicts how foragers could modifypredominately memory-based search to predominately olfactory-basedsearch as the weather changes from dry to wet. When foragersrely on spatial memory, those foragers find only their owncaches, but when they can also use olfaction, they pilfer cachesmade by other individuals. Consequently, the nature of competitiveinteractions among members of the seed-caching guild may changeas the weather changes.     

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.     

Reciprocal pilferage and the evolution of food-hoarding behavior

Current theories of food-hoarding behavior maintain that hoardingcan be adaptive if a hoarder is more likely than any other animalto retrieve its own caches. A survey of the literature indicatedthat the hoarder often has a recovery advantage when searchingfor items it has stored, but levels of cache pilferage are oftenso high (2–30% per day) that at least for some long-termfood hoarders, the caching animal is unlikely to recover a significantamount of its stored food. Except in a few cases (acorn woodpeckersand beavers), kin selection cannot explain the high levels ofpilferage observed. We suggest that some small solitary animalswith overlapping home ranges (e.g., most rodents, chickadees,and tits) are able to tolerate high levels of cache pilferage.Pilferage is not as damaging to these animals as it might otherwisebe because many interspecific and all intraspecific cache pilferersalso cache food. These or similar food caches can be pilferedlater by the original food hoarder. In other words, pilferingin these species is often reciprocal, and because it is reciprocal,it can be tolerated. We argue that caching systems based onreciprocal pilfering can be stable and are not necessarily susceptibleto "cheaters," animals that pilfer food but do not scatter hoardfood themselves, and we introduce a model of food hoarding tosupport this argument. These food-caching systems based on reciprocalpilfering resemble cooperative behavior, but the behavior isactually driven by the selfish interests of individuals. Thistheory of scatter-hoarding behavior based on reciprocity hasimportant implications for the ways that food-hoarding animalsinteract with inter- and intraspecific competitors.