Animal Learning & Behavior1997,25 (4),473-484

The existence and extent ofspatial working memory ability in honeybees

MICHAEL F.BROWN, JONATHAN A. MOORE,CATHERINE H. BROWN, and KRISTEN D.LANGHELD

ViUanova University, ViUanova, Pennsylvania

Honeybees foraged from six locations, each of which was baited with sugar solution prior to eachexperimental trial. Under a variety of conditions, bees exhibited a small but reliable tendency to avoidrevisits to locations that they had visited earlier during the experimental trial. These results replicatethose of Brown and Demas (1994),who concluded that bees use working memory to discriminate pre­viously visited locations from those not yet visited. The present experiments included procedures thatallowed alternatives to this explanation to be more completely ruled out. The extent of spatial work­ing memory performance exhibited by honeybees in these experiments appears to be limited by a pro­cess other than working memory capacity, perhaps the ability of bees to discriminate among severallocations in close proximity to one another.

A distinction fundamental to our understanding ofhuman and animal memory systems is that between ref­erence memory and working memory (see, e.g., Badde­ley, 1986; Honig, 1978). Information that remains moreor less stable over time is thought to be stored in a large­capacity, long-duration reference memory system, whereasa relatively small-capacity and often short-duration sys­tem is used to store information that changes in contentoften or is only temporarily useful. In humans, workingmemory is generally considered to be the cognitive struc­ture in which information is stored while active cognitiveprocesses operate on that information (Baddeley, 1986).

A large number of studies have examined workingmemory in a variety ofvertebrate animals, primarily usingpigeons in the matching-to-sample procedure (e.g., Rob­erts & Grant, 1974) and rats in the radial-arm maze pro­cedure (e.g., Olton & Samuelson, 1976). In both cases,one or more conditional cue(s) must be stored in mem­ory to bridge a temporal gap prior to the critical choiceresponse(s). In matching-to-sample, the identity of thesample determines the test stimulus to which the subjectmust later respond for reinforcement. In the radial-armmaze, the identity of previously visited spatial locationsdetermines the locations that will be baited later in thetrial. The fact that these cues change from trial to trial re­quires a flexible, dynamic system for the temporary stor­age of information (i.e., working memory).

Brown and Demas (1994) recently provided initial ev­idence for working memory in honeybees using a proce-

This research was supported by National SCience Foundation (NSF)Grant I8N-9404120. The participation of Knsten Langheld was madepossible by an NSF Research Expenence for Undergraduates awardWethank Jessica Lease for assistance conductmg Expenment I. Corre­spondence should be addressed to M. F. Brown, Department of Psy­chology, Villanova University, Villanova, PA 19085 (e-mail: mbrown(a)email.vill.edu).

dure that is analogous to the radial-arm maze procedureused with rats and other vertebrates. There is an abun­dance ofevidence for various forms oflearning and mem­ory in honeybees and other invertebrates (for recent re­views, see Abramson, 1994; Bitterman, 1996; Menzel,1990; Papaj & Lewis, 1993). However, Brown and De­mas's data are the first to suggest that honeybees possessa flexible, dynamic, multiple-item memory system, cor­responding to the working memory that has been studiedin humans and other vertebrates.

Ifhoneybees do in fact possess a memory system anal­ogous to, or similar to, vertebrate working memory, thisfact would have wide-ranging implications for our un­derstanding of working memory. For example, workingmemory in vertebrates has been associated with hippo­campal and other brain structures that do not even existin invertebrates. Thus, working memory (or a systemanalogous to working memory) can be implemented in abrain that is very different from those in which it hasbeen studied so far.

Brown and Demas (1994; Demas & Brown, 1995) re­ported a total of four experiments in which honeybeesforaged for small drops ofsucrose solution from a matrixof six closely spaced locations. Bees depleted each loca­tion ofsucrose during the first visit to the location. Thus,just as in Olton's radial-arm maze problem (Olton &Samuelson, 1976), the contingencies ofreinforcement en­couraged one and only one visit to each location duringeach experimental trial. Brown and Demas used a MonteCarlo simulation to estimate the expected tendencies ofthe bees to revisit locations given their location-to-locationtransition probabilities. They found that the tendency ofthe bees to revisit locations was slightly but significantlylower than these estimates. Their experiments includedcontrols for the possibility that a perceptual cue relatedto the presence of the sucrose solution allowed this avoid­ance of revisits and for the possibility that an odor trail

473 Copyright 1997 Psychonomic Society, Inc.

474 BROWN, MOORE, BROWN, AND LANGHELD

left by the bees controlled choice. Thus, Brown and Demasinferred that the bees were using working memory to dis­criminate the locations already visited during an experi­mental trial from those not yet visited.

The present experiments address two issues related tothis recent evidence for working memory in bees. The firststems from recent work by Burmeister, Couvillon, andBitterman (1995), which questions Brown and Demas's(1994) conclusions on both empirical and theoreticalgrounds. The second is related to the magnitude of theeffect reported by Brown and Demas. Their evidence forworking memory was quite reliable, but small in magni­tude relative to working memory effects that have beenreported in vertebrates. This may mean that honeybeeshave only limited working memory ability. If so, it maybe that the bee nervous system can support only limitedworking memory ability. Alternatively, it may be that theexperimental procedures used by Brown and Demas didnot result in an accurate assessment of the magnitude ofbee working memory ability. Most of the work reportedbelow was designed to investigate this possibility.

EXPERIMENT 1

Burmeister et al. (1995; see also Isnec, Couvillon, &Bitterman, 1997; Ohyama, Couvillon, & Bitterman, 1995)recently reported three experiments, using arrangementsof three or four baited locations, in which they failed tofind evidence of any tendency to avoid revisits to loca­tions. They did find systematic biases in their bees tovisit locations according to their spatial properties. De­viations from a priori estimates of chance levels ofchoice accuracy in their bees can be explained in termsof these biases. Although Burmeister et al. did not offeran explanation that would account for the discrepancybetween their results and ours, they did argue that our ex­perimental methods were "highly unsatisfactory" for sev­eral reasons. First, they correctly pointed out (just as wedid) that odor trails can explain the results of our previ­ous free-choice experiments (Brown & Demas, 1994,Experiment 1; Demas & Brown, 1995). That is, when thebee visits a location, it might leave an odor trace and usethis trace as a perceptual cue during subsequent choices.However, such odor trails cannot explain the results ofour previous forced-choice experiments (Brown & Demas,1994, Experiments 2 and 3). In those forced-choice ex­periments, the apparatus was switched with a fresh (andtherefore odorless) one between the third of three forcedchoices and a free-choice phase of each experimentaltrial. The ability ofbees to avoid revisits to the first three(forced) locations during these free choices was com­pared with the tendency to make such revisits predictedby a Monte Carlo simulation.

Second, Burmeister et al. (1995) pointed out that ourprevious experimental technique did not include the bees'feeding to repletion during each trial. They argued thatthis aspect of our technique could have resulted in trialsbeing "given under different motivational conditions and

at different points in the subject's foraging cycle, or, ifthe excluded subject went off to forage elsewhere, atwidely different intertrial intervals" (p. 374). Althoughthey did not specify how these differences could haveproduced the pattern of results we obtained, Burmeisteret al. are correct that this factor could have affected ourresults. Finally, they pointed out that most of our exper­iments (all but Experiment 3 of Brown & Demas, 1994)were conducted quite close to the hive, resulting in manybees other than the subject bee being attracted to the areaof the experimental apparatus. To avoid interference bynonsubject bees, we enclosed our apparatus in a clearplastic container, including a lid that prevented the beesfrom flying higher than 14 em above the surface of theapparatus. Burmeister et al. asserted that this restrictionof subjects' flying room may have affected their behavior.It has also been suggested (by an anonymous reviewer ofa grant proposal) that the Plexiglas lid used to restrict ac­cess to the apparatus may have filtered ultraviolet lightthat might have been involved in the use of spatial cuesby bees.

Although the first ofthese three criticisms applies onlyto some of our previous experiments and it is not clearhow the remaining two criticisms can explain the patternof results we obtained, we believe that the criticismsraised by Burmeister et al. (1995) should be taken seri­ously. There is convincing evidence that an odor trail cancontrol honeybee choice in a laboratory task somewhatsimilar to ours (Giurfa & Nunez, 1992).There is also someevidence that this occurs during natural foraging (Cor­bet, Kerslake, Brown, & Morland, 1984; Free & Wil­liams, 1983; Wetherwax, 1986). Thus, an additional testof the avoidance of revisits by honeybees that cannot beexplained in terms of an odor trail is clearly important.

The experimental procedure of the first experiment inthe present series was very similar to that used by Brownand Demas (1994, Experiment 1), but differed in wayssuggested by the criticisms of Burmeister et al. (1995).First, a technique similar to that used by Burmeister et al.was used to prevent control of choices by an odor trail,in the context ofa free-choice procedure. Specifically, thesix target locations contained small, square dishes, withthe drop ofsucrose solution centered in the dish. Follow­ing the first visit to a target location, the dish was re­placed with a dish containing a drop of water. Thus, anyodor left behind by the bee during the first visit to a tar­get location was removed.

Second, when the bee chose the target location con­taining the last drop of sucrose solution and began feed­ing on that drop, the drop was enlarged so that the beefed to repletion. This was intended to encourage the beesto return to the hive between trials, thereby increasingthe match between the conditions to which the bee wasexposed and the trial structure intended by the experi­mental design. Finally, we conducted the present exper­iment relatively far from the hive, inside a laboratory. Asa result, very few bees that had not been intentionally re­cruited to the experiment entered the area of the appara-

tus. Therefore, there was no need to cover the apparatusand restrict the flying area of the bees, as we did in ourearlier experiments.

MethodSubjects. The subjects were 14 honeybees (Apis mellifera ligus­

fica) from a full-sized hive located mside a campus building, withpassage outside via a tunnel leading from the hive through a win­dow. The hive was three floors above the laboratory where datawere collected.

Apparatus. The apparatus consisted of a 2 X 3 matnx oftargetlocations on a 28.0 X 28.0 X 6 mm (thick) plywood surface. If thematrix is considered to be two columns of three target locationseach, then the two columns of locations were separated by 17.3 em(center to center). The locanons forming the ends of the columnswere separated by 22.7 em, with the middle location ofeach columnbeing 9.8 em from one end of the column and the middle hole oftheother column being the same distance from the other end of theother column. The plywood surface was painted violet. Each targetIocation was defined by the presence ofa 4.4-cm-square, 0.95-cm­deep, translucent polystyrene weighing boat (referred to below as a"dish"; Cole-Parmer No. H-O 1017-05), placed inside a white,porcelain crucible (7 ern in diameter and 1.2 em deep). A red ad­hesive dot (1.3 em in diameter) on the surface of each crucible andvisible through the bottom of the dish was intended as a visual cuedefining the point on each target location where drops of sucrosesolution (or water) were placed. The dish/crucible units definingeach target location could be easily removed from the apparatus sur­face (see Procedure). Six landmarks, consisting of differently col­ored and shaped children's blocks, were glued to the surface of theapparatus, one in close proxrrmtyto each of the target locations. Theapparatus was located on the laboratory window ledge, with thewindow open. Dishes were baited with 50% (v/v) sugar solution atroom temperature, using a microsynnge.

Procedure. Subjects were collected by netting them as they leftthe entrance to the hive. They were taken in individual vials to thelaboratory, and these vials were placed (open end down) over a dishat a target location on the apparatus, which had been baited with alarge drop of sugar solution. Movement of the bee to the bottom ofthe vial (and therefore the dish) was sometimes facilitated by plac­mg an opaque cover over part of the vial. The bee was allowed tofeed from the dish. After feedmg to repletion, it flew out ofthe lab­oratory through the window. If a bee returned to the apparatus, itwas considered to be a subject in the experiment and was markedusing a spot ofTestor's model pamt on the thorax or abdomen.

Following successful recruitment to the apparatus, each tnal beganby placing a freshly baited dish (baited with a 4- to 5-,u1 drop of so­lution in its center) at each of the six target Iocations. When the ex­perimenter detected the bee approaching the window, a camcorderwas turned on to record the subject's behavior. The few nonsubjectbees that entered the laboratory were prevented from interfenngwith the experiment by chasing them away or captunng them.

A choice was defined when any part of the subject bee toucheda dish. This was VIrtually always followed by a full landing andextension of the probOSCIS onto the surface of the dish. When a beeleft a dish following the initial visit, the dish was removed and re­placed with one containing a fresh 4- to 5-,u1 drop of tap water. Thebees spent several seconds Imbibing the sucrose solution followinga correct choice, but immediately rejected the water drop followingan incorrect choice. Because of this, we did not attempt to replacedishes containing water drops with fresh dishes contaimng waterdrops following an incorrect choice. The speed with which mostbees moved from dish to dish toward the end ofa tnal (when manydishes contained water drops) did not allow us to unfailingly re­place dishes before the bee made its next choice. Because of the timetaken to Imbibe sucrose solution during choices made early in the

WORKING MEMORY IN BEES 475

choice sequence, however, we were able to replace dishes follow­ing the initial visit to a target location between the time the bee leftthat location and the time it left the location of its next choice. Thesequence ofchoices made by the bee was recorded, using the video­tape record to resolve any ambiguous cases.

When a bee chose the target location containing the last remain­ing sugar drop, a syringe was used to enlarge the drop so that it waslarger than could be Imbibed by the bee. This resulted in the bee im­bibing a large amount of solution and then flying out the window.A tnal was considered complete when the bee flew out the windowand did not return for at least 3 min. Typically, the bee disappearedfor 10-15 min following a trial and then returned. In the meantime,the apparatus was prepared for the next trial by replacing all sixdishes with freshly baited ones. Trials continued until a bee had com­pleted 20 trials, or until the bee ceased returning for additional trials.

Beginning with the 4th bee that participated in the experiment, ifa bee completed all 20 tnals, the procedure described above wasfollowed by five trials ofa perceptual cue probe procedure, designedto ensure that bees could not discriminate drops contaimng sugarfrom water-only drops. During each ofthese five trials, a randomlyselected set of three locations was baited with sugar solution and theremaining three were baited with water drops. Bees were allowed tochoose from among the six locations until all six had been visited.Dishes were not replaced following a visit.

ResultsThe 14 subjects participated in a mean of 17.2 trials

with six or more choices. Bees visited all six target lo­cations during a mean of 84.0% of these trials. For pur­poses ofdata analysis, trials were considered to be struc­tured into two blocks of 10trials each. Only trials meetinga criterion of including at least six visits were further an­alyzed. If a bee did not participate in at least five suchtrials during a trial block, its data from that block werenot included. This was the case for 4 bees during the sec­ond trial block. These 4 bees did not complete the plannedregimen of 20 trials because the bee ceased visiting theapparatus for unknown reasons.

Two measures of choice accuracy during the first sixvisits were determined for each bee. The first measure isthe number of visits that were to correct (previously un­visited) target locations. Immediate revisits (i.e., revisit­ing a location without an intervening visit to another lo­cation) were not counted as visits because of a potentialartifact. Specifically, we could not be sure that our ma­nipulation ofdishes following initial visits (i.e., removingthe old dish and replacing it with a fresh dish containinga water drop) did not have an effect on the tendency toimmediately revisit. Certainly, the bees did not have thisoption while the manipulation was being carried out. Sothis behavior was considered undefined both in the em­pirical results and in our estimate of chance. This strat­egy eliminates any cuing effect that our manipulation ofthe apparatus to replace target locations might have had.

A second measure of choice accuracy during the firstsix visits was used because of the fact that a fresh dishreplaced a visited one only following the initial visit to atarget location. It is possible that bees may have avoidedvisiting a target location for a third time because ofodorleft there during the second visit. Restricting the analysesto choices made during the first six choices reduces any

476 BROWN, MOORE, BROWN, AND LANGHELD

influence this possibility may have had. Nevertheless, itwas desirable to develop a measure that eliminated anyinfluence of odor left following the second (or subse­quent) visits to target locations. This measure is the meannumber of target locations revisited (one or more times)during the first six visits. Thus, a target location that isvisited three times during the first six choices counts asbeing revisited in exactly the same manner as a target lo­cation that is visited twice. This measure therefore elim­inates the small number ofcases in which a bee visited thesame location for a third (or more) time.

These two measures of choice accuracy were com­pared to estimates of chance performance. Appropriateestimates of chance in this task must take into accountthe nonrandom location-to-location transition biases thatbees demonstrate (Brown & Demas, 1994; Burmeisteret al., 1995). In our apparatus, for example, bees tend totravel from one location to a location in relatively closeproximity. This and other transition tendencies affect thechoice accuracy of the bees in a manner assumed to beindependent of working memory ability. To control forthese effects, Brown and Demas (1994) used Monte Carlosimulations ofbee choice behavior that incorporated thelocation-to-location transition probabilities of individualbees during individual trial blocks. This same techniquewas used to provide comparisons for the empirical re­sults found in the present experiment. First, a transitionprobability matrix was constructed for each bee duringeach of the two trial blocks. Given that the bee was ineach ofthe six locations (or had just started a trial and notyet visited a location), the probability of moving fromthat location (or from entering a trial) to each of the sixlocations was determined. The overall transition proba­bility matrix is shown in Table 1. The transition proba­bility matrices of individual bees for each block of trialswere used in Monte Carlo simulations that chose loca­tions using the same transition probabilities as each bee.Any tendency to avoid revisits to locations beyond whatis shown by these simulations cannot be accounted for bysystematicity in the location-to-location movement pat­terns of the bees.

Table IOverall Transition Probability Matrix

for Choices Made in Experiment 1

Destination Location

Previous Location 1 2 3 4 5 6

Begin trial 6.6 14.9 7.5 33.2 17.8 19.9Location I * 41.9 4.5 33.6 14.5 5.5Location 2 27.6 * 24.8 12.6 21.7 13.3Location 3 5.2 39.8 * 3.6 18.1 33.2Location 4 34.8 14.3 5.1 38.3 7.5Location 5 6.7 16.9 6.7 33.0 * 36.7Location 6 1.6 6.8 49.2 6.3 36.1 *

Note-Each row shows the percentage of transitions (Iocation-to-locationmovements) that were to each of the six locations, given the previouslocation ofthe bee (either the location chosen last or initial entry into theapparatus for a new trial). Data are collapsed across bees and trial blocks.*lmmediate returns to locations are ignored in the analysis. The loca­tions are numbered as two rows of three locations each (e.g., Location 2is in between Locations I and 3, and is also adjacent to Location 5).

The algorithm used to implement the Monte Carlo sim­ulations chose from among six alternatives using thelocation-to-location transition probabilities of individualsubjects during individual trial blocks until all six hadbeen chosen. The estimates produced are based on 1,000simulations using each bee's transition matrix duringeach of the two trial blocks. The data produced by theseMonte Carlo simulations were analyzed in terms of thesame two measures of choice accuracy described abovefor the empirical data. Thus, like the empirical measuresof performance, these estimates did not include any im­mediate revisits to locations.

The top panel of Figure 1 shows the mean number ofcorrect choices made during the first six choices ofeachtrial made by the bees and by the corresponding MonteCarlo simulations. Because 4 bees did not contribute tothe data ofBlock 2, and it is possible that this subject losswas nonrandom, data for the two trial blocks were eval­uated separately.The bees made more correct choices thanestimated by the simulations during both Block 1 [t(13) =3.88] and Block 2 [t(9) = 3.84] (all statistical decisionsreported in this paper were made using a rejection crite­rion ofp < .05). The bottom panel of Figure 1 shows thenumber of target locations revisited (one or more times)during each of the two trial blocks. The bees revisitedfewer locations than estimated by the simulations duringboth Block 1 [t(13) = 2.68] and Block 2 [t(9) = 2.65].

Seven bees participated in the perceptual cue probetrials that followed the primary experimental trials. Foreach trial, the ordinal rank of the initial visit to each ofthe six locations was determined (the first location vis­ited was given a rank of 1, the second a rank of 2, etc.).The mean ordinal ranks of the locations baited withsugar solution and water were determined for each bee.The mean (over bees) ordinal ranks were 3.49 and 3.51for the locations baited with sugar and water, respec­tively [t(6) = .08].

DiscussionThese results confirm the finding ofBrown and Demas

(1994) that bees avoid revisits to locations that have re­cently been depleted of sucrose solution. Because the es­timates of chance to which choice behavior is comparedcontrol for systematicity in the location-to-location tran­sition probabilities, this tendency cannot be attributed toconsistent movement patterns or biases in the bees' be­havior. The avoidance of revisits cannot be explained byan ability to discriminate the presence of sugar solution,as shown by the results of the perceptual cue probe tri­als. Rather, information about the identity of previouslyvisited locations has some control over choices. The ex­perimental and analytic techniques used in the present ex­periment rule out the possibility that this control is basedon a physical trace left by the bee during previous visits,such as an odor trail. Thus, this tendency is best attributedto memories of previous visits. Because the identity ofpreviously visited locations changes both within each trialand from one trial to the next, the memory involved is bestcharacterized as a form of working memory, analogous

WORKING MEMORY IN BEES 477

EXPERIMENT 2

in function to that known to be involved in vertebrate de­layed discrimination performance (see, e.g., Honig, 1978).

Figure 1. Mean values for two measures of choice accuracy ofthe bees and Monte Carlo simulations during the first six choicesof trials in Experiment 1. The measures are the number of cor­rect choices made (top panel) and the number oftarget locationsthat were revisited one or more times (bottom panel). Data areshown separately for the two blocks of trials.

are conditions that support moderate to high levels ofworking memory performance. In addition, determinationof the level ofworking memory performance that can besupported by the bee nervous system may be critical forfurthering a comparative analysis of working memory.

The present experiment examined performance in aprocedure very similar to that ofBrown and Demas (1994;Experiment I) and to that of the present Experiment I. Inthree subexperiments, three different apparatuses wereused that we thought might produce relatively higher lev­els of working memory performance. The first of these(Experiment 2A) was based on the possibility that beeswould discriminate among six baited locations more ef­fectively ifthe locations were separated in space more thanthey have been in our earlier experiments. Thus, in thisexperiment the baited locations were separated by 35 emrather than by 10-15 em, as has been the case in our ear­lier experiments. Ifa limitation on working memory per­formance was produced in the earlier experiments by thedifficulty of discriminating among locations because oftheir proximity, then higher levels of working memoryability might be found under these conditions.

Experiment 2B was motivated by our knowledge that,in rats, spatial working memory performance is increasedwhen a relatively large amount oftime or effort is requiredto visit each baited location (Brown & Huggins, 1993;Brown & Lesniak-Karpiak, 1993). This was implementedin bees by requiring that the bees crawl through a tube toreach each of six baited locations.

Experiment 2C involved a radial-arm maze (Olton &Samuelson, 1976) with six arms. The radial-arm maze hasbeen shown to support very high levels of spatial workingmemory performance in a variety ofvertebrate species, in­cluding rats (see, e.g., Olton & Samuelson, 1976), pigeons(Roberts & Van Veldhuizen, 1985), and corvids (Kamil,Balda, & Olson, 1994). Although the apparatuses used inour earlier experiments were inspired by the radial-armmaze and preserved some of its properties, it is possiblethat other properties ofthe radial-arm maze are critical forits ability to support high levels of performance. In par­ticular, animals choose locations from a central locationin the radial-arm maze, and at least one theory of spatialchoice implies that this common choice point is impor­tant (Brown, 1993). Thus, in Experiment 2C bees visitedsix locations at the ends ofarms ofa radial-arm maze. Themaze arms and central arena were enclosed with mesh.

MethodSubjects. The subjects in these experiments were from the same

hive as those used in Experiment I. They were netted as they left thehive and taken in plastic vials to the laboratory for the recruitmentprocedure (see below). Twenty,20, and 16 subjects participated inExperiments 2A, 28, and 2C, respectively.

Apparatus. The apparatuses used in these experiments are shownin Figure 2. The apparatus used in Experiment 2A (Figure 2, toppanel) consisted of a 94 X 94 ern piece of plywood, painted violet.Six white plastic drinking cups (Solo Co. No. P3A, Urbana, IL)were located as shown. These cups were 5.7 em tall, 3.7 em in di­ameter at the bottom, and 5.3 em in diameter at the top. A red ad­hesive dot (I em in diameter) in the center of the bottom of each cupwas covered by a small piece of clear tape. Drops of sucrose solu-

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The magnitude ofthe effects found in Experiment I andin our earlier experiments (Brown & Demas, 1994; Demas& Brown, 1995) appears rather small and suggests thatthe extent of spatial working memory ability in bees iscorrespondingly small. However, we cannot know whetherthe low level of working memory performance reflects afundamental limitation in the working memory ability ofbees or our failure to allow bees to express the full extentof their working memory ability. The failure of Burmeis­ter et al. (1995) to find any tendency to avoid revisits isconsistent with the possibility that bees possess limitedworking memory abilities.

Both from a theoretical and practical perspective, it isimportant to study conditions that might support higherlevels ofworking memory performance in bees. Examina­tion of the properties of this memory system will requirethat manipulations be used that modulate the level ofwork­ing memory performance. This can be done only if there

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478 BROWN, MOORE, BROWN, AND LANGHELD

Figure 2. Apparatuses used in Experiments 2A (top panel), 28 (middle panel),and 2C (bottom panel). Ruler included to provide seale is 30.4 em long.

tion or water were placed on top of this dot (see Procedure). Cupswere separated from their nearest neighbors by 35 em (center tocenter). The apparatus surface included marks indicating cup loca­tions so that cups could be easily removed and replaced. A set ofchildren's wooden blocks, m a variety of shapes and colored red,blue, or yellow, were placed on the apparatus in the pattern shown inFigure 2. These blocks were intended to serve as spatial landmarks.The apparatus was adjacent to, and at the same height as, the ledgeof the window through which bees gained access to the laboratory.

The apparatus used in Experiment 2B (Figure 2, middle panel)consisted of a 42.5 X 29 ern piece of plywood, painted violet. A2 X 3 matnx of chambers was mounted on the plywood surface,with each chamber separated from Its nearest neighbors by 18 ern(center to center). The chambers were constructed of white PVCplumbing fittings and tubing. A V2-in. threaded fittmg formed thebase of each chamber, into which a 6-cm length of of V2-in. tubingcould be placed. The threaded portion of each fitting was insertedthrough a hole drilled in the apparatus surface. The resulting cham­ber had an interior diameter of 1.4 em and was 9.5 em in verticallength. A bee could enter the chamber at the top and crawl throughthe tube and fitting, which terminated on the surface of one thesame plastic dishes used in Experiment I, six of which were hiddenunderneath the apparatus (one at the terminus of each chamber).The entire apparatus could be easily lifted to allow these dishes andtheir contents to be manipulated (see Procedure). In addition, thetubing lengths forming the top portion ofthe chamber could be eas­ily removed and replaced (see Procedure). SIX children's blocks ofvarious shapes and colors were mounted on the apparatus, asshown. The apparatus was placed on a wmdow sill.

The apparatus used ill Expenment 2C was a radial-arm maze(Olton & Samuelson, 1976) with SIX arms (Figure 2, bottom panel).Each arm was constructed of a wooden frame covered with nylonmesh, and was 39 cm in length, 17 em wide, and 22 cm tall. Thearms were attached to a wooden floor, which was painted white.The central ends of the arms formed a six-sided arena, which was31 ern across. A lid constructed ofa wooden frame and nylon meshcould be used to enclose this central arena. Centered 5 ern from theend of each maze arm was a hole, into which was inserted a %-in.threaded PVC plumbing fitting. These fittings were painted blueand had an interior diameter of 2.5 cm at the top and 1.8 em at thebottom. Bees could crawl through these fittings and onto the sur­face of a plastic dish, mounted below the maze surface at the ter­rrunus of the fitting. These plastic dishes could be easily removedand replaced (see Procedure). The maze was mounted on a tableusing a lazy Susan, allowing it to be easily rotated. The maze wasadjacent to a laboratory window, Its surface was at approximatelythe height of the window sill.

Procedure. The procedures used in these three experiments werethe same except for the exceptions noted. These experiments wereconducted simultaneously by three different experimenters. Eachexpenment was associated with one window ofthe building, throughwhich bees flew to shuttle between the hive and the apparatus andnear which the apparatus was located. These windows were locatedthree floors below the hive, on different sides of the building.

The bee was brought to the laboratory m a small plastic vial. Thevial was opened, turned upside down, and placed over a large dropof sucrose solution. This drop was located either in one of the cupsservmg as a baited location in the apparatus used m Expenment 2A,in one of the SIX chambers of the apparatus used in Experiment 2B(With the tube removed), or on a plastic dish in the central arena ofthe radial-arm maze (Experiment 2C). Typically, several bees wereexposed to this recruitment procedure simultaneously. Some of thebees so treated began Imbibing the sucrose solution, at which pointthe vial was removed. The bee was allowed to feed to repletion,after which It flew out the window. Some of these bees later re­turned to the apparatus, at which time sucrose drops were available

WORKING MEMORY IN BEES 479

in the same or a similar location. Once a bee returned of ItSown ac­cord, it was marked with a spot of paint on the thorax or abdomen(different colors being used to code bees in the three expenments).After a marked bee returned to the apparatus two or three times, itwas considered a subject in the experiment, and the procedure de­scribed below was invoked.

Prior to each trial, each ofthe six locations was baited with a small(approximately 4- to 5-,u1) drop of50% sucrose solution using a sy­ringe. When the bee flew in the window, the camcorder was acti­vated to record behavior. The experimenter recorded the sequenceof locations chosen until the trial was completed. A trial was de­fined as completed when (I) all six locations had been visited atleast once, or (2) 5 min passed without a choice. A choice was de­fined when a bee touched the nm or inside of a cup correspondingto a location (Experiment 2A), the rim or inside of a PVC tube cor­responding to a chamber (Experiment 2B), or the PVC connector atthe end ofa maze arm (Expenment 2C). In Experiment 2C, the cen­tral arena of the maze was covered with the lid during each trial.

After all SIX locations had been visited at least once, the expen­menter provided a large drop ofsucrose solution to allow the bee tofeed to repletion. In Experiment 2A, this large drop was placed ina cup identical to those used on the apparatus, but located on thewindow sill between the apparatus and the window. In Expen­ment 2B, the drop was in a plastic dish held adjacent to the top ofthe tube corresponding to the last choice made by the bee. The beetypically crawled onto the dish after emerging from the tube. In Ex­periment 2C, the drop was III a dish placed in the central arena ofthe maze while the bee visited the last maze arm. In all three ex­penments, bees virtually always imbibed from this repletion dropand then flew out the window after completing a trial.

A procedure was used to eliminate any ability of bees to use anodor trail or "footprint pheromone" (Free & Williams, 1983; Giurfa& Nunez, 1992) as a discriminative cue m Expenments 2A and 2B.This procedure corresponded to that used in Experiment I. Duringthe VISit following the initial visit to a particular location, the cup(Experiment 2A) or tube (i.e., the tube forming the top portion ofeach chamber in Experiment 2B) corresponding to that locationwas replaced with an identical cup or tube. In Experiment 2A, thisreplacement cup contained a drop of tap water (because the con­tents of the cup were potentially visible while choices were made inthat experiment). As in Experiment 1, we did not attempt to replacereplacement cups (or tubes) following third or subsequent visits toa location because the speed with which bees moved from locationto location toward the end of trials (when most locations had beendepleted of food) did not allow us to reliably perform the manipu­lation while the bee remained in a cup (or tube). Thus, any tendencyto avoid a first revisit (i.e., a second visit) cannot be accounted forby an odor cue. However,once a bee revisited a location, an odor mayhave been present at that location.

Each bee was exposed to a series ofsuch trials until 30 trials hadbeen completed.

ResultsThe results were evaluated in terms of two measures

ofperformance. The first was the number oflocations re­visited (one or more times) during the first six choices.This measure, developed in the context of Experiment 1,isolated performance which could be affected by odortraces possibly left after bees made second (or later) vis­its to locations in Experiments 2A and 28. The secondmeasure was the total number ofchoices required to visitall six locations. This measure was not used in Experi­ment I because bees in that experiment often did not visitall six locations. In the present experiment, all trials were

480 BROWN, MOORE, BROWN, AND LANGHELD

tection ofchanges in performance over the course of tri­als, the data were considered to be structured into threeblocks of 10 trials each.

As in our earlier experiments and in Experiment 1,empirical data were compared with an estimate of thechoice accuracy expected on the basis on the location-to­location transition probabilities of each bee during eachtrial block. Each of these estimates was based on 1,000iterations of a Monte Carlo simulation of bee choicesbased on the transition probabilities of individual bees.The transition probabilities in these experiments weresimilar to those found in Experiment 1 in that most beesappeared to show a moderate tendency to choose loca­tions that were spatially proximal to the previously cho­sen location. It should also be noted that, as in all of theexperiments reported in this paper, bees moved from lo­cation to location by flying rather than walking. As in Ex­periment 1, immediate returns to locations were not in­cluded in the transition probabilities or counted in theempirical data because of the inherent ambiguity indetermining whether these behaviors correspond to sep­arate choices and because of the replacement ofcups andtubes following the initial visit to each location (beescould not visit locations while we were in the process ofreplacing the cups or tubes in those locations). The esti­mates were compared to empirical performance using awithin-subjects analysis of variance (ANOVA), whichalso included trial block as a factor.

The bees in Experiment 2A revisited fewer locationsduring the first six choices than indicated by the corre­sponding simulations [F(1,19) = 39.33]. The magnitudeof this difference did not significantly change over thecourse of trial blocks [F(2,38) = 2.98, p = .06]. Therewas no evidence that bees made more correct choicesduring the first six choices than did the Monte Carlo sim­ulations in Experiment 2B [F(1, 19) < 1] or in Experi­ment 2C [F(1,15) = 2.89]. In terms of the number ofchoices required to complete each trial, the bees outper­formed the Monte Carlo simulations in all three experi­ments [Experiment2A, F(1,19) = 303.17; Experiment 2B,F(1,19) = 34.86; Experiment 2C, F(l,15) = 26.73]. Themagnitude of the difference between empirical and sim­ulated performance differed across trial blocks in Experi­ment 2A [F(2,38) = 4.53]. There was no evidence forthis interaction in Experiment 2B [F(2,38) < 1] or in Ex­periment 2C [F(2,30) = 2.97].

DiscussionBees in all three of these experiments revisited loca­

tions at which they had already depleted the sucrose so­lution less often than expected. This ability to avoid re­visits cannot be attributed to systematic patterns or biasesinfluencing the choice of location or the transitions be­tween locations because of our technique of comparingperformance with Monte Carlo simulations incorporat­ing the location-to-location transition probabilities. Fur­thermore, this ability cannot be attributed to visual cuesdistinguishing baited from unbaited locations becausesuch cues were not available in any ofthese experiments.

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completed by the bee visiting all six locations. An advan­tage ofthis measure is that it takes into account all choicesmade by the bees and therefore might be more sensitiveto any tendency to avoid revisits. However, it is open topossible effects of odors or other physical traces that mightbe left by bees following the second visit to a particularlocation. Figure3 shows the results of all three experimentsin terms of both the mean number of locations revisitedduring the first six choices and the mean number of vis­its required to visit all six locations. In order to allow de-

Figure 3. Mean levels of choice accuracy in Experiments 2A,2B, and 2C. Panels on the left show the mean number of loca­tions that were revisited one or more times during the first sixchoices. Panels on the right show the mean number of visits re­quired to visit all six locations. Data are shown in three blocks of10 trials each. Two estimates of chance performance are shown.Estimates of chance performance based on the Monte Carlosimulations using the empirical transition probabilities (M) aredescribed in the text and were used in the statistical evaluationsof choice accuracy. The strict estimates (8) are based on the as­sumption that bees chose randomly from among the six (firstchoice) or five (subsequent choices) available locations. Theywere determined in the same manner as the M estimates, exceptthat the simulation was not constrained by the empirical transi­tion probabilities (instead, it chose randomly from among the al­ternatives). The strict estimate is considered to be a less accurateestimate of chance and is shown for illustrative purposes only.

A physical trace left by the bee, such as odor, couldhave contributed to this ability in Experiment 2C. In Ex­periments 2A and 2B, however, our technique of replac­ing the cup or tube corresponding to a location follow­ing the first visit rules out such cues as an explanationfor bees' ability to avoid the first revisit to a location. Thisability is isolated by one of our performance measures (thenumber of locations revisited during the first six choices).Thus, in Experiment 2A, bees clearly demonstrated anability to avoid revisits to locations, which cannot be ex­plained in terms of physical traces left by the bee. Suchtraces remain possible as explanations for performancein Experiments 2B and 2C, although we have no evidencethat they were employed by bees in those experiments.

The present results replicate those of Brown and De­mas (1994; Demas & Brown, 1995) and those of Exper­iment I under a wider variety of experimental condi­tions. The results therefore provide additional supportfor our earlier conclusion that honeybees possess spatialworking memory. This working memory allows loca­tions visited earlier in the trial to be discriminated fromlocations not yet visited.

However, in the variety of conditions included in thepresent experiment, the level ofworking memory abilityexhibited by the bees remained limited. Although it is dif­ficult to judge what constitutes sizable levels ofworkingmemory performance, by any account, the extent of suchperformance demonstrated by bees so far is small. Forexample, rats are typically found to visit approximately7.5 novel locations during the first eight choices in a ra­dial-arm maze and typically make fewer than two errorsin completing an eight-arm maze. In contrast, bees in thepresent experiment made between four and eight errorsin visiting six locations.

EXPERIMENT 3

Clearly, there are a number of unexamined features ofour apparatus or procedure that might limit the extent ofworking memory ability demonstrated by bees. One ap­proach to understanding the small size of the workingmemory effects we have obtained so far is to examinesome potential constraints on working memory perfor­mance. One possibility is that there is a restriction on theability ofhoneybees to discriminate among the locationsinvolved in the experiments we have conducted.

The present experiment examined this possibility usinga task that required the bees to discriminate among loca­tions identical to those used in Experiments 2A and 2B.Specifically, for each bee there were two locations thatwere never baited, with the remaining four locationsbaited prior to each trial as in the earlier experiments.Bees were thus encouraged to avoid any visits to thesetwo never-baited locations, and their ability to do so wasmeasured. This task can be conceptualized as a reference­memory task (Honig, 1978) in that the identity of thesenever-baited locations remains constant throughout theexperience of the bee. It is also very similar to a number

WORKING MEMORY IN BEES 481

ofdiscrimination learning tasks that have been used withbees by a number of researchers.

Bees have demonstrated high levels ofvisual and spa­tial discrimination ability in both field and laboratorystudies (see, e.g., Cartwright & Collett, 1982; Dyer, 1994;Gould, 1987; Huber, Couvillon, & Bitterman, 1994; Men­zel, Erber, & Masuhr, 1974; Zhang, Bartsch, & Sriniva­san, 1996). The present experiment was intended to re­veal whether similarly high levels of discriminationability could be found in the context of the locations usedin our working memory experiments. If the level ofwork­ing memory ability found in our experiments is limitedby a poor ability of bees to discriminate among the sixlocations used in those experiments, then bees should beable to learn to avoid visits to the never-baited locationonly at low levels of accuracy. On the other hand, if thelow levels of working memory ability we have reportedare due to limitations of working memory itself, thenhigh levels of discrimination ability are possible.

MethodSubjects. The subjects m this experiment were from the same

source and were transported to the laboratory in the same manneras in Experiments 1 and 2. Twelve and 11 subjects parlicipated inExperiments 3A and 3B, respectively.

Apparatus. The apparatuses used in these experiments were thesame as those used in Experiment 2; that used m Experiment 3Awas the same as that used in Experiment 2A, and that used in Ex­periment 3B was the same as that used in Expenment 2B.

Procedure. The procedures used in these experiments were thesame as those used in the corresponding subexperiment of Experi­ment 2, with the exceptions noted below. Prior to each trial of theexperiment, four ofthe six cups (Experiment 3A) or chambers (Ex­periment 3B) were baited. The two never-baited locations were de­termmed randomly for each bee with the restriction that the iden­tity of never-baited locations was distributed as equally as possibleacross the six locations over bees (each location was assigned as anever-baited location for 4 bees in Expenment 3A and for either 3or 4 bees in Experiment 3B). The never-baited locations were deter­mmed prior to the first trial for each bee, and they remained constantover the course ofthe experiment. A trial was considered completewhen a bee had visited all four baited locations. In Expenment 3A,a cup in a never-baited location contained a drop of tap water, andcups were replaced with a fresh cup containing a drop of tap waterfollowing the first VISit to the cup. In Expenment 3B, the tube form­ing the top portion ofa chamber was replaced with a fresh tube fol­lowing the first visit to the chamber. Each bee completed 30 trials.

ResultsTo assess the acquisition of any ability to avoid visits

to the never-baited locations, data from the 30 trials ofeach bee were considered to be structured into six blocksoffive trials each. To isolate the ability of bees to avoidvisits to never-baited locations from the working mem­ory ability used to avoid revisits to locations, only initialvisits to locations made during the first four choices ofeach trial were evaluated. Figure 4 shows the mean per­centage ofthese choices that occurred to the never-baitedlocations in Experiment 3A (top panel) and Experiment 3B(bottom panel). Also shown is the a priori estimate (33.3%)of visits directed toward never-baited locations, assum-

482 BROWN, MOORE, BROWN, AND LANGHELD

Blocks of 5 Trials

Experiment 3A

Figure 4. The mean percentage of choices that were to the un­baited locations and the percentage expected on the basis ofchance in Experiments 3A and 38.

tions were proximal to landmarks that differed in shapefrom the training landmark. Depending on the nature ofthe shape difference, bees chose the correct location with60%-90% accuracy (with 33% being expected by chance).Similarly, Gould found that bees learned to choose onelocation among three on the basis ofthe color oflandmarkswith a choice accuracy of approximately 90% (again,chance is 33%). Similarly, Couvillon, Bitterman, andtheir colleagues have consistently found that bees can learnto choose nearly perfectly between two targets on the basisof color or odor within 10 trials (see Couvillon & Bitter­man, 1991, for a review). The work ofthese and other re­searchers clearly indicates that honeybees are capable ofmuch higher levels of choice accuracy than were foundin the present experiment.

Furthermore, the details of some of these previous ex­periments suggest that the specific discriminative cuesprovided in the present experiments should have been ade­quate to support higher levels of discrimination abilitythan was found. In Gould's (1987) experiments, for exam­ple, locations were arranged in a linear pattern, separatedby 30 em. This spatial separation is similar to that of Ex­periments 2A and 3A, and the shapes and colors we usedare also similar to Gould's. Huber et al. (1994) found highlevels of discrimination accuracy when a baited targetand an unbaited target were separated by 40 em (with orwithout the provision ofa proximal landmark) or by 10em(only when a proximal landmark was provided). Thus, itseems unlikely that discrimination ability in the experi­ment was limited by the nature of the spatial cues or spa­tial separations among the target locations.

One possible explanation for the relatively low levelsof discrimination ability involves the fact that multiplebaited targets (four) were available in the present exper­iments, and they had to be discriminated from multipleunbaited targets (two). Experiments in which bees havedemonstrated high levels ofdiscrimination ability have in­volved a single target location or stimulus, which is dis­criminated from one or more distractor locations or stim­uli. It is possible that the existence of multiple targetslimits the discrimination performance of bees. A secondpossible explanation is suggested by the fact that "baited"targets in the present experiment provided reinforcementonly during the first visit, and were therefore similar tothe unbaited locations (in that they did not contain food)during any subsequent visits. This partial reinforcementofpositive targets may have resulted in lowered levels ofdiscrimination performance.

Wecannot determine the explanation for the limited ex­tent ofdiscrimination ability exhibited by our bees. How­ever, its existence can help explain the low levels ofwork­ing memory performance found in Experiments 1 and 2.Specifically, working memory performance would beexpected to be constrained by any limitation in the abil­ity of subjects to discriminate among the items coded inworking memory. The low levels ofworking memory abil­ity found in our earlier experiments could be explainedby a limited ability to discriminate among the six loca­tions. The results of the present experiments, which did

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ing that bees are insensitive to the identity of never­baited locations.

There was no evidence that the percentage of choicesdirected to never-baited locations changed over the courseof trial blocks in Experiment 3A [F(5,55) < 1] or Exper­iment 3B [F(5,50) < 1].The percentage ofchoices directedto never-baited locations, collapsed over trial blocks, wasreliably less than 33.3% in Experiment 3A [t(11) = 1.8]and in Experiment 3B [t(10) = 2.5].

DiscussionAlthough bees in these two experiments did demon­

strate a reliable tendency to avoid visits to never-baited lo­cations, the magnitude of this tendency was quite smalland did not increase over the course of the experiment.This is surprising given bees' ability to learn to visit abaited target rather than an unbaited target using a vari­ety ofcues, including spatial location in relation to prox­imal or distal landmarks (Cartwright & Collett, 1982;Huber et aI., 1994), color (Menzel et aI., 1974), and shape(Gould, 1987), all ofwhich were available as discrimina­tive cues in the present experiments.

For example, Gould (1987) provided bees with 10 train­ing visits in which food was available between two iden­tical landmarks and then tested them using a three­alternative choice test in which the correct location wasbetween the training landmarks, and the incorrect loca-

not involve working memory but did involve discrimina­tion among the locations, are entirely consistent with thispossibility.

GENERAL DISCUSSION

The results of four experiments (1, 2A, 2B, and 2C)replicate our earlier findings (Brown & Demas, 1994;Demas & Brown, 1995) that bees avoid revisits to loca­tions that have been depleted ofsucrose solution. Inthreecases (Experiments 1, 2A, and 2B), an analysis that ex­amines a subset of the bees' choices provides a means ofruling out odor or other subject-generated cues as an ex­planation for this ability. This is important given thatodor left by bees on visited locations has been implicatedas providing a cue in both natural (Free & Williams, 1983)and laboratory (Giurfa & Nunez, 1992) foraging situa­tions. Intwo ofthese three cases (Experiments 1 and 2A),clear evidence for an ability to avoid revisits that cannotbe explained by such odor trails was found. Because ourestimate ofchance accounts for location-to-location move­ment biases of individual bees, the tendencies to avoidrevisiting locations reported in this paper cannot be ex­plained in terms of systematicity or stereotypy in themovement of the bees among locations. Furthermore, ourexperimental procedures are not open to the criticisms ofour earlier experiments offered by Burmeister et al. (1995).Thus, these data demonstrate convincingly that bees avoidrevisits to locations on the basis ofmemories of the iden­tity of locations visited earlier in the trial.

A word is in order regarding the Monte Carlo simula­tions used to estimate the level of choice accuracy ex­pected on the basis ofchance in Experiments 1 and 2 andin some of our previous experiments (Brown & Demas,1994, Experiment 1; Demas & Brown, 1995). In eachcase, the location-to-Iocation transition probabilities ofindividual bees (e.g., those represented by the means inTable 1) determine the choices of the simulation. Criticshave suggested that a better estimate of chance perfor­mance would include transition probabilities condition­alized not just on the immediately preceding choice, buton choices made earlier in the choice sequence. For ex­ample, bees might have a tendency to move in a straightline across rows of baited locations, and our estimate ofchance would not fully represent this tendency. By defi­nition, however, such a tendency requires that the bee'snext choice be contingent on a previous choice. The iden­tity of that previous choice (or something correspondingto it) is therefore a memory. The first-order transitionprobabilities are included in our estimate of chance be­cause the location ofchoice n-1 serves as a stimulus thatcould very well determine the direction offlight and there­fore the identity of choice n. But any pattern of choicesin which the next location chosen depends on choicesmade in the past (and therefore not currently perceived)meets any reasonable criterion for the involvement ofmemory. It should also be mentioned that Brown andDemas performed two experiments that did not involve theuse ofa Monte Carlo simulation in the estimation ofchance

WORKING MEMORY IN BEES 483

performance (Brown & Demas, 1994, Experiments 2 and3). The results of these experiments supported the con­clusions on the basis ofthe Monte Carlo simulations andtherefore provide independent, converging evidence forthe involvement of memory in honeybee spatial choice.

Inorder to allow previously visited locations to be dis­criminated from those not yet visited, the contents of thismemory must change during the course ofeach trial andbetween trials. Thus, this memory appears to meet the de­fining criteria that are used to identify the working mem­ory ability that has been studied in vertebrates (Badde­1ey,1986; Honig, 1978; Sherry & Schacter, 1987). Demasand Brown (1995) showed that the contents of this mem­ory can be used in a flexible manner, allowing bees to ei­ther avoid additional visits (win-shift, as in the presentexperiments) or selectively revisit locations (win-stay).Inthe latter condition ofDemas and Brown's experiment,bees found sucrose solution only in those locations thatthey had previously visited, and the bees were found toselectively visit those locations. Thus, the memory sys­tem inferred on the basis of our results cannot be under­stood in terms of associative learning, with associativestrength corresponding to each location and being mod­ified by the outcome of visits (see, e.g., Greggers & Men­zel, 1993). Such a view predicts that future responding toa location be determined by the outcome ofprevious vis­its to that location. But, in Demas and Brown's experiment,a reinforced visit to a location could result in either an in­crease in the tendency to visit that location (win-stay) ora decrease in that tendency (win-stay). These considera­tions lend further support to the interpretation of theseresults in terms ofa working memory system, the contentsofwhich are used by other systems to determine behavior.

The working memory ability revealed in our experi­ments mayor may not be related to the bee short-termmemory elegantly demonstrated by the work of Menzel(1979, 1983, 1990). He and his colleagues have providedevidence for at least two different physiological pro­cesses involved in the formation and consolidation ofmemories in honeybees. These processes have differenttime courses, with short-term memory having a durationofapproximately 30 sec. The function of this short-termmemory system as described by Menzel is to provide in­formation to a longer term memory that corresponds toeach stimulus and controls responding to that stimulus.Working memory, on the other hand, is conceived of asholding a limited amount of information being used inthe context of ongoing cognitive processes or behaviors(see, e.g., Honig, 1978). In the case of spatial workingmemory, this information is a set of locations, such asthe locations that have been previously visited within a par­ticular trial of Experiment I or 2 (see also Olton, 1978).Because the identity of visited locations changes bothwithin each trial and from one trial to the next, the con­tent of working memory must change in a dynamic andflexible manner.

Although the present data convincingly demonstratethe existence ofworking memory in bees, they are equallyclear that the extent of the working memory performance

484 BROWN, MOORE, BROWN, AND LANGHELD

exhibited in our experiments is limited.' Experiment 2was intended to provide a range of experimental condi­tions that might support higher levels of spatial workingmemory performance than we had previously found.Bees demonstrated an ability to avoid revisits under thisrange of conditions, but the extent of that ability wasconsistently low relative to the levels of spatial workingmemory performance that has been demonstrated by ratsand a variety of other vertebrates.

It may be that the relatively low levels of spatial work­ing memory performance found in these experiments isa reflection of relatively impoverished spatial workingmemory ability in bees. However, the results of Experi­ment 3 suggest a different interpretation. For reasons thatcannot be determined on the basis of the present results,honeybees appear to have difficulty discriminating amongthe locations used in the present experiments. Thus, anaccurate determination of the extent of spatial workingmemory in bees will require additional work in whichbees choose from among locations (or other response al­ternatives) that can be shown to support high levels ofdiscrimination ability.

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NOTE

I In mterpretmg the level of performance exhibited by the bees, Itshould be kept in mind that cases in which bees left a location and thenImmediately returned to the same location (with no Visitsto other loca­tions intervening) were not considered in the analysis. This is becauseit is not clear what constitutes a unit of choice m this circumstance andalso because of the possibility that switching dishes, cups, and tubes (mExpenments I, 2A, and 2B, respectively) could affect the tendency toImmediately return to the Just-chosen locanon. Such immediate reVIS­its were not included in the empincal results, and so the transinon ma­trices used by the Simulations never produced such Immediate revisitsin the estimates ofexpected performance. However,ifworkmg memorycontributes to a tendency to avoid immediate returns to locations, thenour measures underestimate the levels of working memory perfor­mance. Despite this consideration, however,the extent of workmg mem­ory performance found in these expenments is quite small.

(Manuscript received January 13, 1997,reVISIOn accepted for publication May 14, 1997.)