Journal of Insect Behavior, Vol. 11, No. 2, 1998

The Significance of Odor Cues and Dance LanguageInformation for the Food Search Behavior ofHoneybees (Hymenoptera: Apidae)

Wolfgang H. Kirchner1,2 and Andreas Grasser1

Accepted September 17, 1997; revised September 29, 1997

Although several independent lines of evidence show that bees can make use ofinformation provided by their dance language, there is an ongoing controversyabout the significance of the dance information versus odor cues in the searchbehavior of recruited bees. A series of experiments was performed to assess therelative significance of dance information and odors for the site-specific searchof recruit bees. In these experiments recruit bees were trapped automatically atarrays of artificial flowers at various distances from the hive. The distributionof directions in which the recruits searched for food was compared betweenrecruitment by dancers performing well-oriented dances on the vertical side ofthe comb and dancers performing disoriented dances on a horizontal comb. Theresults show quantitatively that bees use both odor cues and the dance infor-mation. The greater the distance to the feeding site, the greater is the relativesignificance of the dance information.

KEY WORDS: communication; dance language; recruitment; olfaction; Apis mellifera.

INTRODUCTION

On returning from profitable food sources honeybee foragers perform dances bywhich nestmates are recruited. The dances contain correlates of distance anddirection to the food source and most scientists agree that there is strong exper-imental evidence in favor of the hypothesis that this information can be used by

1Universitat Konstanz, Fakultat fur Biologic, D-78457 Konstanz, Germany.2 To whom correspondence should be addressed at Universitat Konstanz, Fakultat fur Biologie,Postfach 5560 M657, D-78457 Konstanz, Germany. Fax: 49-7531-88-3059. e-mail: Wolf-gang.Kirchner@uni-konstanz.de.

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0892-7553/98/0300-0169$15.00/0 © 1998 Plenum Publishing Corporation

the nestmates to localize the food source (see von Frisch, 1967; Seeley, 1985;Gould and Gould, 1988; Kirchner and Towne, 1994). However, experimentsperformed by Wenner and co-workers in the 1960s (Wenner and Johnson, 1967;Wenner et al., 1969) led these authors to conclude that the recruits are guidedexclusively by olfactory cues and never make use of the dance information. Thecontroversy about the significance of the dance communication seemed to besettled when Gould (1976) succeeded in sending recruits into directions differentfrom the direction in which the foragers were collecting food by means ofexperimentally misdirected dances. The dance language hypothesis was furthersupported by experiments in which a computer-driven artificial dancer made thenestmates search for food preferentially in the direction indicated by the dances(Michelsen et al., 1989, 1992). However, the controversy was recently reani-mated by Wenner and Wells (1990) and Wenner et al. (1991). Part of theircriticism seems to be based on a misunderstanding of the dance language hypoth-esis, which seems to be, in the view of Wenner and co-workers, an exclusivealternative to the hypothesis of olfactory food search by recruit bees. Wennerand co-workers seem to ignore the most obvious resolution of the controversy,i.e., that searching bees can make use of both dance information and odor cues.However, the crucial experiments performed so far to test the dance languagehypothesis focused on proving or disproving the existence of a dance language,and none of them tried to assess quantitatively the relative significance of dancelanguage information and odor cues on the search behavior of recruits.

We therefore performed a series of experiments which not only was designedto test the hypothesis of information transfer through the dance language in away which tries to take Wenner and Wells' (1990) criticism of previous crucialexperiments into account, but also allows one to assess the relative significanceof odor cues and dance information and changes in the relative significance ofthese cues with changing distance to the food source.

The basic design of the experiments is similar to one single experimentperformed by von Frisch in the 1940s (described by von Frisch, 1967). Thebasic idea is that the dances normally occur in the darkness of the nest and areoriented relative to gravity. Dances on a horizontal comb are therefore dis-oriented in the dark. Comparison of the spatial distribution of recruits searchingfor food advertised by dances on a vertical versus a horizontal comb thereforeallows assessment of the effect of the dance information.

MATERIALS AND METHODS

A queenright colony of approximately 20,000 honey bees, Apis melliferacarnica, was housed in a six-frame observation hive. Incoming bees were allowedto go only to one side of the combs. The observation hive was placed either inthe normal upright orientation or tilted on its side so that the combs were hor-

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izontal. The exact horizontal orientation of the comb next to the nest entrance(on which dances were performed) was carefully adjusted, and care was takento ensure that no daylight could influence the dances on the horizontal comb.

The dances of foragers were videotaped using an IR-sensitive videocameraand a red darkroom light bulb for illumination. Dance directions were measuredusing a protractor that was mounted on the video screen and analyzed usingcircular statistical techniques (Batschelet, 1981). The experiments were per-formed from July to October, when almost no natural food sources were avail-able for the bees. Groups of 12-15 individually marked bees were trained tovisit a feeding station providing 2 M sucrose at a distance of 10, 100, 250, or750 m from the hive. To prevent any directional information being acquired bythe nestmates, the hive was in its horizontal orientation whenever foragers weretrained to the feeder. The compass direction of the food sources was variedbetween experiments at random. The feeder itself was a standard pneumaticfeeder (von Frisch, 1967) placed inside a wooden box (25 * 25 * 25 cm3). Thisbox was hidden in the ground, and the bees were trained to enter it through asmall (15-mm-diameter) entrance in the lid of the feeding box. This designensured that the feeder was not visually detectable and therefore could not attractthe attention of searching bees by any but olfactory cues.

To assess in which direction nestmates recruited by the foragers searchedfor food, an array of 12 baits was set up in the field, one of them 50 cm infront of the feeder and the others at the same distance from the hive as thefeeder but in different directions (30-330°, in steps of 30°). The design of thebaits was similar to that described by Towne and Gould (1988). They consistedof a flower-like funnel through which the bees entered a feeding chamber, wherethey could feed on a dilute sucrose solution (the concentration was varied betweenexperiments to make sure that the bees did feed on the solution but would notdance should they escape from the bait). The feeding chamber was opaque anddark. Light was visible exclusively through a small one-way door through whichthe bees, which tend to exhibit a strong positive phototactic response after fillingup at a feeding site, entered a second chamber in which they were trapped untilthe end of the experiment.

To attract bees of the experimental colony to the baits, a large group ofbees from this colony was trained to find 2 M sucrose in identical baits whichwere set up in the immediate vicinity (0 to 2-m distance) of the hive. Duringthis pretraining, which was carried out every morning for 30-60 min, the secondchamber of the baits was open. On most days there were 500-1000 bees foragingat the baits. In contrast to previous experiments using similar traps (Towne andGould, 1988), we did not use any scent to attract the bees to the bait and didnot add any scent to the feeding solution at the feeder. At the end of eachrecruitment experiment all recruits captured in the baits were counted and killed.The duration of the recruitment experiments was varied between single trials to

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ensure that the samples of recruited bees were large enough for statistical anal-ysis of the distributions but not so large that the foraging force of the colonywould be depleted.

In addition to the recruitment experiments, in which a group of foragersadvertised a food source, control experiments were performed in which the baitswere set up (for periods of time identical to the durations of the recruitmentexperiments performed using the same distances), but no bees foraged at thefeeding chamber. This would show if and to what extent bees arrive at the baitsindependently of recruitment to the feeder.

The distributions of recruits captured in the baits were analyzed by meansof standard circular statistics (Batschelet, 1981). During the recruitment exper-iments there were also bees attracted by the feeder itself. To make the feederas inconspicuous as possible during the experiments, the feeder was not attended,thus recruits to the feeder enlarged the foraging group there. Their number wascounted after the end of the experiments. The statistics on the directionality ofsearch behavior was calculated in two ways, excluding these bees and alsoincluding them in the group of bees captured in the bait in front of the feeder.

RESULTS

The directions indicated in the dances of the forager group were uniformlydistributed when the hive was laid horizontally (r = 0.06 in 352 wagging runsduring 35 dances of 13 bees, and r = 0.02 in 243 wagging runs during 26dances of 13 bees; P < 0.001). Thus, no directional information about thelocation of the food source was provided by the dances in the experiments usinga horizontal dance floor.

Three recruitment experiments were performed at each of four distancesfrom the hive to the feeder (10, 100, 250, and 750 m) in which the hive wasin an upright position and four experiments at each of these distances in whichthe hive was laid horizontally. The mean number of recruits captured in a singletrial was 52.5 ± 24.8 bees (N = 28 experiments). In four control experiments(one at each of the distances) in which the baits were set up but no foragersvisited the food source, 5.5 ± 4.9 bees (N = 4) were found in the baits. Thedifference is highly significant (P < 0.001). The angular distribution of thebees captured in the baits in these four control experiments was uniform. Theangular distribution (Fig. 1) of recruited bees captured in the traps during theexperiments in which a food source was advertised, however, was significantlyclumped in all cases (P < 0.001). At the shortest distance of 10 m, no significantdifference was found in the distribution of bees recruited by dances on a hori-zontal versus a vertical dance floor, indicating that the horizontal orientation ofthe hive has no disturbing effect on the directional search of the recruits and,also, that directional cues indicated in the dances are not used by the recruits at

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Food Search Behavior of Honeybees

Fig. 1. Distributions of bees recruited by forager bees collectingsucrose solution at four distances. Black bars—combs vertical,dances indicating the feeding site; shaded bars—combs horizon-tal, dances disoriented. The data are normalized with respect tothe direction of the food source and pooled with respect to thecompass direction of the feeding site (indicated as upward direc-tion) for all horizontal-comb (four trials) and vertical-comb (threetrials) experiments at each distance. The percentage of recruitsarriving in the direction toward the feeder is indicated.

this distance (see Discussion). At longer distances, the recruits searched signif-icantly more in the direction of the food source when the hive was verticallyoriented compared to when it was horizontally oriented (P < 0.001 at each ofthe three distances). The directionality of the recruits search behavior can becompared directly by means of the circular standard deviation, which is shownin Fig. 2. This measure of directional scatter shows that there is some direc-

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Fig. 2. Mean circular standard deviations of the recruits' search directions.Black bars—combs vertical, dances indicating the feeding site; shadedbars—combs horizontal, dances disoriented. The mean circular standarddeviation is significantly smaller than in a uniform distribution in all cases.It is significantly smaller when recruits followed well-oriented dancescompared to when recruits followed disoriented dances at distances fromthe feeding site of 100, 250, and 750 m.

tionality in all of the distributions, but that there is a distance-dependent effecton the searching behavior of bees when the hive is oriented vertically. Thefarther the food is from the hive, the more obvious is this difference in searchbehavior. A linear regression of the circular standard deviations on distance ishighly significant in the vertical hive experiments (N = 12 experiments, r =0.72; the slope of -64°/km is highly significant, P < 0.01). No distancedependence can be found in the accuracy of bees recruited by disoriented dances(N = 16, r = 0.1; slope of +3°/km, n.s.).

Some of the searching bees found the hidden food source during the recruit-ment experiments. These individuals are not included in the analyses presentedso far. However, adding these bees to the group of recruits captured in the baitin front of the hidden feeder does not change any of the results significantly. Italso does not change any level of significance indicated above.

The experimental data were also analyzed for possible effects of wind onthe distribution of the recruits. The distribution of recruits pooled with respectnot to the direction of the food, but to the direction of the wind, is uniform,indicating that the bees do not just fly upwind from the hive (N = 25 experimentsin which wind was blowing; circular standard deviation = 77°, n.s.). Thenumber of recruits collected in the baits is also not correlated with wind speed(N = 28, r = —0.01, n.s.). However, some correlation can be found betweenthe number of recruits arriving at the baits and the component of the speed ofwind from the feeder toward the hive (this component is 0 when the wind is

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blowing perpendicularly and negative when it is blowing toward the feeder).The correlation coefficient of r = 0.33 is weakly significant in a one-tailed test(N = 28; P = 0.04). A possible effect of wind is also visible in the small butnonsignificant preference of the recruits for the bait next to the feeder on thedownwind side over the corresponding bait on the upwind side (N = 25; P =0.08).

DISCUSSION

The data presented here allow a number of conclusions, some of whichconfirm previous studies of the search behavior of recruit bees and others thatgive new insight into the behavior of recruits.

The number of bees captured in the baits without dancing is small comparedto the numbers captured during the recruitment experiments. This finding notonly confirms that foraging bees do indeed somehow recruit their nestmates,but shows that the trapping technique can be used to monitor the searchingbehavior of a colony in the presence of other beehives in nearby places. Thehigh number of recruits captured during the recruitment experiments also showsthat it is possible to assess the spatial distribution of searching recruits withoutthe use of artificial scents at the recruit station. All previous studies involvingarrays of recruit stations, whether they counted visits of recruits or capturedthem in traps, used artificial scent oils at the recruit stations and thereby inter-fered with olfactory cues provided by the food source (von Frisch, 1967; Gould,1976; Towne and Gould, 1988; Michelsen et al., 1989, 1992; Waddington etal., 1996).

When the dances provided no directional information because the hive washorizontal, the distributions of recruits at the traps were not uniform. Recruitsexhibited a small but significant preference for the direction of the feeder. Thisshows that odor cues do indeed provide the recruits with directional information.However, the degree of directional orientation is small.

At a distance of 10 m from the hive to the feeder, the searching behaviorsof the recruits with and without directional information are the same. Thisindicates not only the directional information in the dances (which is significantat distances even shorter than 10 m (Kirchner et al., 1988)) is not being usedby the recruits at this short distance to the feeding site, but also, and moreimportant in the present context, that the horizontal orientation of the combobviously has no disturbing effect on the recruits.

At longer distances there is indeed a difference between the distributions,which becomes more and more pronounced with increasing distance. This indi-cates that the search behavior of the recruits is indeed guided by two sets ofcues, one of which is independent of both the distance to the feeding site andthe dance information and the other of which is provided only by oriented dances

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and becomes more and more significant with increasing distance of the feedingsite.

Wind seems to be not of the strong significance that Wenner and co-workershave postulated. However, there is some small effect of wind on recruitment,which supports the view that odors are of some significance for searching bees.

Our results confirm von Frisch's single array experiment (described byFrisch, 1967), array experiments of Gould (1976), and array experiments usinga robot bee (Michelsen et al., 1992), all of which indicated that bees can makeuse of the directional information provided by the dances. These previous exper-iments have been criticized by Wenner and Wells (1990), for they providedscent at the recruit stations and did not exclude directional orientation of therecruits due to asymmetries in the arrays and due to effects of wind. As ourexperiments exclude all of these hypothetical sources of directionality in thesearch behavior of the recruits, we have to conclude that the dance informationis indeed used by searching recruits. This falsifies the exclusive odor searchhypothesis of Wenner and Wells (1990) and Wenner et al. (1991).

On the other hand, the data also show that odor cues are significant. Itmight therefore well be that, under certain experimental conditions, odor cuesare even more important for the recruit bees. Because most of the previous testsof the dance language hypothesis have been performed at distances in the rangeof 150 to 350 m, it is possible that small differences in the experimental designled to the controversial conclusions in the literature about recruitment in honey-bees. However, at long distances it seems to be obvious that the dance infor-mation is the master cue for the directional search behavior of recruits. Thedances lead the recruits into the right area, where odor cues then might becomemore and more important for them finally to find the flowering plants.

The experimental approach of this study allowed us to assess both sets ofcues guiding searching recruits, that is, olfactory cues and the dance information.However, it does not allow one to compare their effects directly, for it comparesrecruitment by odors with recruitment by odors plus dance information. It wouldtherefore be useful to have an experimental approach which allows direct com-parison of these sets of cues. Other than in previous experiments by Gould(1976), one would wish to indicate one direction by the dances but let theforagers fly to exactly the opposite direction. Such an experimental tool hasrecently been developed and will be described elsewhere (Kirchner and Towne,in preparation).

The much more directional search of bees which have followed well-ori-ented dances compared to those recruits which have to rely on olfactory cuesallows honeybee colonies to send their foraging forces to rapidly changing prof-itable resources (Visscher and Seeley, 1982). This advantage of the dances hasobviously driven the evolution of the highly sophisticated dance communicationsystem. Recently Waddington et al. (1994) examined the distribution of foraging

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honeybees in suburban environments with rich floras and compared the spatialpatterns of foraging sites used by colonies located in these environments. Theirresults suggested a reduced role of recruitment communication in habitats withabundant, small patches of flowers and indicate that the individual olfactorysearch behavior is of great significance in those environments. Their data seemtherefore to support that site-specific recruitment by means of the dance com-munication system has evolved to exploit rapidly dispersed patches of resourceswhich are difficult to find by odor search and to concentrate the labor force ofthe colony on some few of the most profitable of such patches. Olfactory searchbehavior and site-specific recruitment through dance communication thus seemto be two strategies that complement each other—on the level of the foragingindividuals as well as on the colony level.

ACKNOWLEDGMENTS

The experimental data presented here are from the diploma thesis of oneof the authors (A.G.). We thank Claudia Dreller and William F. Towne forvaluable comments on the manuscript.

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