Parameters determining the attractiveness of stripe patterns in the honey bee

Anim . Behav ., 1977, 25, 80 -87

PARAMETERS DETERMINING THE ATTRACTIVENESS OF STRIPE PATTERNS IN THE HONEY BEE BY ALUN M . ANDERSON

Animal Behaviour Research Group, Department of Zoology, University of Oxford Abstract. Foraging honey bees were found to approach some stripe patterns more frequently than others, the most attractive patterns being ones with narrow stripes spaced a small distance apart . The attractiveness value of a pattern could be predicted by the function I /(w)(sp) ; where (w) is the stripe width and (sp) the width of the interval between stripes . This measure was applied to the nectar guide patterns found occurring naturally on flowers . The floral patterns had attractiveness values corresponding to the highest levels found experimentally . It is suggested that these patterns act as a near distance signal for the bee to land . Preference for some patterns over others can be demonstrated by first training bees to visit a particular place . When a set of patterns is displayed at this place, arriving bees are found to fly to some much more frequently than others, even though they have never seen them before . Spontaneous preference was first investigated by Hertz (1929a, b) . She found that highly divided shapes were the most attractive and suggested that the attractiveness was related to the quantity of outline a shape possessed ; the greater the quantity of outline the more visits the shape attracted . Highly dissected patterns are also found on bee pollinated flowers, patterns of radial lines and spots being most common . More patterns are present than are seen by us as they are often only visible by their ultra-violet reflectance . Daumer (1958) has photographed many ultraviolet flower patterns and shown that they would be visible to the bee . Sprengel (1793) was the first to suggest that these patterns may serve to attract insects to the flowers and guide them to the nectaries . Later studies with honey bees (Zerrahn 1933) and bumblebees (Manning 1956) showed that the addition of nectar guide patterns to floral shapes caused an increased number of visits from the bees . Free (1970) conducted similar experiments and concluded that disrupted patterns caused the largest increases in attractiveness, flower models with dotted lines were more attractive than ones with continuous lines and groups of dots were more attractive than a central black circle . In the following experiments an attempt is made to describe the parameters of stripe patterns which influence the bees' spontaneous preference. This measure is then related to the kinds of patterns occurring naturally on flowers

and to the results of previous research on pattern discrimination by honey bees . Methods I . Experiments with Honey Bees (i) Paired preference tests Training . A group of 20 individually marked honey bees (Apis melliica) was trained to enter the experimental room and land on a white table . Here they received sucrose solution reward from either of a pair of small (15-mm diameter), yellow dishes . Tests began as soon as the bees were visiting this sugar supply regularly . On average, individual bees returned to the experimental room from the hive every 3 to 5 min . Testing. Before the test, the two dishes containing sucrose solution were removed . Two stimulus cards were then laid out, 50 mm apart on the table . A small yellow dish identical to those used during training was placed on top of each of the cards and filled with water . Sucrose solution reward was never given during a test . As each bee entered the room it flew to one of the stimulus cards . All visits to each stimulus card, including repeated visits by the same bee, were recorded for a period of 5 min . After this test period, the stimuli were removed and the two dishes of sucrose solution again placed on the table . Further tests were carried out at intervals of 30 min . Throughout a series of tests one of the two stimuli was kept unchanged as a control . The other stimulus was changed between tests so that, in a test series, the preference for a number of different test stimuli was found in relation to one particular control stimulus . (ii) Preference tests with multiple stimuli Training . The method was similar to that above . A group of 50 individually marked bees 80



ANDERSON : PATTERN PREFERENCE IN THE BEE

received sucrose solution reward from a set of small yellow dishes in the experimental room . Once the bees were visiting the room regularly tests were carried out. Testing . During a test, up to ten stimulus cards were laid out and water-filled yellow dishes placed upon them. Bees entered the room and their visits were recorded on videotape (Sony CV-21000E videotape recorder) . The test lasted 5 min, the stimuli were then removed and the sugar-filled dishes replaced . The number of visits to each stimulus was found later by slowplayback analysis of the videotapes. Further tests were given at 30-min intervals, the positions individual stimuli occupied were changed in each test. This multiple method differs from the above paired method in that, instead of comparing the preferences of the bee for each stimulus relative to a particular control stimulus, the relative preference among a whole set of different stimuli is found at once . It is assumed that the order of preference will be the same whether it is found by multiple tests or paired tests . However, it cannot be assumed that the percentage preference for a particular stimulus relative to another will be the same in a multiple stimuli test as it will if the same two stimuli were compared in a paired test . When many stimuli are present, the presence of some stimuli may affect the relative number of choices other stimuli receive, e .g . the presence of a very attractive stimulus may make a set of others more similarly unattractive . (iii) Stimulus patterns Patterns of black and white parallel stripes were used . They were made by printing to maximum density on high-contrast, bromide photographic paper . Each stimulus card had the same dimensions (118 mm 2) but the width (w), number (n) and spacing (sp) of the black stripes varied from one stimulus to another . As the total number of stripes on a stimulus card was determined by the width and spacing of the stripes then each stimulus pattern was uniquely defined by its stripe width and spacing. II Examination of Flower Patterns Flowers and tree blossoms were collected from the area within about I km of the experimental room. This area consisted largely of park land and gardens and contained many ornamental species as well as native plants . Each flower and tree blossom collected was photographed twice. The first photograph was taken

81

to show reflectance of visible light, an ultraviolet light absorbing filter (Kodak Wratten no . IA) was placed over the camera lens and a picture taken onto panchromatic film . For the second photograph this filter was replaced with one which transmitted ultra-violet light but absorbed visible light (Kodak Wratten no . 18A) . This photograph recorded the ultra-violet light reflectance of the plants . Patterns were visible in a number of the photographs as either lines, spots or a dark central region . Patterns of contrasting lines, visible in either the ultra-violet or visible regions of the spectrum, were measured . Measurements were taken of the line widths and spacings, mean values being computed from five samples of each species . Spacing was measured as the width of the space between two neighbouring lines . In general, line widths remained constant while the spacing became less as the lines converged towards the centre of the flower . Results I. Experiments with Honey Bees Experiment A. The results and the stimuli used in this experiment are shown in Fig . 1 . The six test stimuli were each separately compared with a plain white control pattern . All the stimuli used had fifteen stripes of the same length, although they all had the same contour length they were not seen as equally attractive . The stimuli may be divided into two groups . Those on the left-hand side of the graph (Fig . 1) are reversed contrast equivalents of those on the right-hand side ; the white spaces between stripes in the stimuli on the left become the black stripes in the patterns on the right and vice versa . Thus, any pattern in one group has an equivalent in the other, group such that stripe width (pattern one) = spacing (pattern two) and spacing (pattern one) = stripe width (pattern two) . It can be seen that the attractiveness of each pattern in one group is the same as its reversed contrast equivalent in the other group . Experiment B. In this experiment the relative attractiveness of ten different stimuli was measured . The stimuli are shown in Fig . 2 numbered from 0 to 9 and the dimensions of their stripe widths and spacings given in Table I. Six different sub-experiments were carried out, naive groups of bees being recruited at the beginning of each . Bl Each of the stimuli was separately compared with stimulus no . 9 in a series of

ANIMAL BEHAVIOUR, 25, 1

82 63

61-

C a) U I-

m CL

57

55 0

71

62

5 3

I5

1 6'

17

3

2

1

I

0

Width Spacing

Fig . 1 . Y-axis : The percentage of responses received by the test pattern in comparison with the control pattern . The percentage of responses a pattern receives is an index of its attractiveness. X-axis : Width and spacing of the stripes comprising the test patterns (mm). As the patterns contain a constant number of lines, the spacing decreases as the width increases . paired choice test . This series of tests was repeated ten times ; to ensure that adaptation to the control stimulus did not influence the results a new group of bees was used in each test series and the order of presentation of stimuli was randomized within each series . A mean figure for the percentage of visits to each test pattern, in comparison with the control

pattern, was computed. In Fig . 2 these percentages are shown beneath the stimulus pattern to which they refer . It can be seen that there are differences in the relative attractiveness of these patterns . B2 Each stimulus was compared in a single paired test with pattern no . 8 . Figures for the



ANDERSON : PATTERN PREFERENCE IN THE BEE

percentage of visits to each pattern were recorded . B3 Single paired tests were carried out as in B2, but stimulus no . 7 was used as the control pattern. B4 A preference test with multiple stimuli was carried out using all ten stimuli shown in Fig . 2 . The test was repeated ten times and mean figures for the percentage of responses to each pattern calculated . B5 This was a repeat of the previous experiment . B6 Another multiple stimuli test was carried out but using only four stimuli ; numbers 3, 5, 6 and 7. The test was repeated 24 times and mean figures calculated for the percentage of visits to each stimulus . The results of these experiments are summarized in Table II . The order of attractiveness found is shown for each of the six sub-experiments . The order is shown in each column with the most attractive stimulus at the top and the least attractive at the bottom . The numbers refer to the stimulus code numbers shown above the relevant stimuli in Fig . 1 .



83

The patterns differed in their stripe widths and spacings . Is it possible to derive some measure of the stimuli which corresponds with Table I. Stimulus Dimensions Stimulus number

Stripe width (mm)

Stripe spacing (mm)

0

0 .4

0 .6

1

2 •o

2 .0

2

3 .5

1 .7

3

2 .5

5 .3

4

1 .8

3 .9

5

2 .0

5 .8

6

1 .5

6.3

7

5 .4

5 .4

8

5 .4

10 .2

9

5.4

20 .2

0 4 3

75 .8

02 .5

55 .3

1

6

7

9

546

50.1

74 .4

2

70 .1

59 .8

5 8

56 .9

52 .8

Fig. 2. The test patterns.



84

ANIMAL BEHAVIOUR, 25, 1

the experimentally found order of attractiveness? In Table III are shown various measures of the patterns and the orders of attractiveness they predict. The first column shows that a contour length measurement (equivalent to n, the num-

ber of stripes) incorrectly predicts that patterns 3, 5 and 6 should be seen as identical in attractiveness . A measure found to predict the observed sequence was that of 1/(w)(sp) ; that is the attractiveness increases as the product of stripe

Table IL Order of Preference of Stimuli Expt . no .

1

Condition No. of Runs Order

2

3

4

5

6

Paired with 9

Paired with 8

Paired with 7

Group of 10

Group of 10

Group of 4

10

1

1

10

10

24

0

0

0

0

0

1

1

1

1

1

2

2

2

4

4

4

4

4

6

6

6

6

6

6

5

5

5

5

5

5

3

3

3

3

7

3

7

7

7

7

3

7

8

8

8

8

8

9

9

9

9

9

2

Table III Observed and Predicted Preference Orders Sequences predicted from Observed preference order

Contour length (n)

Stripe width (1/w)

Stripe spacing (1/sp)

Stripe area (n .w)

1/(w)(wp)

n/w

n/sp

0

0

0

0

2

0

0

0

1

1

6

2

7

1

1

1

2

2

4

1

1

2

4

2

4

4

5,1

4

8

4

6

4

6

3, 5, 6

7

3

6

5

3

5

3

3

4

5

2

5

3

2

5

5

3

3

6

7, 8, 9

6

9

7

7

7

7

7

8

8

8

6

8

8

8

9

9

9

0

9

9

9



85

ANDERSON : PATTERN PREFERENCE IN THE BEE

width and spacing decreases . Thus, the most attractive stimuli are those with narrow stripes spaced closely together . This measure also correctly predicts the results of experiment 1 . In this experiment it was shown that a particular stimulus had the same attractiveness as its reversed contrast equivalent . A pattern and its reversed contrast equivalent have identical (w)(sp) values as the spacing of one is the width of the other and vice-versa . The attractiveness values found in experiment Bl are plotted against (w)(sp) in Fig . 3 . It can be seen that there is little change in attractiveness between (w)(sp) values of 10 and 55 mm2 . At values less than 10 mm2 the attractiveness increases very rapidly until it reaches a peak at the smallest (w)(sp) values studied here . At the highest value of attractiveness the test stimulus was receiving 75 . 6 per cent of all visits . It should be remembered that all these results were obtained by comparing each stimulus in turn with stimulus no . 9 . If these results are compared with those of experiments B2 and B3 in which the control patterns . were nos . 7 and 8 respectively (Fig. 4), it can be seen that this steep rise in attractiveness still occurs at the same (w)(sp) value . This implies that, although these results were derived from choice experiments, the rapid rise in attractiveness does not occur at a particular (w)(sp) difference but at a particular (w)(sp) value. Another measure, closely related to that of (w)(sp) would also correctly predict the observed preference order . This measure is that of the number of stripes (or intervals) divided by the smallest interval between contours (either width or spacing, whichever is the smaller) . This measure approximates to that of 80-

because in these experiments the area of the stimulus card was constant and the size of the width and spacing determined the total number of stripes in a stimulus . Both measures predict that the most attractive stimuli have a large number of narrow stripes placed close together . Neither predicts that the total length of contour should be maximized but rather that a large amount of contour should be distributed so that it is compact locally . This point is made clearer by reference to Fig . 1 . The patterns here all had the same contour length, the most attractive patterns were those which had the narrowest intervals between contours, i.e . the narrowest width or the narrowest spacing. II. Flower Patterns The mean nectar guide width multiplied by the mean spacing was calculated for each of the flower species and is shown in Table IV . In species where a range of values had been recorded, the maximum and minimum values of (w)(sp) found in different regions of the flower were computed. (w)(sp)

Discussion The measure found to predict the attractiveness of stripe patterns can be related both to the patterns found on flowers and to the results of other experiments . The (w)(sp) values of the nectar guide patterns were found to lie between the (w)(sp) values of the two most attractive experimental patterns . The flower patterns thus have (w)(sp) values which place them at the 80 N 5,

c :• a 70

N O

t80.

.;, O 19

.

d 50-

N

60

¢

0

¢¢

20

• 40

•

6b0

80

16o

WxSP

CL

50 0

10

20

30 WxSP

40

5'0

55

Fig. 3 . Y-axis : The percentage of responses received by the test pattern in choice tests with pattern no . 9 . X-axis : The product of stripe width and stripe spacing (mm2) .

Fig . 4 . Y-axis : The percentage of responses received by the test pattern in comparison with the control pattern . X-axis : The product of stripe width and stripe spacing (mm2). The circular points represent comparison with pattern no . 7 as control and the triangular points represent comparison with pattern no . 8 .

86

ANIMAL BEHAVIOUR, 25, 1

highest levels of attractiveness . The most attractive stimuli contained such fine stripes that it is unlikely that they would be visible to the bee at a distance . In the experiments with stripe stimuli, it was noted that even very attractive stimuli did not attract bees from a distance . If the stimuli were moved only 300 mm away from their usual position on the table they received very few visits . Only by first training the bees to come to a specific location is it possible to demonstrate the difference in attractiveness of these patterns . The nectar guide patterns, rather than attracting the bees to the flower, probably function to tell them `alight here' when they are very close to the flower. As the nectar guides lead back to the nectaries this will enable the bees to quickly reach the source of nectar . All the bees used in the experiments could have been visiting flowers before they began to attend experiments . Thus, although the experiments suggest a quantitative measure of the attractiveness of stripe patterns, they do not help to tell us whether the bee learns to find these patterns attractive or whether it would respond to them even on its first flight from the hive . It is probable that the results above only apply to bees which are searching for food . When the bees are returning to the hive, experiments have shown that they have a preference for figures with the simplest contours (JacobsTable IV. Values of Nectar Guide Patterns Species Iris pseudoacorus Geranium ibericum Cerastium tomentosum

w X sp (nunz) 0 . 04 0 . 03-0 .05 0 .04

Geranium pratense

0 . 10-0 •1 1

Meconopsis cambrica

0 .04-0-11

Tropaeolum majus

0 .05-0 •1 1

Viola hybrids

0 . 14-0-16

Delphinium hybrids

0 .06

Cheiranthus cheiri

0 . 02

Malva moschata

0 .20-0 •30

Malva sylvestris

0 .06-0-08

Althaea rosea

0 . 15-0 •2 5

Jessen 1959), presumably figures which resemble the entrance of the hive . The relative attractiveness of patterns can be modified. It is known that, even among a group of highly attractive patterns, it is possible to reinforce visits to one pattern at the expense of others (Anderson 1972) . As bees tend to visit one species of flower at a time (Darwin 1876), it is likely that they will learn the characteristics of a particular nectar guide pattern . This may, in turn, aid flower constancy and increase the efficiency of the pattern as a signal for the bee to land. In previous experiments (Anderson, 1977) a measure of shape was found which described the similarity between two shapes perceived by the bee . The measure consisted of two terms, an area term and a measure of the contour-length-to-area ratio of the shapes (contour density). A number of shapes were found to be visited more frequently than expected on the basis of their similarity to the training shape . Measurements of the (w)(sp) values of these shapes show them all to have values smaller than 10 mm2, the value beneath which a rapid increase in attractiveness begins . None of the other shapes used in the experiments had such small (w)(sp) values . The results from the current experiments would explain the increased number of visits these shapes received as being due to their attractiveness . The number of visits received by a shape in a generalization experiment is thus partly due to its perceived similarity to the training shape and partly due to how attractive a shape it is . We now possess two measures of shape used by the bee, one determining spontaneous attractiveness and one determining the similarity between two shapes . Is it possible that they stem from a common visual measuring process? In the previous paper (Anderson 1977) it was suggested that contour density could be measured by a `flicker' process which measured the contour frequency of shape units . A similar process could be used to produce a (w)(sp) measure . Width could be measured as the mean interval between off (light to dark) responses and on (dark to light) responses and spacing as the reverse . Alternatively, the correct order of attractiveness could be predicted from measurements of the smallest flicker intervals (that is, either width or spacing) and the number of such intervals (the frequency of encountering them) . The results reported here differ from the early predictions made by Hertz (1929a, b) . She



ANDERSON : PATTERN PREFERENCE IN THE BEE

stressed the importance of total contour length as a measure of attractiveness, these results add that the greatest attractiveness is found when the contour is distributed so that within the pattern there are regions of local contour compactness . Acknowledgments Especial thanks are due to Joanna Tagney for making many of the stimulus patterns and to Lindesay Harkness for collecting and identifying most of the flower species . This research was supported by an IBM Junior Research Fellowship. REFERENCES Anderson, A . (1972). Some aspects of learning in insects . Ph .D . thesis, Edinburgh University . Anderson, A . (1976). Shape perception in the honey bee . Anim. Behav., 25, 67-79 . Darwin, C. (1876) . The Effects of Cross and Self Fertilisation in the Vegetable Kingdom . London : Murray.

87

Daumer, K. (1958). Blumenfarben, wie sie die Bienen sehen . Z. vergl. Physiol., 41, 49-110 . Free, J . (1970) . Effect of flower shapes and nectar guides on the behaviour of foraging honey-bees . Behaviour, 37, 269-285. Hertz, M . (1929a) . Die Organisation des optischen Feldes bei der Biene, I . Z. vergl. Physiol., 8, 693-748. Hertz, M. (1929b) . Die Organisation des optischen Feldes bei der Biene, II . Z. vergl. Physiol., 11, 107-145 . Jacobs-Jessen, U. (1959). Zur Orientierung der Hummeln and einiger anderer Hymonopteren. Z. vergl. Physiol., 41, 597-641 . Manning, A . (1956) . The effect of honey-guides . Behaviour, 9, 114-139. Sprengel, C. (1793) . Das entdeckte Geheimnis der Natur im Bau and in der Befruchtung der Blumen . Berlin : Vieweg. Zerrahn, G . (1933) . Formdressur and Formunterscheidung bei der Honigbiene . Z. vergl. Physiol., 20, 117-150 . (Received 12 August 1975 ; revised 11 February 1976 ; MS. number : 1460)