DOI: 10.5061/dryad.08kprr5bb
title: Economic foraging in a floral marketplace: Asymmetrically dominated decoy effects in bumblebees

description: While most models of decision-making assume that individuals assign
 options absolute values, animals often assess options comparatively,
 violating principles of economic rationality. Such ‘irrational’
 preferences are especially common when two rewards vary along multiple
 dimensions of quality and a third, ‘decoy’ option is available. Bumblebees
 are models of decision-making, yet whether they are subject to decoy
 effects is unknown. We addressed this question using bumblebees (Bombus
 impatiens) choosing between flowers that varied in their nectar
 concentration and reward rate. We first gave bees a choice between two
 flower types, one higher in concentration and the other higher in reward
 rate. Bees were then given a choice between these flowers and either a
 ‘concentration’ or ‘rate’ decoy, designed to be asymmetrically dominated
 on each axis. The rate decoy increased bees’ preference in the expected
 direction, while the concentration decoy did not. In a second experiment,
 we manipulated choices along two single reward dimensions to test whether
 this discrepancy was explained by differences in how concentration vs.
 reward rate were evaluated. We found that low-concentration decoys
 increased bees’ preference for the medium option as predicted, whereas
 low-rate decoys had no effect. Our results suggest that both low- and
 high-value flowers can influence pollinator preferences in ways previously
 unconsidered.
description: a)  Study system We conducted experiments
 with bumblebees (<em>Bombus impatiens</em>) (n=103) from
 commercially-reared queenright colonies (n = 8) (Koppert, USA); sample
 sizes per colony are shown in Table S1. Colonies were maintained in small
 boxes (~40cm<sup>3</sup>) on 30% (w/w) sucrose solution and
 honeybee-collected pollen (~0.5g/day, Koppert Biological Systems, USA). We
 connected colonies sequentially to a flight arena (l × w × h: 122 × 61 ×
 61 cm), using a clear plastic tube with sliding doors to control the entry
 and exit of bees into the arena. The floor of the flight arena was lined
 with green laminate and the sides and top consisted of black mesh screens.
 The arena was lit from above by a 40-watt LED light placed atop the arena
 and the room was illuminated with fluorescent light on a 12/12-hour
 light/dark schedule. b)  Experimental arrays and
 floral stimuli The vertical arrays used for
 experiments consisted of black corrugated plastic sheets with 24 holes for
 ‘flowers’ arranged in a 6 × 4 grid. Twelve artificial flowers were
 pseudorandomly arranged to be equally represented across the array, with
 unoccupied holes sealed using black rubber bungs. We constructed
 artificial flowers from 1.5 ml plastic Eppendorf tubes with small holes
 cut in the base of the tubes to pipette sucrose solution. The ‘corollas’
 of these artificial flowers were made from laminated, coloured circular
 disks (5 cm in diameter) placed around the opening of the tube. We used
 three colour stimuli to correspond to our three flower types: blue,
 yellow, and white. To accurately monitor and manipulate
 the reward rate of flowers (see section d below), we attached rotating
 paper disks to the back of our artificial flowers, which corresponded to
 the four rates of reinforcement used across both experiments (Figure S1).
 There were eight positions on the disks that were either marked with an
 “X” or marked with the sucrose concentration used for that flower type
 (e.g., 50%). The spot in the upright position denoted whether a flower was
 empty or full, and whether to refill the flower following a bee visit. The
 experimenter rotated the disk a single position each time a bee visited
 the flower, only refilling it on positions not marked by “X”. Disks were
 only visible to the experimenter and not to the foraging bee.
 c)  Initial training During the initial
 training phase, we first allowed bees to forage for sucrose in the
 foraging arena by giving a whole colony access to a white-wicked feeder
 containing 30% (w/w) sucrose solution. This was a single plastic container
 filled with sucrose solution that had an immersed and saturated wick (a
 braided dental cotton roll, Richmond Dental, USA) protruding vertically
 from the lid, on which bees could land and drink. We initially placed the
 feeder close to where foragers exited their colony. Over 1-2 days, we
 incrementally moved the feeder towards the end of the arena until bees
 readily foraged on it and returned to the colony. We then trained bees to
 visit artificial flowers on a 24-flower array similar to the experimental
 array with the exception that no colour stimuli were used (Eppendorf tubes
 alone). All flowers were provisioned with 30% (w/w) sucrose solution.
 During this initial training, flowers contained an additional hole on top
 which we used to mark individual bees’ thoraces using non-toxic,
 water-based paint markers (POSCA, USA). Bees were each given a unique
 colour combination for identification. Foragers consistently leaving the
 colony to forage from this array (three or more visits) were selected for
 use in experiments. d)  Experimental
 protocol General Protocol Each
 bee was assigned to one of either three (Experiment 1) or four (Experiment
 2) treatments. All treatments were represented across all colonies within
 each experiment. We systematically varied treatment order across bees
 within a given colony to avoid any temporal confounds. All bees
 experienced a total of six consecutive training and testing trials. Trials
 lasted around 10 minutes each with bees making ~ 20-30 flower choices per
 trial. Twelve flowers were used on the array during the training and
 testing trials, and individual flowers could be re-visited since they were
 either re-filled or left empty. For all experiments, a flower ‘choice’ was
 defined as a bee entering a flower and extending her proboscis. Flowers
 varied in concentration (20%, 30%, 40%, and 50% sucrose (w/w)) and reward
 rate (25%, 50%, 75%, and 100% rewarding). Reward volumes were kept
 constant at 4 μl per flower. We used three different flower colors: blue,
 yellow, and white to indicate the different flower types. Blue and yellow
 flowers were always used as the two main flower types for each experiment,
 while decoy flowers were always white. We chose this design since a
 previous study indicated that bees did not show a strong preference
 between this particular blue and yellow, and because counter-balancing
 colors was not feasible in terms of the sample sizes required. Reward
 values were chosen such that bees had roughly equal preferences between
 the two main flower types and were consistent with previous studies
 demonstrating robust preferences in bees for higher nectar concentrations
 and rates of reinforcement. In both experiments, each
 bee was given the following trials: <strong>Trial 1: Binary
 training:</strong> Bees were given a choice between six blue and six
 yellow flowers; this trial allowed bees to gain experience with the two
 main flower types. <strong>Trials 2 and 3: Binary preference
 tests:</strong> These trials allowed us to assess bees’ preference
 between the two main flower types. <strong>Trial 4: Decoy
 training</strong>: Bees in trinary treatments received a trial with
 only decoy flowers to ensure that all bees gained experience with the
 decoy before the trinary preference trials; bees had to visit a minimum of
 10 decoy flowers to move on to the next trial. In place of the decoy
 training trial, bees in binary treatment received another trial with the
 initial two flower types. We did this rather than a ‘blank’ trial to
 ensure that foragers did not lose foraging motivation and to give bees
 across treatments the same number of trials overall. <strong>Trials
 5 and 6: Trinary preference test:</strong> Bees in binary groups
 were given the same choice again, while bees in trinary groups had the
 original two flowers alongside the decoy option. These final two trials
 allowed us to compare bees’ binary vs. trinary preferences both within and
 across treatments. Bees were not trained to the decoy until trial 4, as
 prior experience with decoy flowers may have influenced bees’ initial
 preferences for R vs. C, even if those flower types were absent during
 later choice tests. All trials were recorded using a Canon camcorder on a
 tripod placed near the nest box and facing the array.
 Experiment 1: Are bumblebees subject to decoy
 effects? We tested 55 bees from five colonies. Bees
 were trained and tested individually in one of three experimental
 treatments: a binary choice between two flowers, one higher in reward rate
 (flower R, yellow) and the other higher in concentration (flower C, blue),
 or one of two trinary choice treatments where they also had a decoy flower
 (either DR or DC). Flowers R (30% sucrose, 100% rewarding) and C (50%
 sucrose, 50% rewarding) were designed to be roughly equally preferred,
 with each option being superior to the other on one dimension. Flowers DR
 (20% sucrose, 75% rewarding) and DC (40% sucrose, 25% rewarding) were
 designed to be asymmetrically dominated by flowers R and C,
 respectively. Experiment 2: How do bumblebees evaluate
 reward rate and concentration dimensions separately?
 After finding in Experiment 1 that DR induced the predicted
 effect, but that DC did not affect bees’ relative preferences between
 flowers, we conducted a second experiment to better understand this
 result. In Experiment 2, we tested how bees evaluate each reward dimension
 (concentration and reward rate) separately by determining how the relative
 preference between a medium and high option was affected by the presence
 of a lower (decoy) option. We tested 48 bees from three colonies. Using a
 similar design to Experiment 1, bees were trained and tested individually
 across one of two experimental treatments (binary or trinary) for each
 reward dimension. Bees in binary treatments were given a choice between a
 medium-quality flower (flower M) and a high-quality flower (flower H).
 Bees in trinary treatments were also given experience with a low-quality
 decoy flower (flower L) and subsequently given a three-choice test between
 all three flower types. In this experiment, because there was a clear
 ranking of flowers within each choice set, any initial color preference
 would likely be enhanced by the preference for the higher-quality flower.
 As such, we counter-balanced color (blue or yellow) across the two flower
 types (flowers M and H) to control for any color bias associated with the
 best option.
description: # Economic foraging in a floral marketplace: Asymmetrically dominated
 decoy effects in bumblebees
 [https://doi.org/10.5061/dryad.08kprr5bb](https://doi.org/10.5061/dryad.08kprr5bb) There are two datasets: Decoy.data.csv and decoy4data.csv ## Description of the data and file structure #### Decoy.data.csv The concentration of the flower chosen was used to calculate the proportion of visits to each flower type out of the total number of visits. Column A: Bee identification (number) Column B: Bee identification (color) Column C: Colony identification (letter) Column D: Date of data collection Column E: Treatment name (Binary, Trinary 1, Trinary 2) Column F: Treatment renamed (Binary, Concentration trinary, Rate trinary) Column G: Visit number Column H: Trinary number Column I: Visit number within a trial Column J: Flower color (B=blue, Y=yellow) Column K: Flower state (empty or full) Column L: Concentration of sucrose reward (20%, 30%, 40%, or 50%) Column M: Concentration of reward in previously visited flower Column N: Concentration of reward in the next visited flower Column O: Acceptance (0=reject, 1=accept) Column P: Number of visits to the decoy in the fourth trial Column Q: Actual reward rate for decoy option In column M, na indicates the first flower (i.e., no last flower) In column N, na indicates the last flower (i.e., no next flower) In columns P and Q, na indicates that bees were in the binary treatments and did not encounter a decoy flower type. #### decoy4.data.csv Choice is coded as a binary response. This dataset was used to generate several of the figures in the main text and supplementary. Column A: Bee identification (color) Column B: Bee identification (number) Column C: Date of data collection Column D: Colony identification (letter) Column E: Treatment type (variance or concentration) Column F: Treatment type (binary or trinary) Column G: Treatment by color (1 or 2) Column H: Treatment with color (VB1, VB2, VT1, VT2, CB1, CB2, CT1, CT2) Column I: Treatment color (blue or yellow) Column J: Trial number Column K: Visit number within a trial Column L: Visit number Column M: Flower color (yellow, blue, or white) Column N: Sucrose concentration (0.2 = 20%, 0.3 = 30%, 0.4 = 40%, 0.5 = 50%) Column O: Reward rate (0.25 = 25%, 0.5 = 50%, 0.75 = 75%, 1.0 = 100%) Column P: Choice (0= H chosen, 1=M chosen) Column Q: Flower state (empty or full) Column R: Acceptance (0=reject, 1= accept) Column S: Number of visits to the decoy in the fourth trial Column T: Actual reward rate for decoy option In column R, blank spaces indicate that the flower was empty and acceptance could not be measured. In columns S and T, na indicates that bees were in the binary treatments and did not encounter a decoy flower type. ## Code/software The R code, Hemingway_et_al.Rcode.R can be used to run all reported analyses and generate all published figures.