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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 versus 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. 

Foraging theory assumes that animals assess value based on objective payoffs; however, animals often evaluate rewards comparatively, forming expectations based on recent experience. This form of evaluation may be particularly relevant for nectar foragers such as bumblebees, where individuals can visit thousands of flowers daily that vary in nectar quality. While many animals, including bees, demonstrate reference-based evaluation in experimental contexts, it is unclear whether this occurs in the wild. Here, we asked how daily experience with wildflower nectar influenced wild bumblebees’ reward evaluation. We measured the daily nectar concentration of bee-visited wildflowers (Penstemon spp.), before presenting foragers with conspecific flowers filled with a range of artificial nectar concentrations. We recorded bees’ acceptance of artificial nectar, the probability of subsequent visits to flowers on the same plant, and residence time. While bees had a minimum threshold of nectar acceptability that was unaffected by experience, when there was higher-concentration environmental nectar, they were less likely to accept lower-quality rewards on manipulated plants. Bees also visited more flowers and stayed longer on plants with higher-concentration nectar. This study shows evidence for both absolute and reference-based evaluation in wild bees and points towards differences between bees’ behavior in lab- and wild-foraging contexts. 

While classic models of animal decision-making assume that individuals assess the absolute value of options, decades of research have shown that rewards are often evaluated relative to recent experience, creating incentive contrast effects. Contrast effects are often assumed to be purely sensory, yet consumer and experimental psychology tell us that label-based expectations can affect value perception in humans and rodents. However, this has rarely been tested in non-model systems. Bumblebees forage on a variety of flowers that vary in their signals and rewards and show contrast when rewards are lowered. We manipulated bees' expectations of stimulus quality, before downshifting the reward to induce incentive contrast. We found that contrast effects were not solely driven by experience with a better reward, but also influenced by experience with associated stimuli. While bees' initial response did not differ between treatments, individuals were faster to accept the lower-quality reward when it was paired with a novel stimulus. We explored the boundaries of these label-based expectations by testing bees along a stimulus gradient and found that expectations generalized to similar stimuli. Such reference-dependent evaluations may play an important role in bees' foraging choices, with the potential to impact floral evolution and plant community dynamics.