Wild bumblebees use both absolute and relative evaluation when foraging

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, 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.

Methods
We collected data in June 2021 at two sites in montane meadow and mixed conifer forests. The first site was in the Humboldt-Toiyabe National Forest (Dog Valley Meadow, California), which was lower elevation and bloomed earlier. The second site was in Tahoe National Forest (Van Norden, California). Experiments were conducted using the four most abundant species of bumblebee (n=180): Bombus vosnesenskii (n=88), B. vandykei (n=16), B. centralis (n=52), B. vancouverensis nearcticus (n=5). We focused on two flower species within the genus Penstemon: P. rydbergii at Dog Valley and P. heterodoxus at Van Norden. Each testing day, we calculated the average concentration of nectar in Penstemon in the study meadow, before offering foraging bumblebees Penstemon plants containing artificial nectar that varied across a range of concentrations. This allowed us to determine whether bees’ acceptance of nectar rewards varied based on their short-term foraging experience. To calculate wildflower nectar concentration, we collected nectar each day from 15-30 plants using 1μl microcapillary tubes. These plants were bagged 24 hours earlier (when unopened) using white mesh organza drawstring bags (10 × 15 cm) to prevent visits from other insects prior to nectar extraction. Once nectar was extracted from all flowers on a plant, we cut plants at the base of the stem for use in behavioral experiments, using different plants for each bee. Following nectar extraction, we filled all previously-emptied flowers on each plant with 1 ml of a known sucrose solution of one of six concentrations (w/w): 0, 1, 5, 10, 25, and 50%. Once the bee landed on the manipulated plant, we recorded several behavioral responses. Namely, we stated the time that a bee landed on the plant, how many flowers she visited (typically via walking between flowers, but in one case the bee flew off the plant and then returned to another flower), whether she accepted or rejected the sucrose solution in each flower, and the time that she departed from the plant.