Bumblebees retrieve only the ordinal ranking of foraging options when comparing memories obtained in distinct settings

Are animals’ preferences determined by absolute memories for options (e.g.
reward sizes) or by their remembered ranking (better/worse)? The only
studies examining this question suggest humans and starlings utilise
memories for both absolute and relative information. We show that
bumblebees’ learned preferences are based only on memories of ordinal
comparisons. A series of experiments showed that after learning to
discriminate pairs of different flowers by sucrose concentration,
bumblebees preferred flowers (in novel pairings) with 1) higher ranking
over equal absolute reward, 2) higher ranking over higher absolute reward,
and 3) identical qualitative ranking but different quantitative ranking
equally. Bumblebees used absolute information in order to rank different
flowers. However, additional experiments revealed that, even when ranking
information was absent (i.e. bees learned one flower at a time), memories
for absolute information were lost or could no longer be retrieved after
at most one hour. Our results illuminate a divergent mechanism for bees
(compared to starlings and humans) of learned preferences that may have
arisen from different adaptations to their natural environment.