Bumble bees’ food preferences are jointly shaped by rapid evaluation of nectar sugar concentration and viscosity

Animals are often assumed to follow a strategy of energy maximization, and
therefore should evaluate feeding options based on energy intake rates.
However, at the proximal level, a direct estimate of
energy intake rates, if that is possible at all, might require
postabsorptive senses with relatively longer processing times, whereas an
indirect estimate of energy intake through proxies like pre-absorptive
senses of different sensory food properties might support rapid
foraging decisions. Here, we show that nectar sugar concentration
(sweetness) and nectar viscosity (resistance) drive preferences of bumble
bees, Bombus terrestris, classical models for economic and
foraging decision making. Using a tasteless/odourless biopolymer (Tylose),
we created feeding options that differed in sweetness and resistance,
properties that affect energy intake rate and can be immediately sensed.
When energy intake rates were similar, bumble bees developed preferences
based on sweetness and resistance. When energy intake rates were
different, but sweetness and resistance were balanced against each other,
bees developed no preferences. Decision dynamics during training indicated
that bumble bees simultaneously evaluated sweetness and resistance to make
decisions quickly (in seconds). These results suggest that bumble bees’
food preferences are jointly affected by the immediate sensation of nectar
sweetness and resistance as positively and negatively reinforcing
properties, respectively, indicating a pre-absorptive proximate mechanism
for rapid energy sensitive decisions in bumble bees.