No detectable evidence for metabolic costs of long-term memory formation in honeybees, despite increased energy intake

The brain is energetically expensive. Energy availability may, therefore, determine whether costly cognitive processes such as long-term memory can be expressed. However, there is a limited understanding of the metabolic costs associated with long-term memory formation. Here, we explored the potential induced costs of long-term memory formation using honeybees (Apis mellifera) as a model species. We monitored the sucrose intake of bees over the 20-hour period following a classical spaced olfactory conditioning protocol that induced long-term memory formation, relative to a control group that experienced the same reward schedule but no odour pairing. Bees in the experimental treatment drank significantly more sucrose than controls. We then tested whether the increased energy demands of long-term memory formation showed parallel increases in metabolic rate, by measuring carbon dioxide production in groups of bees at four timepoints following conditioning (1-hour, 4-hours, 24-hours and 72-hours). We found no change in metabolic rate between learning and control groups across all time points, suggesting that long-term memory formation does not impact metabolic rate to an extent that is detectable by our group metabolic rate protocol. While our findings point to dietary costs associated with long-term memory formation, any metabolic consequences may operate at a resolution below that detectable in group-level analyses and may be more effectively examined using individual or cellular-level energy flux approaches.