Behavioral maturation and learning in the bumblebee Bombus occidentalis (Hymenoptera: Apidae)

Age-dependent division of labor is a common feature of social organization in the honeybee Apis mellifera. In this species, young workers (nurses) perform duties inside the nest, whereas their older sisters (foragers) perform tasks outside the nest. Transitions between tasks rely on cognitive abilities such as learning and memory and on maturation of the nervous system. For instance, performance of associative tasks is significantly better in foragers than in nurses or one-day-old bees (Ichikawa and Sasaki 2003 Appl. Entomol. Zool. 38:203-209). Unlike honeybees, bumblebee colonies are characterized by a wide variation in body size of workers. Large workers usually forage whereas small bees serve as nurses. Here, we test whether learning abilities in an olfactory conditioning paradigm correlate with age or body size in the bumblebee B. occidentalis. We trained workers of different sizes and ages (one day to four weeks old) to associate an odor and a sugar-water reward in the proboscis extension reflex paradigm (up to 20 trials). Once established, memory retention was tested after two and five hours. We did not find any statistically significant correlation of learning ability with age or body size. However, we did find increased memory retention in larger bees (p< 0.05). Our data suggest a correlation between a species' division of labor and cognitive abilities. Bumblebees and honeybees differ in their social structure and this difference is clearly reflected at a cognitive level. In contrast to honeybees, young workers of B. occidentalis are mature enough to forage. Moreover, larger bumblebees have a longer memory span than small ones, which is advantageous for foraging, their preferred task. We conclude that in both, A. mellifera and B. occidentalis, learning abilities of the worker caste are correlated with division of labor and probably reflect the cognitive challenge of different tasks.