The Occasional Hymenoptera: Honey bee learning related to circadian rhythm

Honey bees learn best in the morning, and the ability is related to circadian rhythm.

Marina Lehmann and two colleagues in the Neurobiology Department of the University of Konstanz, designed an experiment to investigate the influence of circadian rhythmicity on olfactory learning performance in honey bees (Apis mellifera). The result is in the paper “The early bee catches the flower–circadian rhythmicity influences learning performance in honey bees, Apis mellifera,” published (online July 31, 2010) in Behavioral Ecology and Sociobiology (open access:  HTML, pdf). The experiment involved indoor and outdoor bees and used both natural and artificial light and darkness.

They showed that bees’ olfactory learning is “remarkably” better in darkness (in dim red light) than light. “Possibly, it is just the effect of reduced distracting optical stimuli. Alternatively, or additionally, this might originate from honey bee biology: most of a bee’s life takes place inside the dark hive where odours are a main source of information, including communication between workers and brood … as well as the queen and the workers …. Additionally, odours are used by homing foragers to advertise profitable food sources …. Thus, the better learning performance for olfactory cues in darkness (irrespective of the time of day) could be an adaptation to living within the hive.”

They find that bees learn better during the day than at night and that during the day, they learn best in the morning. “This effect is clearly linked to the circadian rhythm. Our experiments, however, were not designed to evaluate the mechanistic origin of the effect. Thus, an effect of fatigue, of sleeping cycle, or of a direct link to the endogenous clock is possible, without being mutually exclusive. For example, sleep deprivation also affects memory, though its effect is complex with differences between acquisition and retention …. The role of an endogenous rhythm is evidenced by our finding that bees kept in constant darkness also learn better in the morning.”

Although they showed (consistent with other studies) that the circadian rhythm of bees is shorter than 24 hours (as is the length of day for locomotor purposes), they were unable to say whether this has to do with the mechanistic origin of the learning differences. “[A]n effect of fatigue, of sleeping cycle, or of a direct link to the endogenous clock is possible, without being mutually exclusive. For example, sleep deprivation also affects memory, though its effect is complex with differences between acquisition and retention ….”

The authors confirm that this learning is due to circadian rhythm rather than a response to when food is available. (Most nectar sources are more plentiful in the morning.) They did this by training bees to visit particular feeders at particular times of day. But even when trained, the bees showed better learning in the morning. There is also a higher level of foraging activity for bees trained to a morning source compared to those trained to an afternoon source.

The authors speculate that the relation between circadian cycle and learning differences is related to energy allocation “under the assumption that high learning performance is energetically costly …. In this view, the need to learn new odours would be reduced during the afternoon because there are fewer flowering plants at that time. It was shown … that most flowers … blossom and secret nectar and pollen during the morning… . This is also true for aphid sucrose secretion, the so-called honey dew, which is an important food source for bees … . We therefore hypothesise that the increased learning performance has co-evolved with the plant’s nectar and pollen secretion.”

The experiment, however, showed that the bees always arrived at the training sessions before the scheduled feeding time. This suggested an adaption to favor “early” bees “to outcompete possible competitors such as butterflies, flies and conspecifics of other hives. Thus, bees and flowers may have evolved an arms race for earlier times of day, bounded by dawn on one side and by productivity on the other.”

If there is a tight plant-pollinator relationship (as would be expected given the time that specific plants and pollinators coexisted) the authors suggest that the pollinators of plants that have peak nectar secretions as night (such as Silene spp. and Saponaria officinalis) would have better learning performance at night. The pollinators of these plants are nocturnal butterflies and hawk moths. The authors suggest Autographa gamma as the candidate for testing the hypothesis, since it has already been shown to have high appetitive learning ability.

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