The use of numerical information by bees in foraging tasks

The ability of invertebrates to perform nonelemental cognitive tasks is widely debated. Bees utilize the number of landmarks en-route to their destination as navigation cues, but their use of numerical information in other contexts is unknown. Numerical regularity in the spatial distribution of food occurs naturally in some flowers, which contain fixed numbers of nectaries. Nectar foragers on such flowers can increase their foraging efficiency by avoiding return visits to empty nectaries. This can occur if bees base their flower-departure decisions on the number of nectaries they had already visited. We tested, through field observations and laboratory experiments, whether bumblebees adapt their departure behavior to the number of available food resources. In bees that visited Alcea setosa flowers with 5 nectaries in the field, the conditional probability of flower departure after 5 probings was 92%. Visit duration, the flowers' spatial attributes, and scent marks could be excluded as flower-leaving cues. In the laboratory, bees foraged on 2 patches, each with 3 computer-controlled feeders, but could receive only up to 2 sucrose-solution rewards per patch visit. The foragers gradually increased their frequency of patch departure after the second reward. Patch-visit duration, nectar volume, scent marks, and recurring visit sequences in a patch were ruled out as possible sources of patch-leaving information. We conclude that bumblebees distinguish among otherwise identical stimuli by their serial position in a sequence and use this capability to forage efficiently. Our findings support an adaptive role for a complicated cognitive skill in a small invertebrate. Copyright 2011, Oxford University Press.