If you've ever watched a bumblebee move from flower to flower, you might wonder how they decide which flower to choose and how long to stay. Now, researchers reporting in the journal iScience on October 24 have new insight based on their observations of bumblebees' interactions with slippery artificial flowers. They found that the bumblebees make choices to maximize the rate of energy return, or the amount of sugar collected each minute.
"Bumblebees can make decisions 'on the fly' about which nectar sources are the most energetically economical," said Jonathan Pattrick, first author of the study and now based at the University of Oxford, U.K. "By training bumblebees to visit artificial slippery flowers and using different 'nectars' with high, medium or low amounts of sugar, we found that they could make a trade-off between the energy content of the nectar and how difficult it was to access."
In other words, bumblebees choose nectar sources that maximize their immediate return of sugar to the colony, instead of optimizing the energy efficiency of their foraging. That's different from earlier findings in honeybees, which show that honeybees maximize energy efficiency instead.
"Bumblebees have a different lifestyle to honeybees: they only store a small amount of nectar in the nest and so have to make the most of every available opportunity to forage," Pattrick said. "This difference between species may be why the bumblebees adopt a strategy such that their foraging is of more immediate benefit to the colony, even if it means they have to work harder, while honeybees take a relatively more measured approach to how hard they work – this should prolong their working lifespan."
"Identifying the currency used by nectar-foraging bumblebees helps provide a framework for understanding how bumblebees make foraging decisions," said Pattrick. "This information can be used to make predictions about the sorts of flowers the bees are likely to be visiting. In turn, this could inform choices of the flowers to plant in field margins and is also relevant to crop breeders who want to make varieties which are 'better' for bumblebees."
In work carried out at the University of Cambridge, UK, Pattrick and his colleagues used vertically and horizontally oriented slippery-surfaced artificial flowers to test whether bumblebees could make a trade-off between the difficulty in handling a particular flower and its nectar sugar concentration.
A custom computer program allowed them to measure the split-second timing of bumblebee behavior as they decided whether to collect high-sugar nectar from slippery flowers, which required them to expend energy hovering, or lower-sugar nectar from flowers on which they could land. Overall, they captured 60,000 separate behavioral observations, allowing them to precisely estimate foraging energetics. Earlier studies on bumblebee foraging currencies had been based mainly on watching what bees do naturally, making it difficult to distinguish how their decisions were being made.
The data showed that bumblebees chose flowers that were either hard to handle or had a low concentration of sugar, depending on which was more energetically favorable. Their decisions fit with a model in which their critical currency is the rate of energy return as opposed to energetic efficiency, the researchers report.
The researchers say future work should investigate whether bumblebees consistently use this currency, even when the foraging context changes. They are also fascinated by the insects' intelligence: "we find it amazing that, even with a relatively simple brain, bumblebees are able to make such complex energetic decisions," Pattrick says.
