Bumblebees prioritise getting as many calories as possible in the shortest amount of time, new research suggests.
According to the findings, the insects trade off the time they spend collecting nectar with the energy content of that nectar.
The most common bumblebees in the UK, Bombus terrestris, will forage to collect nectar that is difficult to access – but only if the sugar content of that nectar makes it worth doing so.
This approach contrasts with honeybee foraging – honeybees make their decisions by optimising their individual energy expenditure for any nectar they collect, researchers say.
This more measured approach should prolong the honeybee’s working life.
As they forage, bumblebees are making decisions about which nectar sources will give the greatest immediate energetic return, rather than optimising the energy efficiency of their foraging— Dr Jonathan Pattrick, University of Cambridge
Dr Jonathan Pattrick, joint first author of the report, who started the research while in the University of Cambridge’s Department of Plant Sciences, said: “As they forage, bumblebees are making decisions about which nectar sources will give the greatest immediate energetic return, rather than optimising the energy efficiency of their foraging.”
Now based at the University of Oxford, he added: “Our results allow us to make predictions about the sorts of flowers the bumblebees are likely to visit, which could inform the choice of which flowers to plant in field margins to support these important pollinators.
“It’s also relevant to crop breeders who want to make varieties that are ‘better’ for bumblebees.”
Over six months 60,000 behavioural observations were made of the bumblebees, allowing researchers to estimate the energy they used while foraging.
Researchers watched each bumblebee in the study for up to eight hours a day, without taking a break.
The study used vertically and horizontally oriented artificial flowers, slippery surfaces that were difficult for the bumblebees to grip.
A custom computer program was used to measure the split-second timing as the bumblebees flew between the artificial flowers and foraged from them.
Researchers tracked how much energy the creatures spent flying, as well as how much they collected when drinking.
They also identified whether the bumblebees decided to spend extra time and energy collecting high-sugar nectar from slippery flowers, or take the easier option of collecting lower-sugar nectar from flowers they could land on.
It’s amazing that, even with a brain smaller than a sesame seed, bumblebees can make such complex decisions— Dr Hamish Symington, University of Cambridge
Dr Hamish Symington, from the University of Cambridge’s Department of Plant Sciences and joint first author of the report, said: “It’s amazing that, even with a brain smaller than a sesame seed, bumblebees can make such complex decisions.”
He added: “It’s clear that bumblebee foraging isn’t based on a simple idea that ‘the more sugar there is in nectar, the better’ – it’s much more subtle than that.
“And it highlights that there’s still so much to learn about insect behaviour.”
Individual bumblebees were given one of three tests.
In the first test, the nectar on both vertical and horizontal artificial flowers had the same amount of sugar.
The bumblebees made the obvious choice to forage from the horizontal flowers, rather than spend extra time and energy hovering at the vertical ones.
In the second test, the nectar on the vertical flowers was much more sugary than the nectar on the horizontal flowers.
Here the bumblebees chose to drink almost exclusively from the vertical flowers.
In the third test, the vertical flowers offered nectar that was only slightly more sugary than the horizontal flowers.
In this situation the bumblebees had to make a trade-off between the time and energy they spent foraging and the energy in the nectar they were drinking – and they switched to feeding from the horizontal flowers.
According to the findings, bumblebees can choose to spend additional time and energy foraging from hard-to-access nectar sources – but only if the reward is worth it.
Bumblebees offload nectar from flowers – by regurgitation – for use by others in the nest.
Unlike honeybees, they only store a small amount of nectar in the nest, so they need to make the most of every opportunity to forage.
– The research, funded by BBSRC, is published in iScience.
