Bees calculate energy efficiency to choose next flower

If you've ever observed a bumblebee moving from flower to flower, you might wonder how it decides which petals to pick, and how long it should stay. Now, researchers bee-lieve they have the answer. Jonathan Pattrick, from the University of Oxford, conducted the study...

Jonathan - We used artificial flowers and gave bees a choice of visiting these flowers when they were orientated at right angles to the horizontal, or orientated horizontally. Vertical orientation, that's much more difficult for the bees to visit, it takes a lot of energy, it takes a little bit more time. We had three different tests, so we put a little drop of sugar solution in the centre of each flower. In one, the sugar concentration of both the vertical and the horizontal flowers was exactly the same. And as you might expect, the bees very quickly switched to the horizontal flowers. In the second test, we made the sugar concentration of the vertical flowers much higher, so it was much more rewarding than the horizontal flowers. In this one, the bees persisted in visiting the vertical flowers. In the third test, we made the difference in sugar concentration between the horizontal and vertical flower smaller and, in this case, the bees switched from visiting the vertical flowers to visiting the horizontal flowers.

Chris - So they are able really to estimate how much energy they're burning to get that food reward. So they know their net better off by going the extra mile when it does matter in that way.

Jonathan - Yes, but this is where it gets a little bit more complicated because there are two really common foraging currencies you might assume a bee would use. One is the rate at which they can get nectar back to the colony, the second is how efficient they're being about their foraging. And a way of thinking about that is that they're trying to maximise the amount of energy they get out from foraging for any energy they put into foraging. Those two different currencies result in different expected behaviours on our setup and also different expected behaviours from bees in the wild as well.

Chris - We're talking about currency and energy here, but we're not actually sure that that's what the bees are doing because it could just be they like the taste, couldn't it? And that they're willing to go for something a bit sweeter and pay a higher price to get something a bit sweeter.

Jonathan - So the way that our experiment was set up, coming back to these two currencies, was such that visiting these vertical flowers, it took them a little bit longer. So that has a small effect on the currency of rate of energy return to the colony, but it was much more energetically expensive. So it has a big impact on the efficiency of their foraging and so, under those two currencies, what you would predict is that when there's a really large difference between the horizontal and the vertical flowers, if bees are foraging by optimising energy efficiency, they should have switched to the horizontal flowers, where if instead they're trying to optimise the rate at which they get energy back to the colony, they should stick to foraging on the vertical flowers. And because we saw that they did in fact stick to foraging on the vertical flowers, this gives us quite good evidence that this is how they're making their foraging decisions

Chris - Given how important bees are to agriculture, especially bumblebees with buzz pollinated crops that need a big bee like a tomato. Does knowing this now help us to improve the efficiency with which certain crops get pollinated?

Jonathan - So I think knowing the currency in particular is important because it gives us the framework through which bees should be making their foraging decisions. So it just gives you a much better idea about, when faced with a choice between different flower types, which ones the bees are going to be visiting. And yes, that should be useful for guiding choices on if you've got a choice between two varieties, one with a low nectar concentration and one with a high nectar concentration, can you get away with using the one with the lower nectar concentration or does the energetic situation of how you're using bumblebees to pollinate these crops mean that actually you'll get more pollination, a high yield from your crop, by choosing the variety with the higher sugar concentration.

Chris - Or engineering a crop that has a higher nectar concentration that makes it more of a lure for bees?

Jonathan - That would be an option as well. I've previously done some work where we were looking at nectar concentrations in the field bean and we found actually there was a huge variation in nectar concentrations among different varieties of bean. So I think probably in a lot of species that variation is there already and you'd be able to just use standard breeding techniques if you wanted to grow varieties that had a favoured nectar concentration.