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A male bumblebee about to alight on an alumroot. Photo: <a href="">Sjjubs</a>, Creative Commons, some rights reserved
A male bumblebee about to alight on an alumroot. Photo: Sjjubs, Creative Commons, some rights reserved

How bumblebees keep warm

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Bees need to be warm in order to fly. That's usually not a problem, since it takes millions of round trips to flowers to make a pound of honey. But should they fall idle long enough to cool down, bees fire up their wing muscles by shivering. Dr. Curt Stager and Martha Foley, with more about bees.

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Martha Foley: How many times would you say we have talked about bees? I mean we have talked about bees, we have talked about ground bees, we have talked about honeybees, bees, bees, bees. There is always something more to know about bees.

Curt Stager: They are cool.

MF: They are cool, and they are all around, and they are easy to watch because there they are. If you are not messing with them, they will just be there for you.

CS: Well actually, rather than bee "cool," what really got me recently was that they are warm.

MF: Oh yeah?

CS: Bumblebees are, you could say, warm blooded. Their body temperature in their thorax, that middle segment of their body where their wings and legs are attached, is basically the same as human body temperature inside their body.

MF: How do they get warm? They are not wearing down jackets or anything.

CS: Well they kind of are; they are wearing fur coats. Right? Because they are fuzzy. You know that gives them their color, and I guess if you want to pet them, it’s nicer to pet them, but that is not why they have fur. It’s insulation, of course. You do see them as some of the first bees that are out in the spring when it’s still chilly out.

MF: And they shiver, too?

CS: Yeah, they actually shiver, and you can hear them when they shiver—it’s that buzzing sound. Sometimes the buzz is the sound of their wings, but you can sometimes watch and see that their wings are not moving and hear a buzz. That’s the muscles inside their thorax that run their wings. They can just hold their wings steady and have their muscles vibrate.

MF: Kind of idling?

CS: Idling, yeah. That’s it, and it makes a buzzing sound and also generates heat, so they stay warm. In fact, that’s actually sometimes a problem for bumblebees. In order to fly they have to have their muscles warmed up, just like a car engine, so there can be a problem sometimes if they are not just trap-lining from one flower to the other, but they end up going to flowers that have lots of blossoms on them.

MF: And they just sit there for a while and get cold?

CS: Yeah, like goldenrod which has lots of blossoms on it, it’s a great bonanza for them, but after a while they have been sitting there gorging goldenrod nectar and now they are too chilly to take off. Now they have to sit there and buzz for a while before they can take off.

MF: Wow.

CS: So yeah, you can pick these things up and they are not as defensive as honeybees would be with the hive to protect. Every one’s got their own little burrow. So I pick them up and hold them in my hands sometimes and I can feel them buzz, so they are also shivering to warm up.

But I noticed one thing this year, the females have their little burrows and they are usually digging the burrows or out foraging to get pollen to lay their egg on in the burrow. But when they are taking a break, they are sitting there in the entranceway with their head and you could say shoulders visible, and the rest of them in the burrow. You walk up and you look in and they will duck back in their burrow for a while and then come out.

Recently I was noticing—it was a warm sunny day—I was looking at one of the females in her burrow and I felt the warm sun on the back of my neck and I thought, "It’s getting a little warm." And then I realized that her head must be getting a little warm, too. Then I looked closely at her shoulders, and for the first time I noticed that there is really not a lot of fur on the part of her back just behind her head. That’s the part you see and it’s a dark color, so it’s (I’m speculating that it could be) a little solar warming patch right where her flight muscles are, that’s facing out of the burrow where her head is, too.

MF: Collecting the solar heat.

CS: It’s a solar bee.

MF: Or it could be a tan.

CS: Or it could be getting a tan. But that would be great, because she is sitting in this cool, dark burrow, but she could keep her flight muscles warm and just take off whenever she wants to.

MF: I want to ask you for another little factoid about bees. How much work does it take to make honey? Say a jar of honey—what’s that represent in bee labor?

CS: I was able to find that little factoid, and kind of like the hard work of turning maple sap into maple syrup, this work has to be done by the bees. The nectar that they get from the flowers is 15 to 75 percent sugar by weight; honey is like 80 percent, so they have to get rid of that water. But to get that much nectar to make a pound of honey, one calculation says they have to do 17,000 trips to go out foraging among the flowers, and each trip required visiting 500 flowers. So, to make a pound of honey, basically it's 8 to 9 million flower visits.

MF: That is amazing.

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