Bumblebee warm-up

Now that we have had two hard (dahlia-killing) frosts with daytime temperatures hovering in the low 40s all week, most of the insects have disappeared from the backyard.  I went out to photograph fall flowers, and found exactly one bee that was brave enough to venture out in the cold sunlight.

How is this little bumblebee able to fly around in 46 F (7.8 C) temperatures?

How is this little bumblebee able to fly around in 46 F (7.8 C) temperatures?

In fact, this bee doesn't really fly very well at all.  It crawled around on the petal-less flowerhead for a few minutes and then dropped down to a rock nearby.

In fact, this bee didn’t really fly very well at all. It crawled around on the petal-less flower head for a few minutes and then dropped down to a rock nearby.

I'm including this photo just to contrast the bee's posture with the photos below.  Note, wings folded over its back covering its hairy, black abdomen.

I’m including this photo just to contrast the bee’s posture with the photos below. Note, wings folded over its back covering its hairy, black abdomen, the typical bee resting posture.

The bee spread its wings to expose its black abdomen and re-oriented itself so the solar radiation was directly overhead.  Note  how it has positioned its rear legs, in parallel, under its abdomen.

The bee spread its wings to expose its black abdomen and re-oriented itself so the solar radiation was directly overhead.  This is a better posture for maximizing passive heat gain from the sun.

The bee seemed to enlarge the volume of its abdomen somehow, using its rear pair of legs to prop it up.  Wings were maximally spread, and there was just a bare fluttering of the hairs on the thorax.

The bee seemed to enlarge the volume of its abdomen somehow, using its rear pair of legs to prop the abdomen up. Wings were maximally spread, but there was no wing fluttering, like that you might see in moths as they warm up.

Bumblebees can fly at low air temperatures, but only if their thoracic temperature (where the wing muscles are located) is between 30 and 40 C (86-104 F).  To raise their thoracic temperature, they uncouple their flight muscles from their wings, performing a sort of an isometric exercise that generates heat but produces no visible action. However, according to information on the bumblee.org website, it would take 15 minutes for a bumblebee to raise its thoracic temperature from 6 C (like it was today) to 30 C.

In addition to internal heat production by flight muscles, it looks to me like bumblebees might have a secondary system of passive heat gain through the abdomen to speed up the warming process. By spreading their abdominal segments widely, a larger surface area of underlying black cuticle between the rows of hairs is exposed to the sun.

For more information on this topic, BBC Nature Wildlife has a set of videos on hot-bodied Bumblebees from their series on Life in the Undergrowth.

7 thoughts on “Bumblebee warm-up

    • Thanks, Mike. I enjoy watching animals and every now and then you get rewarded with something you’ve never seen before that just makes you wonder…

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