How Honeybees Keep Their Cool

How Honeybees Keep Their Cool

The environment temperature keeps changing.

Why is that a problem?

The structures of many of the body’s enzymes and regulatory proteins rely on weak chemical bonds. 

A change in temperature  can change their shape and disturb their function.

Only a few degrees have a significant impact on metabolism.

Big temperature changes can be fatal.

Terrestrial organisms are endotherms or exotherms:

Endotherms like birds and mammals maintain their body temperature stable (within a very narrow range).

Some invertebrates are also endotherms. 

Maintaining a stable body temperature uses up 90% our food intake.

So, since it's so energy-expensive many organisms are ectotherms. Ectotherms can deal with different body temperatures adjusting their activity and metabolism accordingly.

When the weather gets colder butterflies immediately settle to the ground and wait for the sunlight they need to keep
warm enough to fly.

Not the bees. 

Bees are one of the few invertebrates that are endotherms. 

They keep their body temperature relatively constant.

How does the bee—or any endothermic animal—generate
the heat needed to warm itself?

About 75% of the energy of the chemical bonds in an animal’s food is becoming heat during metabolism anwyay. 

So, no we alter the rate of those metabolic reactions to produce less or more heat and regulate our temperature.

When a bee wants to fly, it first shivers violently
for several minutes.

This burning of glucose generates the heat needed to become airborne.

Then, the flight muscles generate enough heat to maintain the bee’s body temperature good enough to fly.

What happens if there's too much sun?

The butterflies land to shady spots, avoiding sunlight. 

But bees keep flying!
How do they avoid overheating?

Researchers studying bumble bees thought they dissipate the extra heat from their large, un-insulated thorax & abdomen that presumably radiates heat very effectively.

In effect, the bumble bee has an endothermic thorax and an ectothermic abdomen which acts as a heat dissipater.

However,  transfer of heat between thorax and abdomen doesn't happen in flying honeybees.

So how do honeybees keep cool while flying?

In stationary animals, thermoregulation happens changing the amount of metaboling heat (like when shivering)

That mechanism, scientists thought, is not unavailable to flying animals. 

In flights massive amount of energy is needed.

The so far idea was that the metabolic costs of flight are determined by animal’s weight, wing area, flight speed, etc. 

How could a flying animal vary metabolism for thermoregulation WHILE flying?

The Observation

An experiment comparing flight metabolic
rates of African and European honeybees fount that:

Relatively small increases in air temperature were correlated with substantial decreases in flight metabolic rate and wingbeat frequency.

That suggests that honeybees may be able to vary their
metabolic rate changing flight muscle performance. And that's their way of thermoregulation. 

However, the observed correlation may have other explanations.

Perhaps bees that fly in the afternoon, when it is warm, are simply a different age or genetic makeup than bees that fly in cooler weather.

We need a manipulative experiment to rigorously test the hypothesis.

The ExperimentDr. Harrison and his team randomly exposed honeybees to different air temperatures between 20 to 40 oC.

They measured 1. thorax and abdominal temperature, 2. metabolic rate, and 3. wingbeat frequency.

Body temperatures were taken with a tiny, fastresponding microprobe.

Metabolic rates were measured by flowing air through the bee flight chamber at a known rate, and measuring the release of carbon dioxide by the bee.

Wingbeat frequency was measured using a microphone,
tape recorder, and sound-editing software.

The Results

Thorax temperatures varied much less than air temperature. 

Clearly, theses honeybees did not thermoregulate like bumblebees.

Can flying honeybees vary metabolic heat production
with air temperature?

Flight metabolic rates decreased 40 to 50% in bees flying in 40 C air compared to 20 C air (graph).

Honeybees maintain warm, constant thorax temperatures by producing lots of metabolic heat at cool air temperatures, and much less metabolic heat when flying in warm air.

How can honeybees vary metabolic heat production and
still hover?

Harrison's team found that the wingbeat frequency of the hovering honeybees fell by 16% as air
temperature increased.

That suggests that honeybees may vary flight muscle power to vary heat production. Something comparable shivering in mammals.

Bees in colder air use more metabolic fuel and produce more heat because the flight muscles work harder and use more ATP. 

We still more experiments to rigorously test this remarkable mechanism.