The well-known words of Goldilocks—”Not too hot, not too cold. Just right!”—describe perfectly the way a bee hive must be maintained.

The ideal temperature for brood-rearing is about 94ᵒF, but for the adults the preferred range is broader.  In general, honey bees maintain the hive at ~91 to 98.6ᵒF during the active season. 

They monitor temperature, by the way, using sensory receptors on the first five segments of their antennae tips that can detect changes of less than a half a degree Fahrenheit, so they know when the mercury rises or falls (see Bee Biology, “The Bee’s Antennae” on this blog site).

Through both the architecture of the hive and specific behavior, the bees are able to warm the colony if the weather turns cold (down to minus 40ᵒF) and cool it when it is hot (up to 104ᵒF, or even higher). This allows Apis mellifera to live in environments from the coldest temperate zones down to nearly hot desert conditions. 

A wild or feral cavity nest typically has some natural insulation, such as thick wood surrounding a cavity in a tree trunk; the interior would be sealed with propolis against drafts and cold. In a domestic apiary hive, propolis is similarly used to close up cracks, gaps, or unwanted openings in the boxes. Honey bees usually keep the brood at the center of the hive and surround the larvae with honey- and pollen-filled cells, which along with the wax comb, confer a bit of insulation for the brood. 

Not Too Cold

Behaviorally, it is a cooperative effort of the colony members to raise the temperature within the hive during cold weather, but it builds fundamentally on the physical ability of the individual honey bee to keep itself warm. That is, a bee metabolically heats up its flight muscles in the thorax by “shivering” (rapid vibration of muscles), and the  heat is kept contained in this insulated, densely hair-covered section of the body by special features of the circulatory system.  

Thus, the bee is not only always ready and able to fly because the flight muscles are warm, but it also radiates heat from that little thoracic furnace. Certain workers (nurse bees) that are over two days old can function as “heaters” in two different ways: (1) by pressing the warmed-up thorax directly onto a capped brood cell to transfer body heat to the larva inside; or (2) by entering an empty cell among those containing brood and radiating heat in all directions—thus warming six surrounding brood cells at a time.

One theory describes that strategically placed among the larva-containing cells of the brood comb, 5% to 10% of the total cells are deliberately left empty for this purpose.

When the winter temperature drops to 59ᵒF, the bees will start to cluster in the hive.  They form a compact sphere of essentially three layers: at the center is the queen and any remaining brood, plus nurse bees that care for both. Next is a layer of heater bees and fanning bees. The heater bees maintain the core temperature at about 93ᵒF (or less than 85ᵒF if there is no brood), and the fanners disperse the heat and keep the air moving to regulate the amount of CO₂.

The dense outer “shell” consists of bees lined up side by side, close together and facing outward, acting as a shield to prevent heat loss from the cluster.   As the “mantle bees” forming the shell cool down, they are replaced by warm bees from the interior, and this rotation of roles and positions continues for as long as the colony is clustered. 

All this heat-generation requires a lot of calories, supplied by honey that is fed to the heater bees by hive workers freely moving around inside the warm sphere. The cluster can move up or down to reach honey stored in the hive, but typically not side to side. Without adequate honey (around 2 to 4 gallons = 24 to 48 lb) in the hive, the colony could starve over the winter. Also, if a cluster is too small in population to maintain an appropriate temperature, the colony is unlikely to survive the cold season.

Not Too Hot

An individual bee can cool down by holding a droplet of water or nectar on its tongue. That is, if the bee’s thorax heats up to 115ᵒF, it regurgitates a droplet of nectar from its honey crop, and through the evaporation of the droplet’s water content, cools the head immediately, and as a result, pulls heat out of the thorax through its coupled circulation.

Similarly, inside the hive, bees are supplied with water droplets for individual cooling by designated water distributors—-which receive their water supply from special water foragers (see Bee Biology, “Water Foragers” on this blog site).

Cooperative behavior for cooling the hive during hot weather begins with reducing the number of warm bodies inside the hive boxes.  During spring and summer days, foragers and drones are outside for most of the day, and during a very hot night, many bees remain outside, bearding on the outer front hive wall to keep themselves cooler and the hive uncrowded.

To actively cool the hive, bees ventilate it by fanning their wings once the air temperature exceeds 95ᵒF. These fanners are positioned throughout the hive (left, right, top, and bottom) to force air to circulate throughout the hive. As the temperature rises higher, the fanners cluster at the hive’s entrance and on the landing platform, moving warm air out and allowing cooler fresh air to passively move in, which also facilitates exchange of oxygen and CO₂ and controls humidity.

The final method of cooling is analogous to a “swamp cooler” in a building—that is, it employs evaporation of water drops distributed throughout the hive by water recipients starting from the hottest zones, such as the upper area under the lid where the heat is more intense, while fanners create cool, humid air and circulate it throughout the hive.

How do the water foragers know when to go collect additional water for the hive? They are informed by the water recipients that “beg” for water more and more intensely as the hive heats up. The foragers then fly to their sources and return carrying about 80% of their weight in water, filled up like balloons.

Dr. Tom Seeley describes the transfer: “The water carrier comes in looking really fat, and the water receivers start out looking very skinny. Over a minute when the transfer takes place, their forms reverse.”Although some water can be stored in cells of the comb, as is done by bees in the hottest, near-desert environments, it is in more temperate zones stored instead inside the honey crops of living bees that hang like “99 bottles of beer on the wall” in the hive during times of heat stress.

Just Right

The ability of honey bee colonies to regulate the climate within the hive is one of their most important attributes.  It allows them to remain physiologically active when other insects could not do so and might need instead to aestivate during an extremely hot and dry season, or to hibernate or overwinter in a pupal or protected egg state when the weather is too cold.  Our bees are able to emerge and fly on any winter day when the weather allows, and to leap into action at the first signs of spring when early willow pollen appears, after tirelessly heating and fanning through the chilliest months of the year. Their work will continue through the heat of summer, building their stores of honey and pollen to survive the next winter to come.

As Jamie Ellis wrote, “A single bee is a cold-blooded insect, but the honey bee colony is a warm-blooded creature, in which each individual bee is like a single cell of a complex organism.  It can make its temperature different from that of the surrounding environment, and this makes it (along with other similar social insects) rather special within the class Insecta.”

Not just “rather special.”   No . . . I would say they are “just right.”

Technical Reference

Randy Oliver, 2016, Understanding colony buildup and decline, Part 13a: The physics of the winter cluster, American Bee Journal, July 2016. Also available online: http://scientificbeekeeping.com/understanding-colony-buildup-and-decline-part-13a/.