Social homeostasis in termite colonies
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Homeostasis is the tendency of living systems to regulate themselves at a particular state. It is usually cast in terms of organismal homeostasis, Claude Bernard's "la fixité du milieu intérieur ". A classic example of homeostasis is the regulation of body temperature. A mouse adjusts its production of heat from metabolism to maintain body temperature, even in cold environments where heat loss from the body is intense.

Social homeostasis is a phenomenon of social insect colonies, in which the collective activities of the colony's inhabitants regulate the colony environment. In a sense, this is precisely what the cells of an organism's body do when they coordinate their activities to regulate the body's internal environment, what one might call organismal homeostasis. Thus, there is no essential difference between social homeostasis and organismal homeostasis. I have written more extensively on this in my book The Tinkerer's Accomplice.

Social homeostasis is an interesting philosphical problem because it implies the colony is a 'superorganism', with a function, integration, and perhaps even a consciousness that supersedes the organism. This is an outgrowth of the idea, promulgated early in the 20th century by ecologists like Frederick Clements (right), that ecosystems also exhibit organism-like behaviors. The superorganism idea has fallen out of favor in ecology, but it has enjoyed more acceptance among students of social insects, in which homeostasis of colony function is a demonstrable phenomenon, susceptible to experimental manipulation and test.

Social homeostasis of Macrotermes colonies largely involves regulating the composition of the nest atmosphere and its water balance. Many would argue that the mound of Macrotermes is also involved in temperature homeostasis, regulation of the nest temperature. Our results indicate this is very unlikely - the mound is not, as it has sometimes been described, an "air-conditioner" for the nest. You can read more about this here.

What is homeostasis?
Social homeostasis in insect colonies
Social homeostasis in termite colonies


What is homeostasis?

Homeostasis is a fundamental property of living systems. Life cannot be understood without an appreciation of what it is.

In modern times, unfortunately, homeostasis has largely been deposed from its rightful place as a biological first principle. There are many reasons for this. Neodarwinism, for example has elevated natural selection among genes to be the prime organizing principle of life, and often treated other candidates as obscurantist pretenders. Physiologists have been their own worst enemies in this regard, preferring to explore limited operational questions about homeostasis, such as how temperature regulation works.

Homeostasis in fact has a much broader definition. It is the dynamic maintenance of the orderly stream of matter and energy we call an organism. This orderliness can take many forms--temperature gradients, solute concentration gradients, catalytic environments--that degrade through fluxes of energy and matter that are driven by the 2nd Law of Thermodynamics. These are called thermodynamically favored fluxes (TFF). To maintain the orderliness, energy must be mobilized as work to reduce local entropy--to restore heat, mass, or orderliness that is lost through TFFs. These are called physiological fluxes (PF). Homeostasis arises when the following rule is met: TFF + PF = 0. Living systems can only persist if they meet that rule.

This means that homeostasis can exist at any scale of biological organization, from the cellular to the biospheric. Individual organisms can exhibit homeostasis, but so too can assemblages of organisms that cooperate to maintain the dynamic orderliness that is characteristic of life.

You can read more about this in my book The Extended Organism

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Social homeostasis in insect colonies

William Morton Wheeler (right) pioneered the idea of the insect colony as a superorganism, with coordinated function and physiology that cannot be explained by the capabilities of the individuals by themselves. Since Wheeler's original work, numerous studies have shown that the colony environment, usually temperature, of ants, termites, and bees is relatively steady compared to the outside environment.

Mere steadiness of a property is not homeostasis, however. One of the problems that has plagued the superorganism concept has been the inability to distinguish "true" homeostasis - the active regulation of a property - from mere inertia. A large pile of dirt has a steady temperature but it is not homeostatic: no work is done to maintain the temperature.

Recently, the superorganism concept has enjoyed a renaissance among students of the social insects. Honeybee colonies, for example, show coordinated behaviors in which the individuals play specific roles in the assemblage that regulates temperature in a hive. When bees cluster in the hive during winter, for example, the warm core is sustained by layers of bees that generate heat, or link their bodies together to insulate the assemblage.

Respiration in honeybee colonies also is a phenomenon of social homeostasis. Ventilation of bee hives is tidal: air alternately enters and leaves, as it does in our own lungs. In a hive, ventilation is driven by bees that station themselves at the hive entrance and fan in place. This fanning is coordinated among the bees: they alternately fan together, or cease fanning for a time. When the bees fan in place, spent nest air is driven out of the hive entrance. As they fan, they do work against the buoyancy of the warm and moist air. When the bees rest, the warm buoyant air in the nest rises, drawing fresh air into the nest. The rate of ventilation is set by the cycle time between fanning and resting. Just as high CO2 in the blood makes us breathe faster, high CO2 concentration within the nest increases ventilation rate.

Among termites, studies of social homeostasis have focused on how structures built by workers act as "air conditioners", devices to regulate temperature. As you can see here, it is unlikely that temperature is the regulated property. Social homeostasis among termites is directed more to regulating the nest atmosphere, the concentrations of carbon dioxide, oxygen, and most importantly, water.

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Social homeostasis in termite colonies

In the 1960's, the Swiss entomologist Martin Lüscher proposed an ingenious mechanism for social homeostasis in colonies of Macrotermes natalensis. In a nutshell, Lüscher proposed that the mound and colony comprise a colossal heart-lung machine, driven by the colony's considerable heat production (roughly 55-200 watts). In Lüscher's conception (right), the heat produced by the colony's metabolism imparts buoyant forces to air in the chimney. This buoyant air is lofted upwards to the top of the mound. As this spent air accumulates, it is forced to percolate downwards through tiny surface channels in the surface of the mound. Here, oxygen, carbon dioxide, heat and water vapor in the spent nest air are exchanged with the fresh outside air. Because the density of air in the mound increases as a result of this exchange, gravity draws the air forcibly downward into a space, the cellar, that opens beneath the colony. Once there, the air ready to be powered on another circuit through the mound.

Lüscher's elegant scheme provided a mechanistically sound model for social homeostasis. It is even self-adjusting to a degree: more heat imparts stronger buoyant forces to the nest air, prompting a faster circulation of air.

Martin Lüscher's model is more beautiful than correct, however. The Macrotermes michaelseni colony exhibits social homeostasis, but it does not work quite the way Martin Luscher thought it did.

For a detailed discussion of how the mound really works as a gas exchanger, go here.

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