Studying ants to find out how colony size affects patterns of behavior, energy use

Studying ants to find out how colony size affects patterns of behavior, energy use
Arizona State University researchers James Waters and Tate Holbrook seek to discover how size affects the organization and physiology of superorganisms such as bacterial communities, insect colonies or human cities. Here, three queens along with brood and a young worker ant of the pleometrotic California seed-harvester ant, Pogonomyrmex californicus. Credit: James S. Waters

( -- How does size affect the organization and physiology of superorganisms such as bacterial communities, insect colonies or human cities? James Waters and Tate Holbrook, graduate students in the School of Life Sciences at Arizona State University, work on answering this question by studying how colony size affects the patterns of behavior and energy use in ant colonies.

Social insect are excellent study organisms because, despite the lack of either physical connections between individuals or any kind of centralized control system, the whole colony can exhibit impressive feats of organization including the division of labor, extensive foraging networks and elaborate nest architecture.

For their studies, Waters and Holbrook mainly focus on the California seed-harvester ant, Pogonomyrmex californicus. Queens of this species can be collected following the ants' annual mating flights and brought back to the lab to start new colonies. Within a month or two, eggs laid by the queens develop into larvae, pupae and adult workers. Over the course of a year, the colonies may grow as large in size as to include 1,000 ants.

Studying ants to find out how colony size affects patterns of behavior, energy use
Metabolic rates of whole ant colonies were estimated by measuring oxygen consumption and carbon dioxide production after placing artificial colony nests within a custom-designed respirometry chamber. Credit: James S. Waters

One question that interested Waters is whether colonies become more efficient as they get bigger. The first step in figuring this out was to estimate the power demands of the colonies as a function of their size. One way to think of an animal is as an engine that burns oxygen as a fuel to power all of life's processes, from to communication. Waters used a tool called respirometry to measure the amount of oxygen being consumed by entire colonies as they breathed within special chambers.

While larger colonies obviously needed more energy overall compared to smaller colonies, as colonies grew, they surprisingly needed less energy per ant. When groups of ants were taken out of their colonies, however, they all required the same relative amount of energy. These patterns suggest that there is something special about being in the environment of the colony that regulates energy use by individual ants, and this energy use changes, or scales, with the size of the colony.

The scaling of may be associated with colony size-related changes in behavior. An important behavioral pattern in colonies of ants and other social insects is the division of labor--when different workers specialize in different jobs, like brood care and foraging.

Holbrook investigated how colony size influences division of labor in P. californicus. First, he carefully painted ants with unique color combinations so he could identify individual workers within each colony. He then watched colonies of different sizes for many hours, recording which ants performed which jobs. Holbrook discovered that as colony size increases, so does division of labor. In smaller colonies, individual workers perform a variety of jobs, but in larger colonies, workers tend to specialize in specific jobs. It remains to be tested whether higher division of labor makes larger colonies more efficient.

The studies of Waters, Holbrook and their colleagues indicate that colonies of seed-harvester , and probably other social insects as well, are more than the sum of their parts. Social interactions between colony members give rise to colony-level properties that vary with colony size and shape the physiology and behavior of individuals.

These results may extend to broader contexts, including the regulation of cells within organisms and the organization of individuals within societies. In fact, these studies question the very nature of what it means to be an individual. If the basic biology of a single ant is so strongly influenced by the composition of the colony in which it lives, perhaps the ant is not itself an individual so much as it is a part of an entity existing on a higher level of biological organization, the superorganism.

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Jan 27, 2011
Very interesting research. I wonder what factors determine how ants are specialized after birth, and how individuals determine what tasks to perform for the benefit of the colony. If ants become more specialized as the colony grows, does it mean there is some sort of delegation going on between the insects on who does what or do they just figure it out individually. It's fascinating that a massive ant colony can be efficient and everyone knows what their role is, without overdoing one thing or the other.

Jan 27, 2011
Dear PlasticPower,
Your comment brings up so many interesting questions, from the temporal development of task specialization to how decisions are made within colonies... there are surely many different answers. In some ant colonies, genetic diversity can play a major role in determining the kind of tasks workers will later go on to do. In other colonies, workers can manipulate how the brood develop in an environmental (i.e., non-genetic) fashion to affect how they will grow and what kind of tasks they may ultimately do. (continued...)

Jan 27, 2011
... And decision making is such an exciting field too -- there's some really interesting work on quorum sensing and how ants communicate about making decisions being done with Temnothorax ants (e.g., by Stephen Pratt's lab at ASU among others). One of the major themes underlying all of this is the idea of self-organized complexity. It's possible that given a certain set of relatively "simple" rules... e.g., ones that each worker follows, that by the way they all interact, you can generate some really impressive patterns... from the kind of metabolic scaling we found to some really neat nest architecture and behavioral division of labor. There's so much more to learn, but it's a fun process!

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