Ant Colonies & Metabolic Scaling, Part 2

In a recent post, Dr. Dolittle reviewed some of our recent findings about the metabolic rate of ant colonies. We focused on the harvester ant, Pogonomyrmex californicus, a species with wide geographic distribution across the southwestern United States:
Ant 1.jpg
If you are interested, there is a large collection of Pogonomyrmex resources available online thanks to Bob Johnson and the Social Insect Research Group at Arizona State University. From this website, you can find out about how to identify seed harvester ants, find detailed range maps, and also learn how to culture colonies in the lab.

Although the majority of ant species have colonies with a single queen, one of the interesting features of Pogonomyrmex californicus is that some populations are pleometrotic, that is, they have multiple queens even at mature colony sizes. Here is a photograph of a large group of recently mated queens who are collectively beginning to excavate a nest in Pine Valley, San Diego:
ant 2.jpg

When we started to try to understand how the metabolic rate of ant colonies should depend on their size, we put together a model that added up the individual metabolic rates expected for each queen, worker, larvae, and pupae within the colony. This model predicted an isometric scaling relationship. When we measured the colony metabolic rates however, we found an allometric relationship, as you can see by the two lines in this plot of metabolic rates as a function of colony size:
ant 3.jpg
The non-linear scaling pattern we found for the colonies (with the ¾-power exponent) means that although larger colonies burned more energy than smaller colonies, they didn't burn as much on a mass-specific (or average per-capita) basis. When you take the log-transform of both axes, this curved nonlinear relationship becomes effectively straight, and what used to be the exponent becomes the slope of the line. And surprisingly, insects, mammals, and social insect colonies all show very similar relationships between body size and metabolic rate:
ant 4.jpg
How can we explain these scaling patterns? Many prominent theories attempt to explain the so-called "universal" scaling of metabolic rates with body size based on how the geometry of organisms (and their circulatory systems) change with size. One characteristic that was interesting to us was how ant behaviors change with size, so we set out to see if the ants change their patterns of movement depending on colony size. We did this by paint-marking ants and digitally tracking their coordinates:
ant 5.jpg
We found that the distributions of activity changed with colony size in an interesting way. While overall variance in walking speeds decreased with increasing colony size, the distributions became increasingly skewed. This suggests to us that the division of labor within colonies may change with colony size, much like the division of labor between cells changes as organisms grow during development and across evolutionary time scales.

You can see a video of Pogonomyrmex californicus colony-founding.

You can also see a video (with music) of ant colony activity in the lab:

Click here to read our published findings.
James S. Waters, C.T. Holbrook, J.H. Fewell, and J.F. Harrison (2010). Allometric scaling of metabolism, growth, and activity in whole colonies of the seed harvester ant, Pogonomyrmex californicus. The American Naturalist 176(4).

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Interesting stuff.
How many ants in a 1-g "colony"?
What is the mass of real colonies?
Does the metabolic rate of a group of ants depend on the area or volume in which they are enclosed?

By Sven DiMilo (not verified) on 08 Dec 2010 #permalink

Dear Sven, thanks for your comments! The average worker weighs about 4mg, so a 1-g colony may have 250 workers. Some of the colonies in our study had over 600 workers. This is a moderate size for the colony, but it is still in the "ergonomic" growth stage as discussed by Wilson & Holldobler and is pre-"mature" at which stage it would be producing sexuals. In our paper we showed that the metabolic rate of worker groups (i.e., not colonies) scaled isometric with colony size, and also showed no effect of area or volume. Anna Dornhaus' lab has shown that space may affect worker group energetics in Temnothorax ants.

One of the neat things about publishing with AmNat was their suggestion to deposit the raw data for our work in an online database. We obviously deposited all of the model and metabolic rate data, but we also included the colony demographics (breakdown of castes with colony sizes). Feel free to check it out, it should be available to everyone as an Excel Workbook without needing to pay or log in or anything complicated: