a Pogonomyrmex worker contemplates the superorganism

Ecologists have long known that an animal’s metabolism is related to its size in a rather predictable way. Large animals process more energy than small animals, but they do so with an efficiency such that, pound for pound, they use less. A cow may weigh as much as 15,000 mice, but an ounce of cow muscle has a lower metabolism than an ounce of mouse muscle. Kleiber’s Law describes the relationship: metabolism normally scales at 3/4 the rate of body mass.

One way to evaluate the controversial superorganism concept– the notion that an insect colony is more like an organism itself than just a sum of many individuals- is to ask whether colonies follow Kleiber’s law. That is, as ant colonies get bigger does the relationship of metabolism to mass scale at the slower 3/4 (like an organism), or at an isometric 1:1 (like an aggregate of individuals)?

This week, a team of myrmecologists at Arizona State University led by James Waters published a clever test in the journal American Naturalist.

Waters created fake Pogonomyrmex “colonies” that were aggregates of single ants, measured their metabolic rate at difference sizes, and compared their scaling with that of functioning colonies. Sure enough, the aggregates scaled at 1/1, while the colonies scaled at the organismal 3/4 . This finding complements, in greater detail, an earlier paper by Hou et al (2010) that more broadly surveyed social insect species. Here’s the abstract:

The negative allometric scaling of metabolic rate with body size is among the most striking patterns in biology. We investigated whether this pattern extends to physically independent eusocial systems by measuring the metabolic rates of whole functioning colonies of the seed-harvester ant Pogonomyrmex californicus. These intraspecific scaling data were compared to the predictions of an additive model developed to estimate collective metabolic rates. Contrary to the prediction of the additive model, colony metabolic rate allometry resembled the pattern commonly observed interspecifically for individual organisms, scaling with colony mass0.75. Among the same-aged colonies, net growth rate varied by up to sevenfold, with larger colonies exhibiting higher net growth efficiency than smaller colonies. Isolated worker groups exhibited isometric metabolic rate scaling, suggesting that the social environment of the colony is critical to regulating individual patterns of work output. Within the social environment, individual worker locomotor velocities exhibited power-law distributions that scaled with colony size so that larger colonies exhibited a greater disparity between active and inactive ants than did smaller colonies. These results demonstrate that behavioral organization within colonies may have a major influence on colony-level metabolism and in generating intraspecific variation in growth trajectories.

As far as energy and harvester ants are concerned, the Superorganism appears to hold.

Incidentally, it seems I picked the wrong week to quit sniffing glue. This has been an enormous week for ant science and I can barely keep up. Can’t you myrmecologists take a break for a few days and let me regroup?

source: Waters, J.S. et al 2010. Allometric Scaling of Metabolism, Growth, and Activity in Whole Colonies of the Seed?Harvester Ant Pogonomyrmex californicus. American Naturalist DOI: 10.1086/656266