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Metabolic rate is a measure of the "fire of life" burning within an organism – the spark that enables it to breathe, run, reproduce and live fully. While it is commonly accepted by scientists that this rate scales according to an animal's size – for example, a mouse has a relatively higher metabolic rate than an elephant – a century-old puzzle remains: what controls this relationship? Scientists at Arizona State University have cracked a window into this long-standing physiological conundrum – the role that social evolution plays in an individual's energy equation.
The study, published in The American Naturalist, investigates the scaling of metabolic rate in whole colonies of the seed harvester ant Pogonomyrmex californicus. The authors, graduate students James Waters and C. Tate Holbrook and professors Jennifer Fewell and Jon Harrison, discovered that the metabolic rate of ant colonies could not be accurately predicted by simply summing up the metabolic rates of individuals within the colony. More surprisingly, the researchers, with ASU’s School of Life Sciences and the Center for Social Dynamics and Complexity in the College of Liberal Arts and Sciences at ASU, went on to find that larger colonies consume less energy (per-mass) than smaller colonies. These findings suggest not only that metabolic rate scales with colony size, following the same pattern it does with body size of individual organisms, but also that colony size influences patterns of behavior and the amount of energy individual ants expend.
The answer to the question of how size relates to metabolic output has the potential to revolutionize thinking throughout biology, from understanding the evolution of animal body sizes to the development of methods for using medication in clinical treatments.
Waters, lead author and doctoral candidate in the laboratory of Harrison in ASU’s School of Life Sciences, points out that "it's hard to figure out, on a mechanistic level, why size affects metabolic rate because it's not easy to change an animal's size. With a colony though, it's as easy as adding or removing individual ants … and since these super-organisms show the same scaling pattern as individual animals, the basic energy regulation mechanisms could be the same, either for distributed systems like these colonies or more physically connected systems such as individuals made up of many cooperating cells."
Waters and his coauthors suggest that social insect colonies could be used as model systems to test hypotheses about the fundamental regulatory networks that influence metabolism across levels of biological organization, from cells to societies.