Group Size, Scaling of Work, and Metabolism in Ant Colonies

Arizona State University, Scottsdale AZ

Investigators

Abstract

The pattern of lower energy use per gram with increasing size (known as metabolic scaling) occurs across all animal groups, yet the mechanisms behind it are still not understood. Intriguingly, social insect colonies show the same level of reduced metabolism with increasing colony size. This project will combine experiments and mathematical models to determine the relationships between energy use and the organization of work in ant colonies. Ant colonies serve as an exemplar for this question, because their highly coordinated societies are organized around the work needed to sustain colony growth and maintenance, analogous to metabolic processes in organisms. Understanding the mechanisms underlying metabolic scaling has potential applications in physiology, medicine, and agriculture. The energy-based models and experiments in this project will be relevant well beyond social insect colonies, including informing human population scaling issues. The project will connect this research with a Mathematics and Social Biology Co-mentoring Program, to train research teams of undergraduates together in biology and mathematics. Students will participate in a formal training program on social biological principles and simulation modeling, and then participate as cross-disciplinary teams in collaborative research. Existing connections with research and minority programs at ASU will be used to recruit under-represented students into the program. Teaching modules on group size, social behavior, and metabolism for online K-12 use will also be developed in collaboration with ASU's Ask-a-Biologist outreach program. This project is co-funded by the Animal Behavior program in the Division of Integrative Organismal Systems, the Mathematical Biology program in the Division of Mathematical Sciences, and the BIOMAPS program for proposals at the interface of Biology, Math and the Physical Sciences. Group size is one of the most fundamental attributes of sociality, with important effects on social organization and fitness. Previous work has found that larger harvester ant colonies (Pogonomyrmex californicus) exhibit allometric changes in task performance and activity patterns, including increased worker specialization. They additionally shift the allocation of effort, possibly away from more expensive tasks. Potentially coupled with this, colonies also show a consistent pattern of hypometric metabolic scaling, which matches that of organismal scaling patterns. Because task performance is directly linked to colony growth and metabolism, these results lead to the hypothesis that larger colonies achieve economies of scale in energy use from scaling changes in the organization of work. To examine how colony size influences work organization, the distribution of workers across tasks, individual task specialization, and worker activity levels will be measured in laboratory colonies of varying size. Colony metabolism, brood production and growth rates will be measured simultaneously to link behavioral organization to colony metabolic consequences. Additionally, colony demographics, including worker size and age distributions, will be assessed for their contributions to variation in metabolism. These parameters will be measured for: (a) colonies that vary in size but not age; (b) colonies changing in size ontogenetically over time; (c) size-manipulated colonies. The empirical research will be combined with simulation, differential equation and optimization models to ask how components of task organization and activity might interact to generate scaling changes in colony metabolism and productivity.