Ant colonies possess a “societal metabolism,” acquiring, transforming, and allocating resources through a network of foragers (Moses, 2005). Ant foraging- trail networks channel foragers to known food resources and away from competing colonies (Jun et al., 2003). Computer models suggest the spread of information occurs faster in larger colonies of harvester ants, genus Pogonomyrmex (Adler and Gordon, 1992), providing a possible mechanism of differentiation. Does the ability to utilize and share information scale super-linearly with a colony’s size? Within colonies, do foragers recruit more to denser sources of food, using information transfer to increase forager efficiency and harvest seed caches before competing colonies find them? To address these questions, we studied three sympatric species of Pogonomyrmex in central New Mexico that differ in average colony size: P. rugosus, P. maricopa, and P. desertorum. We hypothesized a) that across colonies recruitment to dense food resources scales positively with colony size, and b) that within colonies recruitment scales positively with seed density. We observed baited colonies for 1 hr, tracking the capture of dyed seeds arranged in piles of different densities and of native seeds. We generated a model of idealized effects of recruitment on foraging patterns and compared the output to our observations. We did not find support for hypothesis a, that recruitment scales positively with colony size, but did find support for hypothesis b, that recruitment does scale positively with increasing seed density. These findings highlight a key intersection between the metabolism of energy and of information.
To explore whether large ant colonies are "smarter" and how colonies use recruitment.
Experimental Design: Our experimental design consisted of testing potential differences in forager recruitment by baiting actively foraging colonies with dyed seeds arranged in different distributions (i.e. piles of different numbers of seeds) around each colony. Differences in the rate foragers collect seeds of different colors indicate potential differences in the rate foragers are recruited, by chemical pheromone trails, to piles of different sizes. We conduct these experiments on a number of colonies of three species of Pogonomyrmex that differ in average colony size, and then compare the rates those species retrieve seeds of different distributions.
Field Methods: We located actively foraging Pogonomyrmex colonies between 8 and 9am and distributed dyed millet seeds in a wide circular swath around each colony. We used 256 millet seeds of each color for P. rugosus and P. maricopy and 32 sesame seeds of each color for P. desertorum, which is smaller and has smaller colonies. For each color, we divided those seeds into piles as follows: red = 1 pile; purple = 4 piles; green = 16 piles; blue = "piles" of one seed each (i.e. scattered at random within the circular swath). We then sat near the nest entrance and recorded the seeds brought in by foraging ants. Using a Java program (SeedCounter) we recorded the color and time of each seed retrieved by the focal colony during an observation period of 1 to 1.5 hours. We generated seed uptake curves from this data.
Laboratory Procedures: We also developed an ant foraging simulation. Virtual ants start at a nest entrance at the center of a bounded lattice. The lattice contains 256 randomly distributed blue seeds and a single pile of 256 red seeds. A foraging ant searches at random until it collects a seed, then delivers it to the nest. To model patch fidelity, a successful forager returns to location it found its previous seed and begins a new random search.