Complexity of searching movement in the European harvester ant Messor wasmanni: effect of temperature and body size

Abstract

Individual search behavior is an important feature of ant ecology that can be influenced by many factors including environmental constraints, competitive interactions, and morphology. In this study, I investigated the effects of temperature, body size, and leg allometry on search movement in the polymorphic European harvester ant, Messor wasmanni. Temperature is known to increase movement speed, but how it interacts with morphology to influence path sinuosity is unknown. Ants in this study were grouped according to dimension (small, medium, and large) and were filmed moving individually in a neutral arena at three different temperatures: 10, 20, and 30 °C. Movements were analyzed by evaluating mean speed, path length, and path complexity using 5 different indexes to survey tortuosity: Mean Square Displacement (MSD), Straightness (ST), Intensity of Habitat Use (IU), Sinuosity (SI), and Fractal Dimension (Fractal D). Search movement seemed not to be strongly affected by size, even though extreme temperatures faintly diversified the path complexity among castes. Finally, the relationship between body weight and leg length was evaluated to test the ‘size-grain’ hypothesis among dimensional castes. Mean speed was strongly affected by temperature, but not by the size of the ants. Complexity indexes revealed different outcomes through their relationship to speed or path length. Interestingly, leg length and body size were negatively correlated, indicating that legs are proportionally shorter in large rather than in small ants, causing rejection of the ‘size-grain’ hypothesis for this species. Thus, individuals experience a similar roughness of soil surface irrespective of their size.