The effect of ground surface rugosity on ant running speed is species-specific rather than size dependent

Abstract

Foraging is one of the main reasons for ants to walk. Foraging speed is mainly dependent on morphological traits, but also on the surface structure. The size-grain hypothesis (SGH) suggests that (1) relative leg length should increase allometrically with body size, and (2) smaller, shorter legged species have an advantage with increasing habitat complexity in comparison to larger and longer legged species. In general, it is thought that leg length is a good predictor for running speed. We performed morphological analyses and running experiments with differently sized ant species along an artificial gradient of surface rugosity. We measured running speed, number of steps, body size and leg length to answer the question, if morphological traits can be used to predict the running speed of ants in complex habitats, as predicted by the SGH. We found that (1) leg length increased allometrically with body size, and that (2) the largest species were among the fastest and could sustain their speed with increasing surface rugosity. The smallest species with the relatively shortest legs were the fastest on the flattest surface, but their speed decreased rapidly with increasing rugosity. Leg length was not a good predictor for running speed. Similar-sized species responded idiosyncratically to high surface rugosity, which might be related to species-specific habitat preferences. Species-specific behavior or stride frequency influenced running speed strongly, which hampers precise predictions on their running speed, based on morphological traits exclusively.