The Organization of Soil Disposal by 1 Ants

28 29 Colonies of Pheidole ambigua ants excavate soil and drop it outside the nest entrance. 30 The deposition of thousands of loads leads to the formation of regular ring-shaped piles. 31 How is this pattern generated? This study investigated soil pile formation on level and 32 sloping surfaces, both empirically and using an agent-based model. We found that ants 33 drop soil preferentially in the direction in which the slope is least steeply uphill from the 34 nest entrance, both when adding to an existing pile, and when starting a new pile. Ants 35 respond to cues from local slope to choose downhill directions. Ants walking on a slope 36 increase the frequency and magnitude of changes in direction, and more of these changes 37 of direction take them downhill than uphill. We found that ants carrying soil on a slope 38 wait longer before dropping their soil compared to ants on a level plane. These 39 mechanisms combine to focus soil dropping in the downhill direction, without the 40 necessity of a direct relationship between slope and probability of dropping soil. These 41 empirically determined rules are used to simulate soil disposal. The slight preference for 42 turning downhill we had measured empirically was shown in the model to be sufficient to 43 generate biologically realistic patterns of soil dumping when combined with memory of 44 the direction of previous trips. From simple rules governing individual behaviour an 45 overall pattern emerges, which is appropriate to the environment, and allows a rapid 46 response to changes. 47 48 3 Dynamic unpredictable environments pose great challenges to the organisms inhabiting 52 them. Behaviours which are appropriate in one situation may become inappropriate when 53 conditions change. Social insects provide many examples of behaviours which are 54 modified to meet environmental changes, from foraging patterns in ants (Sendova-Franks 55 and Franks 1993; Detrain et al. 2001) to brood care in honey-bees (Schmickl and 56 Crailsheim 2002). The self-organized behaviour of many social insects means that the 57 colony's response to the changing environment is often based on changes in the 58 behaviour of individual workers in response to local cues and interactions (Bonabeau et 59 al. 1998; Johnson et al. 2003; Théraulaz et al. 2002). In particular, a single set of local 60 behavioural rules (followed by workers individually) can lead to differing global results 61 depending on environmental conditions …