Route selection but not trail clearing are influenced by detour length in the Australian meat ants

Animals travelling through the environment often face trade-offs between environmental parameters such as risk, travel speed and ease of movement when selecting their routes. Route selection is of particular importance for central place foragers like ants, which collectively and repeatedly use trails to exploit stable sources of food. We investigated how colonies of meat ants (Iridomyrmex purpureus) select and clear trail routes when faced with semi-permeable obstructions (strips of grass turf) that substantially slow their travel speed. Meat ant colonies usually re-routed their trails to avoid obstructions when short strips of turf were laid across existing trails, but always travelled directly across the turf when avoiding the turf would have significantly increased travel time. No significant difference in trail clearing activity was found between the short and long obstruction treatments. On binary mazes, meat ants were equally likely to choose paths obstructed with turf and equal length smooth paths, despite much higher time costs associated with the obstructed route. Colonies always chose the shorter, turf-covered path on mazes where the length of the smooth path was increased by 50%, suggesting that meat ants prioritise the minimisation of travel distance when selecting new trail routes. Meat ant route selection and clearing behaviour may reflect a long-term foraging optimisation strategy whereby colonies pay high short-term costs to minimise long-term travelling costs by selecting relatively direct, short distance trail routes which can be cleared of obstructions over time.