Ergonomics of load transport in the seed harvesting ant Messor barbarus Linnæus, 1767: morphology influences transportation method and efficiency

Abstract

We studied in the field the load transport behaviour of workers of the polymorphic Mediterranean seed harvester ant Messor barbarus. Individual ants used two different methods to transport food items: carrying or dragging. The probability of dragging instead of carrying varied significantly with both the mass of the item transported and its linear dimension. Moreover, the values of item mass and length at which dragging began to occur increased with increasing size of the workers. However, ants of increasing sizes began to drag for decreasing values of the relative mass represented by the items, which reflects different biomechanical constraints due to allometric relationships between the different parts of their body. Transport rate was significantly higher in big ants but varied in the same way for workers of different sizes with the relative mass of the item transported. Nevertheless, although big ants were individually more efficient than small ants in transporting food items, the relative transport rate, defined as the ratio of transport rate by the mass of the ant, was higher for small ants than for big ants. Colonies should thus have a greater benefit in investing in small ants than in big ants for the transport of food items. This may explain why the proportion of big ants is so small on the foraging columns of M. barbarus and why big ants are most often employed in colonies for other tasks than transporting food items.