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We studied in the field the load transport behavior of workers of the polymorphic Mediterranean seed harvester ant Messor barbarus. Individual ants used two different methods to transport food items: carrying and 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, larger ants began dragging at decreasing values of the relative mass represented by the items transported, which reflects different biomechanical constraints resulting from allometric relationships between the different parts of their body. Transport rate was significantly higher in large ants but varied in the same way for workers of different sizes with the relative mass of the item transported. Nevertheless, although large ants were individually more efficient than small ants in transporting food items, the relative transport rate, defined as the ratio of transport rate to the mass of the ant, was higher for small ants than for large ants. Colonies should thus have a greater benefit in investing in small ants than in large ants for the transport of food items. This may explain why the proportion of large ants is so small on the foraging columns of M. barbarus and why large ants are most often employed in colonies for tasks other than transporting food items. [ABSTRACT FROM AUTHOR]

In social insects, food collection for the entire colony relies on a minority of its workers. How can the colony choose between resources, determine the task allocation of workers and exhibit a flexible food storage strategy from the foraging decisions taken only by a minority? We addressed this question by posing nutritional challenges to trap-jaw ants, Odontomachus hastatus, and explored their response in terms of survival, foraging behaviour and energy storage. In the first challenge, ants alternated between long periods of confinement to a high-protein diet and short periods of confinement to a highcarbohydrate diet. In the second challenge, ants alternated between long periods of confinement to a high-carbohydrate diet and short periods of confinement to a high-protein diet. In the third challenge, ants were given simultaneously a high protein and high-carbohydrate diet. First, we showed that (1) mortality increased with protein consumption, (2) a brief exposure to a high-carbohydrate diet lessened the negative consequence of high protein consumption and (3) ants given a choice of complementary diets regulated intake and minimized mortality. We also demonstrated that ants used an energy-saving strategy to overcome challenging nutritional environments. In addition we showed that the ants had an extraordinary capacity to regulate the amounts of food entering the nest both at the collective level by allocating more workers to foraging on a high-protein diet and at the individual level by collecting more food on a high-carbohydrate diet. Our study provides new insights into the strategies used by ants facing nutritional challenges and deepens our understanding of the nutritional ecology of ants and, thereby, their vast ecological success. [ABSTRACT FROM AUTHOR]