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 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.

Walking kinematics in the polymorphic seed harvester antMessor barbarus:influence of body size and load carriage

“…In M. barbarus big ants have, relative to their size, bigger heads than small ants (Heredia and Detrain, 2000; Bernadou et al, 2016). This means that their center of mass is located in a more anterior position compared to small ants.…”

“…Messor barbarus is a seed harvester ant whose mature colonies can shelter several tens of thousands individuals (Cerdan, 1989). Its workers display a polymorphism characterized by a continuous monophasic allometry between head mass and thorax length (Heredia and Detrain, 2000; Bernadou et al, 2016). Individuals range from 1 to 40 mg in mass and from 2 to 15 mm in length.…”

“…Ants offer a very good model to study the effect of changes in total mass on walking kinematics for at least three reasons. First, they are famous for their load carriage performances and can routinely carry loads (prey items, seeds, nest material, nestmates and brood) weighing more than ten times their own mass over tens of meters (Bernadou et al, 2016). In addition, the food they collect can be transported internally or externally.…”

“…a high variability in the size of individuals within the same colony. In addition, this polymorphism is characterized by allometric relationships between the different parts of the body (Bernadou et al, 2016), which means that big workers are not an enlarged copy of small workers but that the growth of some of their body parts during development is different from that of small workers (Bonner, 2006). In fact, relative to their mass their legs are shorter and their head bigger than those of small workers.…”

“…In fact, relative to their mass their legs are shorter and their head bigger than those of small workers. Here, we hypothesized that this allometry could lead to differences in unloaded and/or loaded locomotion in different sized workers and thus could explain the differences observed in their load carriage performances (Bernadou et al, 2016). To test this, we ran an experiment in which we compared the kinematics of workers tested first unloaded and then loaded with loads whose relative mass we varied in a systematic way across different sized ants…”