Ants are well known for their load carrying performances. For example, a single worker of the seed harvester ant Messor barbarus is able to lift and carry more than ten times its own mass, and much more if the load is dragged instead of carried (Bernadou et al. , 2016). Our project aims at characterizing the load carrying performances of ants from a biomechanical point of view and at investigating the structures and mechanisms that allow them to achieve these performances.
In order to characterize load carriage in ants we use a motion analysis platform designed by the company R&D Vision. This platform is adapted to the small size of the ants. It allows to track in 3D the position of the center of mass of the ants and of 23 strategic points on their body segments with five synchronized high resolution high speed video cameras. The software Vicon Peak Motus then allows to reconstruct a 3D model of the ants, to characterize very accurately the kinematics of their displacement and to compute the mechanical power they generate during their locomotion with a load.
The objective of our project is to establish a link between the load carriage energetic performances of workers of different sizes/shapes and the division of labor observed within the colonies. We could then test whether the size distribution of loaded ants observed on foraging trails is linked to their load carriage performances (theory of caste ergonomics). The species we work with, M. barbarus, is particularly suited to answer this question because it is characterized by a strong size polymorphism: the mass of the workers within the same colony can vary by a 10 fold factor. This polymorphism is due to allometric relationships between the different parts of their body: in M. barbarus big ants have a much bigger head compared to their body than small ants (Bernadou et al., 2016). Their center of mass is thus shifted forward, which has probably a strong impact on their locomotor behavior and on their load carrying performances.
A- Workers of the seed harvester ant Messor barbarus carrying fragments of alimentary pasta back to their nest. B- Relationship between head mass and thorax length in M. barbarus workers. Head mass was linearized using the cube root of the values. Head mass increased significantly faster than thorax length. The dashed line has a slope of 1 and would correspond to an isometric growth between head mass and thorax length. N=57 ants. From Bernadou et al. (2016)- D- Snapshot of a video showing the rajectories of the points tracked by the software Kinovea.