Abstract
Ants are well-known for their amazing load carriage performances. Yet, the biomechanics of locomotion during load transport in these insects has so far been poorly investigated. Here, we present a study of the biomechanics of unloaded and loaded locomotion in the polymorphic seed-harvesting ant Messor barbarus (Linnaeus, 1767). This species is characterized by a strong intra-colonial size polymorphism with allometric relationships between the different body parts of the workers. In particular, big ants have much larger heads relative to their size than small ants. Their center of mass is thus shifted forward and even more so when they are carrying a load in their mandibles. We investigated the dynamics of the ant center of mass during unloaded and loaded locomotion. We found that during both unloaded and loaded locomotion, the kinetic energy and gravitational potential energy of the ant center of mass are in phase, which is in agreement with what has been described by other authors as a grounded-running gait. During unloaded locomotion, small and big ants do not display the same posture. However, they expend the same amount of mechanical energy to raise and accelerate their center of mass per unit of distance and per unit of body mass. While carrying a load, compared to the unloaded situation, ants seem to modify their locomotion gradually with increasing load mass. Therefore, loaded and unloaded locomotion do not involve discrete types of gait. Moreover, small ants carrying small loads expend less mechanical energy per unit of distance and per unit of body mass and their locomotion thus seem more mechanically efficient.
Highlights
Locomotion is a crucial aspect of animal behavior
We investigated the dynamics of locomotion of unloaded and loaded individuals of the polymorphic ant M. barbarus
Small ants were more erected on their legs than big ants and their CoM showed greater vertical oscillations
Summary
Locomotion is a crucial aspect of animal behavior. It is essential to accomplish tasks such as searching for food or a shelter, hunting for prey, looking for a mate or escaping a predator. Insects are of particular interest for the study of locomotion due to their outstanding performances, as attested by the maximum speed some insects can reach, e.g. about 40 body lengths per second for the ant Cataglyphis bombycina (Pfeffer et al, 2019) or about 35 body lengths per second for the cockroach Periplaneta americana (Full and Tu, 1991). This probably explains why insects have been for decades a source of inspiration for the design of legged robots (Kar et al, 2003; Koditschek et al, 2004; Dupeyroux et al, 2019)
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