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

Abstract

Ants are famous in the animal kingdom for their amazing load carriage performances. Yet, the mechanisms that allow these insects to maintain their stability when carrying heavy loads have been poorly investigated. Here we present a study of the kinematics of loaded locomotion in the polymorphic seed-harvesting ant Messor barbarus. In this species big ants have larger heads relative to their size than small ants. Hence, their center of mass is shifted forward, and the more so when they are carrying a load in their mandibles. We tested the hypothesis that this could lead to big ants being less statically stable than small ants, thus explaining their lower load carriage performances. When walking unloaded we found that big ants were indeed less statically stable than small ants but that they were nonetheless able to adjust their stepping pattern to partly compensate for this instability. When ants were walking loaded on the other hand, there was no evidence of different locomotor behaviors in individuals of different sizes. Loaded ants, whatever their size, move too slowly to maintain their balance through dynamic stability. Rather, they seem to do so by clinging to the ground with their hind legs during part of a stride. We show through a straightforward model that allometric relationships have a minor role in explaining the differences in load carriage performances between big ants and small ants and that a simple scale effect is sufficient to explain these differences.

Dynamics of locomotion in the seed harvesting antMessor barbarus:effect of individual body mass and transported load mass

“…Note that the data presented in this paper are part of the data collected in the study presented in Merienne et al (2020) . The studied species, experimental setup and experimental protocol are thus the same.…”

“…After letting the glue dry for 15 min and the ant recover for half an hour, the ant was placed again on the platform and its locomotion was recorded in loaded condition. We retained only the recordings in which the load did not touch the ground during the transport (see Merienne et al, 2020 ). At the end of the recording, the ant was captured and weighed a second time.…”

“…For the unloaded condition, we expressed all variables Y as a power law function of ant mass M , i.e., Y = a ∗ M b ( Merienne et al, 2020 ). For each variable, the values of the coefficients a and b , as well as the value of the variable predicted by the statistical model for the mean mass of the tested ants (12.5 mg), are given in a table, along with their 95% confidence interval.…”

“…The changes induced by load carriage on the locomotion of ants have so far been investigated only with a kinematic approach, through the analysis of stepping pattern ( Zolliköfer, 1994 ; Moll, Roces & Federle, 2013 ; Merienne et al, 2020 ). In the seed harvesting ant Messor barbarus for example, load carriage has been found to decrease locomotory speed (through a decrease in stride frequency but not of step amplitude), to increase the mean number of legs in contact with the ground, as well as to induce a change in leg positioning, with ants spreading their legs further away from their longitudinal body axis in order to maintain their stability ( Merienne et al, 2020 ). On the other hand, the impact of load carriage on the exchanges of mechanical energies and on the mechanical cost of locomotion in ants is poorly documented.…”

“…Specifically, we tested the assumption that, ants, in the same way as humans ( Heglund et al, 1995 ), could be able to decrease, or at least compensate, the additional mechanical cost of carrying a load by improving the pendulum-like behavior of their CoM through a better transfer between the gravitational potential and kinetic energy of their CoM. Moreover, since large ants have a less stable locomotion than small ants ( Merienne et al, 2020 ) due to the forward shift of their CoM, we predict that their locomotion when transporting loads representing the same amount of individual body mass should be less mechanically efficient than that of small ants, and the more so for loads of increasing mass.…”