How Load-Carrying Ants Avoid Falling Over: Mechanical Stability during Foraging in Atta vollenweideri Grass-Cutting Ants

Abstract

BackgroundForaging workers of grass-cutting ants (Atta vollenweideri) regularly carry grass fragments larger than their own body. Fragment length has been shown to influence the ants’ running speed and thereby the colony’s food intake rate. We investigated whether and how grass-cutting ants maintain stability when carrying fragments of two different lengths but identical mass.Principal FindingsAnts carried all fragments in an upright, backwards-tilted position, but held long fragments more vertically than short ones. All carrying ants used an alternating tripod gait, where mechanical stability was increased by overlapping stance phases of consecutive steps. The overlap was greatest for ants carrying long fragments, resulting in more legs contacting the ground simultaneously. For all ants, the projection of the total centre of mass (ant and fragment) was often outside the supporting tripod, i.e. the three feet that would be in stance for a non-overlapping tripod gait. Stability was only achieved through additional legs in ground contact. Tripod stability (quantified as the minimum distance of the centre of mass to the edge of the supporting tripod) was significantly smaller for ants with long fragments. Here, tripod stability was lowest at the beginning of each step, when the center of mass was near the posterior margin of the supporting tripod. By contrast, tripod stability was lowest at the end of each step for ants carrying short fragments. Consistently, ants with long fragments mainly fell backwards, whereas ants carrying short fragments mainly fell forwards or to the side. Assuming that transporting ants adjust neither the fragment angle nor the gait, they would be less stable and more likely to fall over.ConclusionsIn grass-cutting ants, the need to maintain static stability when carrying long grass fragments has led to multiple kinematic adjustments at the expense of a reduced material transport rate.

Ergonomics of load transport in the seed harvesting ant Messor barbarus Linnæus, 1767: morphology influences transportation method and efficiency

“…A reduction in speed in foraging ants transporting external loads has been reported in several ant genera (Eciton: Bartholomew et al, 1988;Atta: Rudolph and Loudon, 1986;Burd, 2000;Röschard and Roces, 2002;Moll et al, 2013;Rhytidoponera: Nielsen, 2001;Dorymyrmex: Torres-Contreras and Vasquez, 2004;Messor: Bernadou et al, 2011). In ants carrying external food items in their mandibles, the load is located on the anterior part of the body, resulting in a forward shift of the center of mass of the whole system composed of the ant and the load it carries.…”

“…On encountering a large food item, ants first attempt to lift it from the ground and carry it in an upright position (Sudd, 1960(Sudd, , 1965Czaczkes and Ratnieks, 2013). If, in doing so, the position of their center of mass is shifted forward so that it falls outside the polygon formed by the legs in contact with the soil (Moll et al, 2013), they shift to a dragging strategy, unless, as in some ant species (e.g. Oecophylla smaragdina : Federle et al, 2001), they are able to avoid falling forward because they are equipped with specialized adhesive organs that allow them to cling to the substrate.…”

“…Ants are a notable exception to this rule, and their ability to transport loads several times their own mass over distances equivalent to several thousands of times their size has always been an object of amazement and fascination for naturalists. Thanks to specific morphological and anatomical features, such as the efficient and dynamic control of tarsal pad adhesion (Federle et al, 2001;Federle and Endlein, 2004), and the extreme resistance of their neck joints (Moll et al, 2013;Nguyen et al, 2014), ants are able to lift, hold and carry extremely heavy loads with their mouthparts. For example, individual workers of the species Azteca andreae are able to hold loads weighing up to 5700 times their own mass while clinging on a leaf margin (Dejean et al, 2010).…”

“…The carrying method and carrying limits of each species are probably partly imposed by biomechanical constraints linked to the morphology (e.g. leg lengths relative to body size: Zollikofer, 1994;Moll et al, 2013) and/or the anatomy of the workers (e.g. neck joint muscles: Keller et al, 2014).…”

Behavioral and mechanical determinants of collective subsurface nest excavation

“…undesirable because of their effects on locomotion and sensing: as supplementary material Movie 4 demonstrates, large pellets affect climbing stability and speed (Moll et al, 2013) because of slips and mis-steps. We posit that large pellets also hinder antennal contacts with tunnel walls, which are important for tactile-and chemo-sensing in subterranean environments, as well as for slip and fall recovery .…”