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

Karin Moll,Flavio Roces,Walter Federle

doi:10.1371/journal.pone.0052816

Karin Moll, Flavio Roces + Show 1 more

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https://doi.org/10.1371/journal.pone.0052816

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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.

Highlights

The fitness of an animal primarily depends on the quality and quantity of food [1]
In 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
Fragment length was chosen based on field measurements of naturally carried grass fragments by A. vollenweideri [18]. 30 mm is within the length range of fragments carried by workers of 3.5– 5.5 mg body mass

Summary

Introduction

The fitness of an animal primarily depends on the quality and quantity of food [1]. In social insects, individual workers are responsible for the nutrient gain of the entire colony and its survival. The influence of load mass on locomotion and transport rates has been studied in many social insects (e.g. Veromessor pergandei [3], Atta colombica [4], Atta cephalotes [5,6], Dorymyrmex goetschi [7], Apis mellifera [8,9]). In addition to the direct effects of the load mass on carrying speed, large off-centre loads may increase the insects’ risk of falling over. Such stability constraints during the transport of loads, as previously suggested by Zollikofer [10], have not been investigated, stability during locomotion represents a fundamental problem for pedestrian animals [11]. 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

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