How to find home backwards? Locomotion and inter-leg coordination during rearward walking ofCataglyphis fortisdesert ants

Abstract

For insects, flexibility in the performance of terrestrial locomotion is a vital part of facing the challenges of their often unpredictable environment. Arthropods such as scorpions and crustaceans can switch readily from forward to backward locomotion, but in insects this behaviour seems to be less common and, therefore, is only poorly understood. Here we present an example of spontaneous and persistent backward walking in Cataglyphis desert ants that allows us to investigate rearward locomotion within a natural context. When ants find a food item that is too large to be lifted up and to be carried in a normal forward-faced orientation, they will drag the load walking backwards to their home nest. A detailed examination of this behaviour reveals a surprising flexibility of the locomotor output. Compared with forward walks with regular tripod coordination, no main coordination pattern can be assigned to rearward walks. However, we often observed leg-pair-specific stepping patterns. The front legs frequently step with small stride lengths, while the middle and the hind legs are characterized by less numerous but larger strides. But still, these specializations show no rigidly fixed leg coupling, nor are they strictly embedded within a temporal context; therefore, they do not result in a repetitive coordination pattern. The individual legs act as separate units, most likely to better maintain stability during backward dragging.

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“…Interleg coupling dependent on walking direction: As illustrated in Fig 6, walking backward shows stable footfall patterns similar to those observed during forward walking. This seems to be in contrast to the results of Pfeffer et al (2016), who stated that coordination in backward walking ants is weaker than in forward walking. The reason for this weak coordination may be that in these experiments ants had to carry higher load when walking backwards which provides irregular sensory input that may disturb the controller or, that the ants simply adopt a lower velocity (see above).…”

How to find home backwards? Navigation during rearward homing of Cataglyphis fortis desert ants

“…This is also true for backward walking, where an even weaker correlation was found between stride length and stride frequency (see fig. S3 in Pfeffer et al, 2016). Despite the weak correlation between stride length and stride frequency, the efficiency of the stride integrator during backward homing comes close to that of forward homing.…”

“…This is interesting, because the physiological basis of the odometer module was hitherto unclear (Wittlinger et al, 2007;Wittlinger and Wolf, 2013). Therefore, it was also difficult to predict a priori what influence the irregular stepping pattern would have on distance gauging (Pfeffer et al, 2016). It has been hypothesized that stride length might be deduced from stride frequency, because the relationship between these two parameters is relatively stable during forward walks (Bässler and Büschges, 1998).…”

“…All data were obtained during the summer months of 2013, 2014 and 2015. We presented the ants with a heavy food load to induce them to walk backwards (for details, see Pfeffer et al, 2016). Homing behaviour was examined in channel and open field experiments (see Fig.…”

“…Whereas general walking and forward homing with a small food crumb is characterized by robust tripod coordination, the stepping pattern during backward dragging shows remarkable irregularities. Body and leg movements are unsteady and the inter-leg coordination is less tightly coupled (Pfeffer et al, 2016). Also, in terms of muscle activity and motor output, it seems that backward leg movements are not only a simple reversal of the forward motion sequence (Graham and Epstein, 1985).…”