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

Abstract

Cataglyphis ants are renowned for their impressive navigation skills, which have been studied in numerous experiments during forward locomotion. However, the ants' navigational performance during backward homing when dragging large food loads has not been investigated until now. During backward locomotion, the odometer has to deal with unsteady motion and irregularities in inter-leg coordination. The legs' sensory feedback during backward walking is not just a simple reversal of the forward stepping movements: compared with forward homing, ants are facing towards the opposite direction during backward dragging. Hence, the compass system has to cope with a flipped celestial view (in terms of the polarization pattern and the position of the sun) and an inverted retinotopic image of the visual panorama and landmark environment. The same is true for wind and olfactory cues. In this study we analyze for the first time backwardhoming ants and evaluate their navigational performance in channel and open field experiments. Backward-homing Cataglyphis fortis desert ants show remarkable similarities in the performance of homing compared with forward-walking ants. Despite the numerous challenges emerging for the navigational system during backward walking, we show that ants perform quite well in our experiments. Direction and distance gauging was comparable to that of the forwardwalking control groups. Interestingly, we found that backward-homing ants often put down the food item and performed foodless search loops around the left food item. These search loops were mainly centred around the drop-off position (and not around the nest position), and increased in length the closer the ants came to their fictive nest site.

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

“…Furthermore, the mass of the food load is often not as symmetrical as in our study, which might give leg and body movements a biased direction. Additionally, the natural substrate is more uneven than our sanded, plain aluminium floor (even the desert ground, see movie 1 in Pfeffer and Wittlinger, 2016). One can easily assume, therefore, that we would find even more flexibility of locomotion under natural conditions with even less coupled leg coordination and more unsteady leg and body movements.…”

“…That fact that the stability could be maintained during backward dragging indicates a strong influence of sensory feedback overriding the seeming rigidity of the tripod leg pattern. Here, the exciting question arises of if and how a challenging task such as odometry is accomplished with such a flexibility of the walking apparatus (Pfeffer and Wittlinger, 2016). This might also be an interesting aspect for the implementation of insect walking in robotics.…”

Path integration: how details of the honeybee waggle dance and the foraging strategies of desert ants might help in understanding its mechanisms

“…Holonomic control also occurs in ants walking backwards while dragging heavy prey to the nest. Part of their strategy for keeping on course and updating PI state is to stop occasionally and face in their usual direction of travel (Pfeffer and Wittlinger, 2016a). Facing forwards seems to improve the precision of the path when they return to moving backwards (Pfeffer and Wittlinger, 2016a).…”

Path Integration: Combining Optic Flow with Compass Orientation

“…In contrast to the process of path integration accumulation, which acts independently of body orientation to create the home-vector, the guidance control system proposed for using the home-vector is limited to producing movement for which the insect faces along its direction of travel. Ants, however, can follow their home-vector even when travelling backwards to drag a heavy food-item behind them [11]. The model does not account for this ability, so does it mean that the proposed control system is wrong?…”

“…But another way to reconcile the model with behaviour (and which could more generally allow an insect to face in any direction when following a PI home-vector) is through a mechanism that was proposed to explain how backwards facing ants can follow their retinotopically encoded visual routes [12], and that may well be used more generally [13]. When ants follow their home-vector travelling backwards, they sporadically drop their food item briefly to peek forwards [11]. The comparison proposed by Stone et al [3] could occur during these peeks.…”