Homing distance in desert ants, Cataglyphis fortis, remains unaffected by disturbance of walking behaviour and visual input

Evidence of learning walks related to scorpion home burrow navigation

The navigation by chemo-textural familiarity hypothesis (NCFH) suggests that scorpions use their midventral pectines to gather chemical and textural information near their burrows and use this information as they subsequently return home. For NCFH to be viable, animals must somehow acquire home-directed ‘tastes’ of the substrate, such as through path integration (PI) and/or learning walks. We conducted laboratory behavioral trials using desert grassland scorpions (Paruroctonus utahensis). Animals reliably formed burrows in small mounds of sand we provided in the middle of circular, sand-lined behavioral arenas. We processed overnight infrared video recordings with a MATLAB script that tracked animal movements at 1–2 s intervals. In all, we analyzed the movements of 23 animals, representing nearly 1500 h of video recording. We found that once animals established their home burrows, they immediately made one to several short, looping excursions away from and back to their burrows before walking greater distances. We also observed similar excursions when animals made burrows in level sand in the middle of the arena (i.e. no mound provided). These putative learning walks, together with recently reported PI in scorpions, may provide the crucial home-directed information requisite for NCFH.

Highlighted Article: Evidence that sand scorpions perform looping walks immediately after establishing a burrow and the possible significance of these putative learning walks in terms of scorpion navigation.

Rapid recovery of locomotor performance after leg loss in harvestmen

Animals have evolved adaptations to deal with environmental challenges. For instance, voluntarily releasing appendages (autotomy) to escape potential predators. Although it may enhance immediate survival, this self-imposed bodily damage may convey long-term consequences. Hence, compensatory strategies for this type of damage might exist. We experimentally induced autotomy in Prionostemma harvestmen. These arachnids are ideal to examine this topic because they show high levels of leg loss in the field but do not regenerate their legs. We video-recorded animals moving on a horizontal track and reconstructed their 3D trajectories with custom software tools to measure locomotor performance. Individuals that lost either three legs total or two legs on the same side of the body showed an immediate and substantial decrease in velocity and acceleration. Surprisingly, harvestmen recovered initial performance after 2 days. This is the quickest locomotor recovery recorded for autotomizing animals. We also found post-autotomy changes in stride and postural kinematics, suggesting a role for kinematic adjustments in recovery. Additionally, following leg loss, some animals changed the gaits used during escape maneuvers, and/or recruited the ‘sensory’ legs for locomotion. Together, these findings suggest that harvestmen are mechanically robust to the bodily damage imposed by leg loss.

Accuracy and spread of nest search behaviour in the Saharan silver ant, Cataglyphis bombycina, and in the salt pan species, Cataglyphis fortis

Desert ants of the genus Cataglyphis are renowned for their navigation abilities, especially for their beeline homing after meandering foraging excursions reaching several hundreds of meters in length. A spiralling nest search is performed when an ant misses the nest entrance upon completing its homebound travel. We examined the nest search behaviours of two desert ant species dwelling in different habitats—Cataglyphis bombycina living in the dunes of the Sahara and Cataglyphis fortis found in the salt pans of North Africa. The two species show distinct differences in walking behaviour. C. bombycina performs a strict tripod gait with pronounced aerial phases, high stride frequencies, and extremely brief ground contact times. In view of these peculiarities and the yielding sand dune substrate, we hypothesised that homing accuracy, and namely distance measurement by stride integration, should be lower in C. bombycina, compared to the well-studied C. fortis with less specialised walking behaviour. We tested this hypothesis in ants’ homebound runs from a feeding site in a linear channel setup. Surprisingly, the accuracies of nest searches were similar in the two ant species, and search accuracy was also independent of the walking substrate, soft dune sand or a hard floor. The spread of the nest search, by contrast, differed significantly between the two species, C. bombycina exhibiting a larger search spread. This may be interpreted as an increased path integration uncertainty due to the above locomotor specialisations, or as a compensation strategy accounting for the silver ants’ particular environmental and behavioural situation.

Two distance memories in desert ants-Modes of interaction

Navigation plays an essential role for many animals leading a mobile mode of life, and for central place foragers in particular. One important prerequisite for navigation is the ability to estimate distances covered during locomotion. It has been shown that Cataglyphis desert ants, well-established model organisms in insect navigation, use two odometer mechanisms, namely, stride and optic flow integration. Although both mechanisms are well established, their mode of interaction to build one odometer output remains enigmatic. We tackle this problem by selectively covering the ventral eye parts in Cataglyphis fortis foragers, the eye regions responsible for optic flow input in odometry. Exclusion of optic flow cues was implemented during different sections of outbound and inbound travel. This demonstrated that the two odometers have separate distance memories that interact in determining homing distance. Possible interpretations posit that the two odometer memories (i) take on different relative weights according to context or (ii) compete in a winner-take-all mode. Explanatory values and implications of such interpretations are discussed. We are able to provide a rough quantitative assessment of odometer cue interaction. An understanding of the interaction of different odometer mechanisms appears valuable not only for animal navigation research but may inform discussions on sensor fusion in both behavioural contexts and potential technical applications.

A Neotropical armored harvestman (Arachnida, Opiliones) uses proprioception and vision for homing

Animals use external and/or internal cues to navigate and can show flexibility in cue use if one type of cue is unavailable. We studied the homing ability of the harvestman Heteromitobates discolor (Arachnida, Opiliones) by moving egg-guarding females from their clutches. We tested the importance of vision, proprioception, and olfaction. We predicted that homing would be negatively affected in the absence of these cues, with success being measured by the return of females to their clutches. We restricted proprioception by not allowing females to walk, removed vision by painting the eyes, and removed the odours by removing the clutch and cleaning its surroundings. We found that vision is important for homing, and in the absence of visual cues, proprioception is important. Finally, we found increased homing when eggs were present, and that the time of the day also influenced homing. We highlight vision as a previously overlooked sensory modality in Opiliones.

Role of the different eyes in the visual odometry in the wolf spider Lycosa tarantula (Araneae, Lycosidae)

The wolf spider Lycosa tarantula returns home by means of path integration. Previous studies demonstrated: (i) that the angular component of the outbound run is measured using a polarized-light compass associated with the anterior median eyes; (ii) changes in direction of the substratum are detected by the anterior lateral eyes (ALEs); and (iii) in relation to the linear component of the outbound run, an increase of optic flow, in either the lateral or ventral fields of view, caused spiders to search for the burrow at a point nearer to the goal. However, the role of the secondary eyes [ALEs, posterior lateral eyes (PLEs) and posterior median eyes (PMEs)] in the perception of this optic flow and the importance of them for gauging the distance walked is still unknown. In this study, lateral or ventral gratings of wavelength λ=1 cm were used, with two groups of spiders in each setup: (1) PLEs+PMEs covered and (2) ALEs covered. The largest reduction in the distance walked to return to the burrow was observed with the ventral grating/ALEs covered. These results show the importance of the previously neglected ALEs for the visual behavior of these spiders. The possibility of gathering information for locomotion from the three pairs of secondary eyes in the mushroom bodies is discussed.

Optic flow odometry operates independently of stride integration in carried ants

Cataglyphis desert ants are impressive navigators. When the foragers roam the desert, they employ path integration. For these ants, distance estimation is one key challenge. Distance information was thought to be provided by optic flow (OF)-that is, image motion experienced during travel-but this idea was abandoned when stride integration was discovered as an odometer mechanism in ants. We show that ants transported by nest mates are capable of measuring travel distance exclusively by the use of OF cues. Furthermore, we demonstrate that the information gained from the optic flowmeter cannot be transferred to the stride integrator. Our results suggest a dual information channel that allows the ants to measure distances by strides and OF cues, although both systems operate independently and in a redundant manner.

How to find home backwards? Locomotion and inter-leg coordination during rearward walking of Cataglyphis fortis desert ants

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.

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

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 backward-homing 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 forward-walking 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.

Visual odometry in the wolf spider Lycosa tarantula (Araneae: Lycosidae)

The wolf spider Lycosa tarantula homes using path integration. The angular component of the displacement is measured using a polarized-light compass associated with the functioning of the anterior median eyes. However, how L. tarantula estimates the linear component of the displacement was not known prior to this investigation. The ability of L. tarantula to gauge the distance walked after being displaced from its burrow was investigated using experimental channels placed in an indoor setup. Firstly, we manipulated the perception of visual stimuli by covering all the spider's eyes. Secondly, we changed the optic flow supplied by a black-and-white grating (λ=2 cm) perceived either in the lateral or in the ventral field of view. Finally, the period of the lateral or ventral grating was changed from λ=2 cm to λ=1 cm. Our results indicate that visual information contributes to distance estimation because when the spider's eyes were covered, the spiders tended to search for the burrow at very variable distances. This visual information is created by the motion of the image as the spider walks, the motion in the lateral field of view being the most important. The preference of a lateral optic flow over the ventral flow can be explained by the difference in the resolution capacity of the posterior lateral eyes and the anterior lateral eyes.