What makes a visual scene navigable?
In considering the sensory ecology of an animal, we need to understand the information provided by the environment for a specific task. Within the insect navigation field, Jochen Zeil has been at the forefront of using image derived metrics to analyse the information in visual scenes. Here, the authors present a novel metric from work on navigation in humans, and investigate its utility for analysing the informational content of scenes for insect navigation.
Abstract: Recent work on virtual reality navigation in humans has suggested that navigational success is inversely correlated with the fractal dimension (FD) of artificial scenes. Here we investigate the generality of this claim by analysing the relationship between the fractal dimension of natural insect navigation environments and a quantitative measure of the navigational information content of natural scenes. We show that the fractal dimension of natural scenes is in general inversely proportional to the information they provide to navigating agents on heading direction as measured by the rotational image difference function (rotIDF). The rotIDF determines the precision and accuracy with which the orientation of a reference image can be recovered or maintained and the range over which a gradient descent in image differences will find the minimum of the rotIDF, that is the reference orientation. However, scenes with similar fractal dimension can differ significantly in the depth of the rotIDF, because FD does not discriminate between the orientations of edges, while the rotIDF is mainly affected by edge orientation parallel to the axis of rotation. We present a new equation for the rotIDF relating navigational information to quantifiable image properties such as contrast to show (1) that for any given scene the maximum value of the rotIDF (its depth) is proportional to pixel variance and (2) that FD is inversely proportional to pixel variance. This contrast dependence, together with scene differences in orientation statistics, explains why there is no strict relationship between FD and navigational information. Our experimental data and their numerical analysis corroborate these results.
Zahedi, M. S., & Zeil, J. (2018). Fractal dimension and the navigational information provided by natural scenes. PloS one, 13(5), e0196227.
They do it with magnets.
The animal kingdom has a breadth of sensory modalities that can be used by different navigational behaviours. One of the most evocative is the potential use of the earth’s magnetic field and many animals have been shown to be able to use magnetic cues to set directions or even know their latitude. Strong evidence however, has been absent from studies of ant navigation. No longer. We now have a fantastically elegant example of how magnetic cues are used by ants as part of their navigation. As we have seen here recently, some ants perform a particular pirouette behaviour as part of their learning walks. That is, as the learning walk proceeds the ant will occasionally pause and then turn to accurately fixate the nest entrance. Here, Fleischmann et al show that these pirouettes are under the control of magnetic cues; by rotating their direction with use of a magnetic coil.
This is a simple but profound finding, and it is worth applauding the cleverness of observing the pirouettes as an indicator of which cues might be being used by ants. By their nature, pirouettes are dissociable from the rest of the learning walk and it is this that gives a chance to observe the influence of a particular sensory cue. Lovely stuff.
Pauline Nikola Fleischmann, Robin Grob, Valentin Leander Müller, Rüdiger Wehner, Wolfgang Rössler (2018) The Geomagnetic Field Is a Compass Cue in Cataglyphis Ant Navigation. Current Biology. doi.org/10.1016/j.cub.2018.03.043
Ants need space to learn
We know that over the early foraging life of an ant, the ability to navigate using path integration is then complemented by a building visual knowledge of the world, which is initially facilitated by learning walks. Initially, these learning walks are useful in learning about the next position. Using an ingenious system of water moats, Fleischmann et al. constrained the area over which ants could move near their nest, thus disrupting natural learning walks. It was found that a large area was required, if learning walks were to be effective in providing visual knowledge that could compete with path integration in cue conflict experiments. Further analysis of the learning walks could be useful in determining whether it is the learning walk per se, that is disrupted by the restrictive moat. An alternative is that an enclosed nest surround means that ants cannot store views from far enough away from the nest. Having stored views from opposite sides of the nest that are different enough to be distinguished may be a fundamentally important to the implementation of visual homing.
Fleischmann, P., Rössler, W. and Wehner, R. (2018) Early foraging life: spatial and temporal aspects of landmark learning in the ant Cataglyphis noda. Journal of Comparative Physiology A
Studying ants with simple mazes
There has been a lot of knowledge gained from experiments using T or Y mazes with bees (e.g. for pattern choice experiments) and ants (e.g. for pheromone trail experiments). Maze experiments allow for neat logical experimental designs and can address big questions in a low-tech way. This paper is both a how-to guide for T-maze experiments and a review of their strengths and weaknesses. It could be a fantastic resource for student projects.
Czaczkes, T. J. (2018). Using T-and Y-mazes in myrmecology and elsewhere: a practical guide. Insectes Sociaux, 1-12.
Biorobotics
Many of us are advocates for Biorobotics, where both engineering can learn from biology and robotics can be a tool for testing biological ideas. This virtuous interplay is the focus of a new Opinion article from Gravish and Lauder, who use examples of physical robots that have been beneficial from a biological perspective. A more specific case study comes from Julien Serres, who has an open access book Chapter looking at the history bio-inspired optic flow solutions for robotic control.
Julien R. Serres (2018). Taking Inspiration from Flying Insects to Navigate inside Buildings, Interdisciplinary Expansions in Engineering and Design With the Power of Biomimicry, Dr. Gulden Kokturk (Ed.), InTech, DOI: 10.5772/intechopen.72918. Available from: https://mts.intechopen.com/books/interdisciplinary-expansions-in-engineering-and-design-with-the-power-of-biomimicry/taking-inspiration-from-flying-insects-to-navigate-inside-buildings
Gravish, N., & Lauder, G. V. (2018). Robotics-inspired biology. Journal of Experimental Biology, 221(7), jeb138438.
What goes in … … part 2
In the same way that the sensory system of an animal can influence navigational behaviour – the information available in the world can also influence navigational behaviour. Here we have two papers that highlight this point, one through behavioural experiments and one through modelling. Hunt et al observed the trajectories of colonies of Temnothorax ants as part of a nest migration. In their experimental variations to arena within which ants were migrating was surrounded by black wall-like landmarks, with either a flat or sloped top. The paths of migrating where quantifiably different in the two conditions, with ants producing straighter paths when one of the wall-like landmarks had a sloped top. Straighter routes are presumably more efficient and adaptive for the ants. This suggests that the sloped landmark reduces visual homogeneity and either directly provides more information to the ants or allows for symmetry breaking.
Müller et al address the question of how the environment can influence navigation in a more theoretical way. They ask the question of whether some locations are easier to find than others, when using visual information. Not surprisingly, their modelling shows that this is the case. The important piece of the story is that information that can be gained locally, correlates with the ability to navigate to a place from further away. This means that it is theoretically possible for an insect (or robot) to choose locations that will be easier to visually navigate back to. An idea that has biological implications in terms of nest choice, especially for those species that do not have conspicuous nests.
Hunt, E. R., Kendall, C., Stanbury, E., Sendova-Franks, A. B., & Franks, N. R. (2018). Two route landmarks are more useful to navigating ant colonies when they are dissimilar. Behavioural processes.
Müller, M. M., Bertrand, O. J., Differt, D., & Egelhaaf, M. (2018). The problem of home choice in skyline-based homing. PloS one, 13(3), e0194070.
What goes in … …
Although it is self-evident that the sensory input available to an insect is going to influence subsequent behaviour, it is not true that there will be a simple relationship between the two. Ecological constraints and evolutionary history might result in complex relationships between sensors and navigational behviours. We have two papers here that address this issue in terms of resolution: Palavalli-Nettimi and Narendra; and also, colour: Aksoy and Camlitepe.
Abstract: “Evolution of smaller body size in a given lineage, called miniaturisation, is commonly observed in many animals including ants. It affects various morphological features and is hypothesized to result in inferior behavioural capabilities, possibly owing to smaller sensory organs. To test this hypothesis, we studied whether reduced spatial resolution of compound eyes influences obstacle detection or obstacle avoidance in five different species of ants. We trained all ant species to travel to a sugar feeder. During their return journeys, we placed an obstacle close to the nest entrance. We found that ants with higher spatial resolution exited the corridor, the area covered between either ends of the obstacle, on average 10 cm earlier suggesting they detected the obstacle earlier in their path. Ants with the lowest spatial resolution changed their viewing directions only when they were close to the obstacle. We discuss the effects of miniaturisation on visual navigational competence in ants.”
—Palavalli-Nettimi, R., & Narendra, A. (2018). Miniaturisation decreases visual navigational competence in ants. Journal of Experimental Biology, jeb-177238.
Abstract: “Ants constitute one of the most intriguing animal groups with their advanced social lifes, different life histories and sensory modalities, one of which is vision. Chemosensation dominates all other modalities in the accomplishment of different vital tasks, but vision, varying from total blindness in some species to a relatively well-developed vision providing ants the basis for visually-guided behaviors, is also of importance. Although studies on ant vision mainly focused on recognition of and guidance by landmark cues in artificial and/or natural conditions, spectral sensitivities of their compound eyes and ocelli were also disclosed, but to a lesser extent. In this review, we have tried to present current data on the spectral sensitivities of the different ant species tested so far and the different methodological approaches. The results, as well as the similarities and/or discrepancies of the methodologies applied, were compared. General tendencies in ants’ spectral sensitivities are presented in a comparative manner and the role of opsins and ant ocelli in their spectral sensitivity is discussed in addition to the sensitivity of ants to long wavelengths. Extraocular sensitivity was also shown in some ant species. The advantages and/or disadvantages of a dichromatic and trichromatic color vision system are discussed from an ecological perspective.”
—Aksoy, V., & Camlitepe, Y. (2018). Spectral sensitivies of ants–a review. Animal Biology. doi: 10.1163/15707563-17000119
