Multisensory navigation and neuronal plasticity in desert ants

Cataglyphis ants have a polarity-sensitive magnetic compass

Spatial orientation based on the geomagnetic field (GMF) is a widespread phenomenon in the animal kingdom, predominantly observed in long-distance migrating birds,1 sea turtles,2 lobsters,3 and Lepidoptera.4,5 Although magnetoreception has been studied intensively, the mechanism remains elusive. A crucial question for a mechanistic understanding of magnetoreception is whether animals rely on inclination or polarity-based magnetic information. Inclination-based magnetic orientation utilizes the angle between the magnetic field lines and gravity, indicating poleward and equatorward. In contrast, polarity-based magnetic orientation allows animals to detect the polarity of the GMF, the north and south direction of the field vector. Cataglyphis desert ants are excellent experimental models for testing whether magnetic inclination or polarity of the magnetic field is used for navigation. Desert ants are solitary foragers with exceptional navigational skills.6 When the ants leave their underground nest for the first time to become foragers, they perform learning walks for up to three days to learn the visual panorama and calibrate their compass systems.7,8 The ants repeatedly stop their forward movement during learning walks for performing turns (pirouettes), interrupted by stopping phases. Gaze directions during the longest stopping phases are directed toward the nest entrance.9 We experimentally manipulated look-back behavior systematically by altering polarity or inclination of the GMF. We demonstrate that Cataglyphis ants, contrary to most other insects studied,10 possess a polarity-sensitive magnetic compass, making them ideal experimental models for narrowing down the evidence for particle-based mechanisms underlying magnetosensation in this insect.

The Budding Neuroscience of Ant Social Behavior

Ant physiology has been fashioned by 100 million years of social evolution. Ants perform many sophisticated social and collective behaviors yet possess nervous systems similar in schematic and scale to that of the fruit fly Drosophila melanogaster, a popular solitary model organism. Ants are thus attractive complementary subjects to investigate adaptations pertaining to complex social behaviors that are absent in flies. Despite research interest in ant behavior and the neurobiological foundations of sociality more broadly, our understanding of the ant nervous system is incomplete. Recent technical advances have enabled cutting-edge investigations of the nervous system in a fashion that is less dependent on model choice, opening the door for mechanistic social insect neuroscience. In this review, we revisit important aspects of what is known about the ant nervous system and behavior, and we look forward to how functional circuit neuroscience in ants will help us understand what distinguishes solitary animals from highly social ones.

Investigating visual navigation using spiking neural network models of the insect mushroom bodies

Ants are capable of learning long visually guided foraging routes with limited neural resources. The visual scene memory needed for this behaviour is mediated by the mushroom bodies; an insect brain region important for learning and memory. In a visual navigation context, the mushroom bodies are theorised to act as familiarity detectors, guiding ants to views that are similar to those previously learned when first travelling along a foraging route. Evidence from behavioural experiments, computational studies and brain lesions all support this idea. Here we further investigate the role of mushroom bodies in visual navigation with a spiking neural network model learning complex natural scenes. By implementing these networks in GeNN-a library for building GPU accelerated spiking neural networks-we were able to test these models offline on an image database representing navigation through a complex outdoor natural environment, and also online embodied on a robot. The mushroom body model successfully learnt a large series of visual scenes (400 scenes corresponding to a 27 m route) and used these memories to choose accurate heading directions during route recapitulation in both complex environments. Through analysing our model's Kenyon cell (KC) activity, we were able to demonstrate that KC activity is directly related to the respective novelty of input images. Through conducting a parameter search we found that there is a non-linear dependence between optimal KC to visual projection neuron (VPN) connection sparsity and the length of time the model is presented with an image stimulus. The parameter search also showed training the model on lower proportions of a route generally produced better accuracy when testing on the entire route. We embodied the mushroom body model and comparator visual navigation algorithms on a Quanser Q-car robot with all processing running on an Nvidia Jetson TX2. On a 6.5 m route, the mushroom body model had a mean distance to training route (error) of 0.144 ± 0.088 m over 5 trials, which was performance comparable to standard visual-only navigation algorithms. Thus, we have demonstrated that a biologically plausible model of the ant mushroom body can navigate complex environments both in simulation and the real world. Understanding the neural basis of this behaviour will provide insight into how neural circuits are tuned to rapidly learn behaviourally relevant information from complex environments and provide inspiration for creating bio-mimetic computer/robotic systems that can learn rapidly with low energy requirements.

Minimal circuit motifs for second-order conditioning in the insect mushroom body

In well-established first-order conditioning experiments, the concurrence of a sensory cue with reinforcement forms an association, allowing the cue to predict future reinforcement. In the insect mushroom body, a brain region central to learning and memory, such associations are encoded in the synapses between its intrinsic and output neurons. This process is mediated by the activity of dopaminergic neurons that encode reinforcement signals. In second-order conditioning, a new sensory cue is paired with an already established one that presumably activates dopaminergic neurons due to its predictive power of the reinforcement. We explored minimal circuit motifs in the mushroom body for their ability to support second-order conditioning using mechanistic models. We found that dopaminergic neurons can either be activated directly by the mushroom body's intrinsic neurons or via feedback from the output neurons via several pathways. We demonstrated that the circuit motifs differ in their computational efficiency and robustness. Beyond previous research, we suggest an additional motif that relies on feedforward input of the mushroom body intrinsic neurons to dopaminergic neurons as a promising candidate for experimental evaluation. It differentiates well between trained and novel stimuli, demonstrating robust performance across a range of model parameters.

Monarch butterflies memorize the spatial location of a food source

Spatial memory helps animals to navigate familiar environments. In insects, spatial memory has extensively been studied in central place foragers such as ants and bees. However, if butterflies memorize a spatial location remains unclear. Here, we conducted behavioural experiments to test whether monarch butterflies (Danaus plexippus) can remember and retrieve the spatial location of a food source. We placed several visually identical feeders in a flight cage, with only one feeder providing sucrose solution. Across multiple days, individual butterflies predominantly visited the rewarding feeder. Next, we displaced a salient landmark close to the feeders to test which visual cue the butterflies used to relocate the rewarding feeder. While occasional landmark displacements were ignored by the butterflies and did not affect their decisions, systematic displacement of both the landmark and the rewarding feeder demonstrated that the butterflies associated the salient landmark with the feeder's position. Altogether, we show that butterflies consolidate and retrieve spatial memory in the context of foraging.