Multisensory navigation and neuronal plasticity in desert ants

Cataglyphis desert ants are skilled visual navigators. Here, I present a brief overview of multisensory learning and neuronal plasticity in ants, with a particular focus on the transition from the dark nest interior to performing first foraging trips. This highlights desert ants as experimental models for studying neuronal mechanisms underlying behavioral development into successful navigators.


Multisensory navigation and neuronal plasticity in desert ants
Wolfgang Rössler 1, * Cataglyphis desert ants are skilled visual navigators. Here, I present a brief overview of multisensory learning and neuronal plasticity in ants, with a particular focus on the transition from the dark nest interior to performing first foraging trips. This highlights desert ants as experimental models for studying neuronal mechanisms underlying behavioral development into successful navigators.

Behavioral transitions and the acquisition of navigational information in ants
Spatial orientation is essential for most animal species. The discoveries of place and grid cells in the mammalian hippocampus and entorhinal cortex represent milestones in uncovering neuronal mechanisms underlying spatial orientation. How do small-brained animals process multisensory information and acquire the competence for efficient long-distance navigation in unpredictable environments? Desert ants of the genus Cataglyphis are valuable experimental models to address these questions.
Ants live in complex societies that may comprise up to millions of individuals per colony. Reproductive division of labor between female castes (queen-worker polymorphism) and adult behavioral flexibility among groups of workers (worker polyethism) are essential features underlying the organization of ant societies.
For cooperative food provisioning of the queen's offspring, experienced workers venture out on far-ranging foraging excursions from which they return to their common nest. Many ant species employ trail pheromones, much like an external memory system, for olfactory guidance of nestmates to profitable food sources and back to the nest. However, to lay new chemical trails, pioneering ants must rely on other senses when they return home from previously unfamiliar terrains.
Thermophilic desert ants of the genus Cataglyphis ( Figure 1A) are solitary foragers, scavenging for food (mostly dead insects) during the hottest time of the day. Ground temperatures of more than 60°C render trail pheromones impossible. Earlier work had shown that experienced Cataglyphis foragers use the position of the sun, particularly the associated polarized-skylight cues, as a compass. The ants integrate this compass input with distance information from a step-integrator to estimate a home vector pointing to the nest along the shortest way possible (path integration) [1]. To correct for cumulative errors inherent to path integration over longer distances, experienced foragers use panoramic landmarks, whenever available, as additional visual guidance cues.
Young ants undergo an age-related polyethism. They start with storing food and feeding larvae to then switch to nest maintenance tasks. After weeks in darkness, the ants eventually leave the underground nest for the first time. How do naïve ants acquire navigational information? As both the daily course of the sun (solar ephemeris) and the panoramic scenery are unpredictable for naïve ants, these navigational cues must be learned and cannot be fully encoded genetically. Before heading out on first foraging trips, Cataglyphis ants perform structured learning walks for 2-3 days, extending in small loops into different directions close to the nest. The ants perform body rotations (pirouettes) interrupted by brief stops when their body axis is oriented towards the nest entrance [2]. As the ants cannot see the nest entrance from their positions, these nest-directed views provide an ideal experimental readout for the ants' path integration abilities. Manipulation experiments demonstrate that in contrast to experienced foragers, young ants, during learning walks, exclusively rely on the Earth's magnetic field as their reference compass for sampling nest-related panoramic views and calibrating their sky-compass systems.

The ant brain: plasticity triggered by internal and external factors
The marked interior-forager transition in Cataglyphis demands high levels of behavioral flexibility. Naïve ants must learn and memorize complex navigational information with a brain of about a millimeter in diameter and an estimated number of less than one million neurons. Recent studies highlight the neuroanatomical and neurochemical complexity of Cataglyphis' brain ( Figure 1B). A confocal imagingbased 3D-neuroanatomical atlas identified 33 synapse-rich brain compartments (neuropils) and 30 connecting fiber tracts in Cataglyphis nodus (see Insect Brain Database i ). Comprehensive transcriptomic and peptidomic analyses characterized 71 peptides with likely neuroactive function in C. nodus brain; 22 of those were spatially mapped at high resolution using mass spectrometric imaging [3]. Together with recent comparative genomic analyses in facultatively parthenogenetic Cataglyphis hispanica [4], these data bases provide valuable resources for integrative studies on plasticity in the ant brain.
What are the internal drivers for the interiorforager transition, how does the ant brain deal with drastic changes in multisensory input, and where is navigational information stored? Investigations in the basal (ponerine) ant Harpegnathos saltator revealed that the Trends in Neurosciences OPEN ACCESS neuropeptide corazonin triggers behavioral and physiological caste transitions between adult workers and queens [5]. In Harpegnathos, workers remain fertile and potentially transform into substitute queens (gamergates). In Cataglyphis, corazonin mRNA levels and the volume of four corazonergic neurons in each brain hemisphere increase in foragers, suggesting that in this case corazonin is involved in the interior-forager transition of workers ( Figure 1C) [6].
How is visual navigational information processed in the ant brain? Two prominent pathways lead from the primary optic lobes to integration centers: one targeting the mushroom bodies, centers for sensory integration and memory formation, and one terminating in the central complex, a center computing sky-compass and movement information for path integration [7] (Figure 1B). Compared with Drosophila fruit flies, desert ants and other visually oriented social Hymenoptera like bees and wasps possess voluminous mushroom bodies with prominent visual input, in addition to olfactory innervation. A similar situation in solitary parasitoid wasps suggests that advanced spatial orientation was a major driver for the evolution of elaborate mushroom bodies [8]. In line with this notion, specific brain lesions in wood ants show that the mushroom bodies are required for panoramic view memories [9].
A recent study on circuit plasticity involved in long-term memory in Drosophila mushroom bodies revealed an increase in presynaptic boutons of olfactory projection neurons mediating the conditioned (learned) odor stimulus [10]. Behavioral experiments in the natural habitat followed by quantitative neuroanatomical analyses in Cataglyphis demonstrated an increase in synaptic boutons of visual projection neurons in the mushroom bodies and a significant volume increase in the central complex following the performance of learning walks under natural skylight [11]. The results suggest that both long-term storage of panoramic view memories in the mushroom bodies and calibration of sky-compass circuits in the central complex depend on the experience of a rotating sky-polarization pattern during active performance of learning walks.
How are view memories in the mushroom bodies integrated with path-integration information in the central complex? Recent connectome studies in Drosophila using serial electron microscopy revealed that approximately 50% of the mushroom body output neurons synapse onto specific neurons in the fan-shaped body, a subdivision of the central complex [12]. During navigation in Cataglyphis, this pathway could potentially signal panoramic view matches and olfactory context from the mushroom bodies to the central complex to combine this input with path integration information. How magnetic information is received and integrated into these navigational circuits is currently being investigated.

Concluding remarks
Cataglyphis desert ants exhibit profound behavioral flexibility when becoming experienced navigators. Experimental manipulations in the natural habitat combined with neurocircuit analyses during the interior- forager transition revealed high levels of structural plasticity in visual circuits of the ant brain. This triggers many fundamental questions that remain to be addressed. How do neuropeptide modulators orchestrate the behavioral transitions? What is the nature of the magnetic sensor and the associated magnetosensory pathway? Are sky-compass and magnetic cues integrated with the endogenous clock for timecompensation? Cataglyphis ants are established experimental models for studying neuroethological mechanisms underlying their remarkable behavioral development into skillful navigators. Future investigations will benefit from combining neurocircuit analyses with behavioral, molecular, physiological, genetic, and comparative approaches. It will be important to closely link these multidisciplinary analyses to the ants' natural environment.