The acquisition and expression of memories of distance and direction in navigating wood ants

Abstract

Wood ants, like other central place foragers, rely on route memories to guide them to and from a reliable food source. They use visual memories of the surrounding scene and probably compass information to control their direction. Do they also remember the length of their route and do they link memories of direction and distance? To answer these questions, we trained wood ant (Formica rufa) foragers in a channel to perform either a single short foraging route or two foraging routes in opposite directions. By shifting the starting position of the route within the channel, but keeping the direction and distance fixed, we tried to ensure that the ants would rely upon vector memories rather than visual memories to decide when to stop. The homeward memories that the ants formed were revealed by placing fed or unfed ants directly into a channel and assessing the direction and distance that they walked without prior performance of the food-ward leg of the journey. This procedure prevented the distance and direction walked being affected by a home vector derived from path integration. Ants that were unfed walked in the feeder direction. Fed ants walked in the opposite direction for a distance related to the separation between start and feeder. Vector memories of a return route can thus be primed by the ants' feeding state and expressed even when the ants have not performed the food-ward route. Tests on ants that have acquired two routes indicate that memories of the direction and distance of the return routes are linked, suggesting that they may be encoded by a common neural population within the ant brain.

A Neurocomputational Model of Goal-Directed Navigation in Insect-Inspired Artificial Agents

“…Our proposed encoding of GVs is validated by recent findings from a behavioral study on wood ants (Fernandes et al, 2015). The authors carried out a series of novel experimental paradigms involving training and testing channels.…”

“…For GVs, the context is determined by the foraging state, which we model as a binary unit. Indeed, behavioral studies on desert (Wehner et al, 2006) and wood ants (Fernandes et al, 2015) have shown that expression of spatial memories is controlled by an internal state in a binary fashion. The association of the context with a reward signal, received at the feeder, drives synaptic weight changes corresponding to the difference between the current PI state and the respective weight.…”

“…Another example of vector memory is the waggle dance of honeybees (De Marco and Menzel, 2005; Menzel et al, 2005), in which the distance and direction to a goal are encoded by the duration and direction of the dance, respectively. After returning from a successful foraging run, insects re-apply this vector information in subsequent foraging runs (Capaldi et al, 2000; Wolf et al, 2012; Fernandes et al, 2015). …”

Neural mechanisms for reward-modulated vector learning and navigation: from social insects to embodied agents

“…Our proposed encoding of GVs is validated by recent findings from a behavioral study on wood ants [76]. The authors carried out a series of novel experimental paradigms involving training and testing channels.…”

“…For GVs, the context is determined by the foraging state, which we model as a binary unit. Indeed, behavioral studies on desert [75] and wood ants [76] have shown that expression of spatial memories is controlled by an internal state in a binary fashion. The association of the context with a reward signal, received at the feeder, drives synaptic weight changes corresponding to the difference between the current PI state and the respective weight.…”