Optimistic ants: Positive cognitive judgement bias but no emotional contagion in the ant Lasius niger

Understanding the emotional states of animals is key for informing their ethical treatment, but very little attention has been directed towards the emotional lives of invertebrates. As emotions influence information processing, one way to assess emotional states is to look for an individuals cognitive bias, i.e., their tendency to make optimistic or pessimistic judgements. Here we developed a free-running judgment bias task for the ant Lasius niger, and applied the judgement bias to assess ants reactions towards positive and negative stimuli. After an initial learning phase in which individuals were trained to associate two odour stimuli with positive or negative reinforcement, their reaction towards ambiguous stimuli, i.e., a mixture between both odours, was assessed. We also explored our study species capacity to socially transmit emotional states ( emotional contagion) by investigating whether social information could elicit emotional responses. We find L. niger to be optimistic, showing a baseline positive judgement bias, with 65-68% of ants preferring an ambiguous 1:1 mix of positive and negative cues over no cues. Providing an unexpected food reward prior to the judgement bias task increases positive judgement bias (c. 75% positive). There was a non- significant tendency towards a negative judgement bias after experiencing a mild electric shock (c. 75% negative). Neither positive nor negative social information (trail and alarm pheromones, respectively) affected the ants judgement biases, thus providing no indication for emotional contagion. The development of a powerful, simple, and ecologically relevant cognitive judgement task, deployable in the lab and in the field, opens the door to systematic comparative studies of the evolutionary and ecological causes of judgement bias.