Loss of attraction for social cues leads to fungal-infected Myrmica rubra ants withdrawing from the nest

In social insects, individuals infected by pathogens withdraw from the nest, preventing the spread of diseases among genetically related nestmates and thereby contributing to the 'social immunity' of the colony. Here we investigated the extent to which the isolation of sick ants correlates with changes in their behavioural responses to environmental stimuli that serve as nest-related cues, including light, colony odour and physical presence of nestmates. Myrmica rubra ant workers infected by Metarhizium brunneum fungus showed a weak but constant attraction to light. By contrast, the progressive withdrawal of moribund workers from the nest appeared to be concomitant with a decline in their attraction towards nestmates or colony odour, which started on the third day after infection. We hypothesized that the fungus impaired the olfactory system of infected ants, preventing them from adequately reacting to chemical blends involved in colony marking and nestmate recognition. Instead of being an active behaviour, the social seclusion of sick ants appears to be the simple outcome of their increasing difficulty in orienting themselves towards nest-related cues. This phenomenon was reinforced by minor positive phototropism and the progressive dysfunction of motor skills as the infection progressed.