Insects have played a significant role in the study of learning and memory. They possess a relatively simple nervous system, allowing for a cellular and even molecular approach. Most research on insect learning and memory, however, has so far focused on the fruit fly and honey bee. Ants have evolved what can be considered among the most advanced and complex societies. Despite this, they have generally been neglected in studies addressing cognitive processes.
All ants are eusocial and are characterized by cooperative brood care, reproductive division of labour and overlapping generations. Because ant workers usually do not reproduce, fitness benefits are gained only if cooperative acts are directed towards nestmates, which are usually related, and not towards alien individuals. This requires an effective recognition system, allowing reliable discrimination of nestmates from non‐nestmates. Nestmate recognition is therefore essential for the stability and success of insect societies.
This recognition system can be divided into three components: production, perception and action. In ants, the production component involves colony‐specific chemical cues (label), located on the cuticle of each individual. The label is a blend of long‐chain hydrocarbons, varying in quality and in quantity between different colonies and species. When an individual ant encounters another individual, it detects the label of the encountered individual with its antennae, and evaluates the similarity between this label and its own neural representation of the colony odour, called the template (perception component).
If this perceived label is too dissimilar compared to the template, the ant will reject the encountered individual (action component). In this thesis, I focus on all three components, with an emphasis on perception, which involves cognitive processes. First, we review current evidence on the possible role of learning in acquiring the recognition template and we present a novel model for the perception component of the nestmate recognition system. In the experimental papers, we describe a novel way in which ants can distribute their label between individuals in order to homogenize the colony odour. We elucidate how chemical structure of stimuli, the context wherein they are perceived, and the individual ant’s learning experience, influence the way she perceives the chemical world surrounding her. We test how worker specialisation, i.e. the task that workers perform, such as brood care and foraging, influences their nestmate recognition abilities. We show that ants performing tasks inside the nest, which are not likely to encounter non‐nestmates, have a less efficient recognition system than workers spending most of their time outside the nest.
We also investigate whether recognition of identity plays a role in social immunity and we find a negative result. Ants that become terminally sick do not signal their physiological status to their nestmates, but instead actively leave the nest to die in isolation, thus preventing the disease from spreading through the colony. As these moribund ants have nothing to lose anymore (they will die anyway), they increase their aggressive behaviour against non‐nestmates, possibly to repel any potential invaders from the colony, thus expressing their ultimate cooperative behaviour towards nestmates.