A harnessed ant with open mandibles. Photo credit: Paul Devienne, Laboratoire d'Ethologie Expérimentale et Comparée.
The sudden appearance of a smell can instantly comfort us or make us uneasy. Pleasant smells may trigger happy childhood memories, and smells we find unpleasant may set us on edge and drive us to avoid whatever is malodorous – sometimes for reasons we don't fully understand. This reaction to smell is not unique to humans. Insects rely heavily on their attraction to and repulsion by smells, based on their past experiences. ‘We usually think that insects are little robots, following rules that are innate, and they're not plastic in their behaviour,’ says Patrizia d'Ettorre, at the University of Paris 13, France, ‘but, this is absolutely not true. They do learn a lot.’ For decades, honey bees were the insects we turned to for insights into learning and memory. However, d'Ettorre is convinced that ants, being more diverse in ecology and life history than honey bees, will open up a broader view into the mechanisms behind learning and memory. Her team, including researchers from the University of Toulouse, France, wanted to study aversive conditioning in ants – training them to associate an otherwise neutral smell with an unpleasant experience. The problem? ‘This has not been done so far with ants,’ d'Ettorre says. Why not? ‘I think it's because it's difficult’, she laughs, ‘We really had to be stubborn to make this work.’
Lucie Desmedt, a Master's student in d'Ettorre's lab, set to work on a way to train carpenter ants to learn and remember a new unpleasant odour association. She placed individual ants into a harness and presented them with one of two natural odours. To help the ant learn a bad association with the odour, she touched the ant's back legs with a hot probe, an experience not unlike that which an ant would encounter walking on open ground in the summer. This unpleasant sensation caused the ant to open its mandibles wide in an aggressive display.
Desmedt tested the ant's ability to learn in two contexts. She trained one group to associate a single odour with the heated probe. For the second group, she provided the ants with two odours, one associated with a dab of the hot probe and another where no heat was applied. She then tested the ant's ability to remember the negative association by presenting the ‘hot’ odour, or the other odour, to each ant 10 min later and watching the ant's reaction.
Not surprisingly, the ants learned a negative association with the hot odour after just a few trials. And, mandibles wide, they displayed their discontent when they encountered the hot odour again 10 min later. Also, the ants learned and remembered best when they were presented with the additional odour (that was not accompanied with heat), which is similar to the behaviour of ants trained with food rewards and for honey bees and Drosophila. d'Ettorre suspects that it is easier for the animals to learn to remember the odour that is associated with the uncomfortable experience when it is trained in parallel with a second odour, and says ‘The stimulus is more relevant’, probably because they have another experience to compare it with. In other words, context makes a difference.
With a successful new way to train ants, d'Ettorre lists potential future projects: ‘We can now study mixtures of odours; we can use a pharmacological approach to study the dynamics of memory, the underlying brain circuits that are working in different types of learning…’. And she hopes to continue to pursuing the nitty gritty of those learned smell associations that treat or torment animals of all sizes.
