Foraging insects need to exploit every available cue to track down food fast. This is why many insects associate certain scents with food in order to home in on tasty treats. They can also be trained to associate specific odours with sweet or bitter tastes, and scientists have taken advantage of this skill to learn about the mechanisms of memory formation. ‘They learn simple and complex associations and possess a relatively simple nervous system that allows retracing associative phenomena to the cellular and molecular level,’ explain Fernando Guerrieri and his colleagues from the University of Copenhagen, Denmark and the University of Toulouse, France. According to the team, the strength of a memory depends on the way the ants were trained, with short memories lasting no more than an hour and long-term memories holding for days. Also, new proteins have to be synthesised in the brain for a memory to be truly long term. Knowing that Camponotus fellah ants seem to be able to retain memories for at least 3 days, Guerrieri and his colleagues Patrizia d'Ettorre, Jean-Marc Devaud and Martin Giurfa decided to find out whether the memories were genuinely long term by testing to see if ants' brains synthesise new proteins after long-term memory training ().
‘A hungry harnessed ant extends its maxilla-labium [mouth parts] when its antennae are stimulated with a sucrose solution,’ the team explains, and adds, ‘the maxilla-labium extension reflex can also be elicited by odours if these are appropriately paired with sucrose solution.’ Training ants to associate sweet sucrose with the scent of the hydrocarbon docosane – extending their mouth parts when presented with the odour – the team also trained the ants to associated bitter-tasting quinine with another hydrocarbon, octacosane, to keep their mouths immobile in response to the scent. Next, they tested the insects' memories 1 h, 12 h and 3 days after training to see if the insects remembered when to extend their mouthparts and when to keep them retracted. Sure enough, the ants remembered to extend their mouthparts in response to docosane while remaining immobile when their antennae picked up octacosane. Their long-term memory seemed to be primed, but had they consolidated the memory by synthesising new proteins in their brains?
The team repeated the experiment, training another group of ants to associate the odours and hydrocarbons, but this time they fed the ants a chemical, cycloheximide, before training to prevent them from synthesising new proteins. If the 72-h-old memories were genuinely long term, the ants that had been fed cycloheximide would be unable to distinguish between the two odours 3 days later: and they didn't. However, the ants had no problem forming short- and mid-term memories. They still remembered the odours 1 h and 12 h after testing; they just couldn't consolidate their long-term memories by synthesising proteins.
So, having shown that protein synthesis is the key to ants' long-term memories, the team is keen to find out whether ants use the same memory structures and circuits as that other champion of insect memory, the bee, and to learn more about how insects use their memories to navigate their complex environments in search of food.
