I used to work in Edinburgh’s Butterfly and Insect World. While I was there, my favourite animal was not the chameleon, which changed colour when it was angry. Nor was it the royal pythons that loved human body heat, and would sometimes squeeze unnervingly tight. No, my favourite exhibit was the leafcutter ant.
We had quite a remarkable system with the leafcutters. Every morning I would go for a wander outdoors and collect bits of plants. These ranged from hedge clippings, to flowers, to holly branches (I would usually choose plants that involved climbing a tree or two). I would then take these plant cuttings and place them in an area that the ants had access to via a hanging rope. The ‘forager’ leafcutters would swarm on to the plants and start cutting off small pieces (living up to their name). While they were doing this, the ‘guard’ ants with their bulking jaws would stand guard, waving their mouthparts threateningly.
The ants would then start marching back up the rope, suspended from the roof. Carrying their hoard, they followed this rope walkway all the way into their nest. They would then take the plant cuttings deep into the nest, into a specific room. In this room there were ‘worker ants’ farming the ants’ harvest: fungus. The fungus was nurtured and cared for by these farmers, and was then used to feed the young of the colony. The fungus grew on some plants very well, whilst other plants seemed to have a natural resistance to it.
What was really incredible about this was that the ants learned which plants were good for their fungus farm and which were not. If one day I gave them a plant that the fungus did not grow on easily, they wouldn’t take it the next time I gave it to them. As time went on I learned which plants the ants liked the most and would do my best to get these ones for them, though I would usually throw in the odd new one for them to try out. I have to admit, it started to feel a bit like I was working for them… I’d love to go off on a tangent about the excellent Ant horror movie, ‘Them’ right now, but I will resist, however do check out the trailer at the end.
So, how did the ants know which plants made good fungus-food and which did not? A study I came across the other day looked into just this, using the leafcutters Acromyrmex ambiguus 1. In this experiment, some ant colonies were given blackberry and plum leaves, which both are good for growing fungus. Other colonies were given the same two sets of leaves, but this time with fungicide (undetectable to the ants) on the blackberry leaves.
After the ants were given a chance to try and grow fungus on these two plants, individuals were tested to see which type they would then choose when given an option. When an ant had experienced untreated blackberry and plum, they preferred the blackberry the next time they were offered it. However, ants which were given blackberry with fungicide on it chose it less often the following time, after their experience of it being useless for their farm.
The really interesting bit came next, when the scientists gave the ants a choice between just the odours of plum and blackberry. Amazingly, they found the same pattern, only slightly less pronounced: ants still showed a preference for the best fungus-grower.
So, it seems that ants use the odour of plants to learn which ones to avoid in the future. However, other cues like taste may also be important, which is why the ants avoided the ‘bad’ plant even more when they had more to go by than just the smell.
The exact mechanism for how the ants learn to associate the poor result of certain plants in their garden to their choices made the next day is still not known, but I’m very much looking forward to seeing the upcoming experiments on these fascinating little creatures.
1 Saverschek, N. & Roces, F. Foraging leafcutter ants: olfactory memory underlies delayed avoidance of plants unsuitable for the symbiotic fungus. Animal Behaviour. doi:10.1016/j.anbehav.2011.05.015