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First published online February 15, 2006
Journal of Experimental Biology 209, v (2006)
Copyright © 2006 The Company of Biologists Limited
doi: 10.1242/jeb.02099
Outside JEB |
FOOD FOR THOUGHT
University of Cambridge
jen22{at}hermes.cam.ac.uk
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Many animals, including humans, devote considerable amounts of time to searching for a nutritious meal. Once food is found, it's a good idea to remember that spot so that the food can be exploited until it runs out. Some animals, like jays or squirrels, go to considerable lengths to cache food for future consumption, remembering the precise locations of food items for many weeks. For most animals, however, recalling the location of a food source is only important if they're hungry. It might be expected, therefore, that hunger could cause an animal to recall the memory of how to get to a particular food source; a bit like our hunger pangs inspiring us to remember the way to the fridge.
Ants use numerous cues, including visual and olfactory, to navigate around their environments and locate both their nests and food sources. One species, the wood ant Formica rufa, can learn the locations of particular visual landmarks and use these to navigate. This makes wood ants an ideal system to test whether the feeding state of an animal could induce visual memory recall that enables the animal to reach either a food source (if it's unfed) or its nest (if it's fed). Robert Harris, Tom Collett and co-workers in Sussex University, UK, decided to test wood ants to see the effect that feeding state had upon the insect's visual memory recall.
In the first experiment, the wood ants were trained to forage in the presence of a wall that was on their left as they walked to the foraging site and on their right when they returned home. So, when the ants walked towards food, they saw the wall with their left eye but when they returned they saw it with their right eye. To test whether their feeding state could induce the recall of a different visual memory (wall seen by the left or right eye), ants were placed midway along the wall. Those that hadn't fed walked towards food, seeing the wall with their left eye just like an ant leaving the nest to get food, but those that had fed headed for home with the wall on their right. To check whether the ants had learnt fixed motor patterns, fed and unfed ants were placed midway between two parallel walls. The unfed ants walked along one of the walls keeping the wall always on the left, a strategy that should lead them to food, whereas the fed ants adopted the opposite strategy, which should lead them home.
In a second experiment the wood ants were tested in a `Y' maze with two arms, one of which led to home and the other to food. The two arms were marked by different visual patterns. The patterns were switched between the two arms, allowing Harris and his co-workers to separate the visual pattern that the ants chose from the exact route that they took. The wood ants learnt to choose the visual pattern leading to home or food depending on whether they were fed or unfed, respectively. After training, even when the ants weren't rewarded, the unfed ants chose the visual pattern that should lead to food whereas the fed ants chose the pattern that led towards home.
These experiments provide strong evidence that the feeding state of an ant can contribute to it choosing between alternative paths leading to different goals. In the wood ant, these paths must be visually discriminated, suggesting that feeding state is capable of altering which visual memories are acted upon. However, many questions remain about this system. It would be interesting to know whether ants can learn and select between food sources with different nutritional qualities such as carbohydrate and protein. Other insects can certainly adjust their intake of different nutrients. Another issue that is outstanding is the actual mechanism that assesses feeding state and affects the recall of different memories – both mechanosensory feedback from the gut and feedback from energy stores (such as the fat body) could play important roles. With so many intriguing questions to be answered, studies on the wood ant are certain to continue to provide insights into the recall and use of memories.
References
Harris, R. A., Hempel de Ibarra, N., Graham, P. and Collett, T. S. (2005). Priming of visual route memories. Nature 438,302 .[CrossRef][Medline]
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