What drives choice? Which conditions do we base our decisions upon: those at the time we learn the information needed to make the choice, or those at the time of choice itself? It might be wiser to base one's decision on the conditions at the time of choice and not be influenced by the past. However, Lorena Pompilio and colleagues from the University of Oxford have recently shown that a grasshopper's choice is dependent upon how much it gained at the time of learning, not at the time of choice. A similar effect has been demonstrated in mammals and birds, but this is the first demonstration in an invertebrate species of remembered gains driving future preferences. This implies that the mechanism of making decisions based on past conditions (state-dependent valuation) was not an early vertebrate acquisition but is a wider phenomenon and perhaps universally present.
The team investigated how a grasshopper's nutritional state at the time they learned to recognize a scent affected their subsequent choices when re-presented with the scent. First the team restricted the grasshoppers' food intake so that they were in a `poorly fed' state. Next, they trained the `poorly fed' insects to recognize a scent, lemon grass, while rewarding them with a wheatseedling treat. Once the insects had learned to recognize the lemon grass odour, the team switched the insect's nutritional state to `well-fed' by giving them access to unlimited food and retrained them to recognize a peppermint odour associated with a wheat-seedling reward. Next Pompilio and her colleagues tested whether or not the insect's nutritional state at the point that they learned to recognize each scent influenced their choice. The team placed the grasshoppers in a Y-maze; one arm of the Y-maze arms had the peppermint odour and the other was scented with lemon grass, and both scents were accompanied with a wheat-seedling reward. The insects always chose the odour that they associated with the `poorly fed' state during training, supporting the hypothesis that choice is governed by past gains. Thus, food in the `poorly fed' state is more rewarding and the odour paired with the `poorly fed' state becomes linked with this rewarding experience, prompting the grasshopper to choose the odour associated with the `poorly fed' state since it elicited an expectation of greater reward.
The authors suspect that nutrient levels in the hemolymph of underfed insects drop and the taste receptors become increasingly sensitive to key depleted nutrients resulting in greater feedback when the insect contacts a food item. The team explains that this probably alters the insect's memory. Instead of remembering the attributes of each option and weighing them against the current nutritional state, state-dependent valuation, situations where the insect makes a decision based on its previous experience may be computationally more efficient. It may reduce errors and make decision-making quicker by limiting the amount of information that is processed during decision-making. In the event of competition for food resources it could aid in initiating a faster and more successful dash to the prize.
- © The Company of Biologists Limited 2006