Abstract
Animals must often decide between exploiting safe options or risky options with a chance for large gains. Both proximate theories based on perceptual mechanisms, and evolutionary ones based on fitness benefits, have been proposed to explain decisions under risk. Eusocial insects represent a special case of risk sensitivity, as they must often make collective decisions based on resource evaluations from many individuals. Previously, colonies of the ant Lasius niger were found to be risk-neutral, but the risk preference of individual foragers was unknown. Here, we tested individual L. niger in a risk sensitivity paradigm. Ants were trained to associate one scent with 0.55 M sucrose solution and another with an equal chance of either 0.1 or 1.0 M sucrose. Preference was tested in a Y-maze. Ants were extremely risk-averse, with 91% choosing the safe option. Based on the psychophysical Weber–Fechner law, we predicted that ants evaluate resources depending on their logarithmic difference. To test this hypothesis, we designed 4 more experiments by varying the relative differences between the alternatives, making the risky option less, equally or more valuable than the safe one. Our results support the logarithmic origin of risk aversion in ants, and demonstrate that the behaviour of individual foragers can be a very poor predictor of colony-level behaviour.
Highlights
Organisms were assumed to maximise energetic gains while minimising costs, on the basis that evolution should drive animals to have optimal behavioural strategies
We found an effect of the first presented feeder (GLMM Analysis of Deviance, Chisquare = 5.4, DF = 1, p = 0.02); more ants chose the safe alternative in the test if they had experienced it first in the training (GLMM post hoc with estimated means, odds ratio = 0.266, SE = 0.148, z = − 2.38, p = 0.017)
We predicted, based on psychophysical principles, that logarithmically balanced rewards should be perceived as having equal value
Summary
Organisms were assumed to maximise energetic gains while minimising costs, on the basis that evolution should drive animals to have optimal behavioural strategies. The optimal foraging theory framework (Pyke et al 1977) fails to fully describe behaviour— organisms do not always behave optimally. Such deviations are often described as “irrational”, it is broadly acknowledged that such deviations often follow bounded or ecological rationality (Fawcett et al 2014). We define risk as a situation in which the probabilities associated with an option (e.g. food source) are known, but the exact payoff which will be received is not known. Rolling a 6-sided die hoping for a 1 is a risky proposition, with a known success chance of 1/6. Rolling a die with an unknown number of sides hoping for a 1 is a proposition under uncertainty
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