Subjective value of risky foods for individual domestic chicks: a hierarchical Bayesian model
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For animals to decide which prey to attack, the gain and delay of the food item must be integrated in a value function. However, the subjective value is not obtained by expected profitability when it is accompanied by risk. To estimate the subjective value, we examined choices in a cross-shaped maze with two colored feeders in domestic chicks. When tested by a reversal in food amount or delay, chicks changed choices similarly in both conditions (experiment 1). We therefore examined risk sensitivity for amount and delay (experiment 2) by supplying one feeder with food of fixed profitability and the alternative feeder with high- or low-profitability food at equal probability. Profitability varied in amount (groups 1 and 2 at high and low variance) or in delay (group 3). To find the equilibrium, the amount (groups 1 and 2) or delay (group 3) of the food in the fixed feeder was adjusted in a total of 18 blocks. The Markov chain Monte Carlo method was applied to a hierarchical Bayesian model to estimate the subjective value. Chicks undervalued the variable feeder in group 1 and were indifferent in group 2 but overvalued the variable feeder in group 3 at a population level. Re-examination without the titration procedure (experiment 3) suggested that the subjective value was not absolute for each option. When the delay was varied, the variable option was often given a paradoxically high value depending on fixed alternative. Therefore, the basic assumption of the uniquely determined value function might be questioned.
KeywordsRisk Chick Choice Bayesian estimation Value
We express our sincere gratitude to Dr. Takuya Kubo (Hokkaido University) for his generous guidance and instruction on statistical computations, and Dr. Michael Colombo (University of Otago, New Zealand), Dr. Giorgio Vallortigara (University of Trento, Italy), and Dr. Tiaza Bem (Polish Academy of Science, Poland) for their critical comments on the manuscript. We would also thank anonymous referees for their critical reading, generous comments, and instructive suggestions, which were valuable in revising the paper. This study was supported by grants from the Japan Society for the Promotion of Science (JSPS; grant-in-aid for scientific research (C), #19500260) and the Japanese Ministry of Education, Culture, Sports, Science, and Technology (MEXT; grant-in-aid for scientific research on priority areas—Mobiligence #20033001) to T.M. Experiments were conducted under the guidelines and approval of the Committee on Animal Experiments of Hokkaido University. The guidelines are based on the national regulations for animal welfare in Japan (Law for the Humane Treatment and Management of Animals; after partial amendment No. 68, 2005).
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