動物学：確率を理解しているオウム

The kea, a parrot species from New Zealand, is capable of understanding and acting upon probabilities, according to a study published this week in Nature Communications . This finding is the first known report of statistical inference in an animal outside of the great apes.

Amalia Bastos and Alex Taylor conducted a series of experiments designed to test several criteria for statistical understanding, mirroring previous studies done with primates and human infants. First, they trained six kea (named Blofeld, Bruce, Loki, Neo, Plankton and Taz) to associate the colour black with a reward and the colour orange with no reward. The authors varied the relative frequencies of orange and black tokens in two transparent jars, and offered the kea tokens from both jars, concealing each token in a closed fist.

The authors found that the kea preferred tokens from jars with a higher relative, but not necessarily absolute, frequency of rewarding objects. When a horizontal barrier was placed in the jar and changed the fraction of rewarding tokens accessible above the barrier, the kea detected this physical constraint and selected the jar with the highest probability of accessible rewarding tokens. Finally, kea also preferred selecting tokens from an experimenter who had previously demonstrated a ‘bias’ towards offering a higher fraction of rewarding tokens.

Previously only humans and the great apes have been found to be capable of true statistical inference. Showing this type of complex, higher order cognitive process in a bird may help us to understand the evolutionary history of statistical inference.