Monkeys (Sapajus apella and Macaca tonkeana) and great apes (Gorilla gorilla, Pongo abelii, Pan paniscus, and Pan troglodytes) play for the highest bid

Many studies investigate animals’ decisions under risk focussing on individuals’ attitudes towards risk. Yet, little attention has been paid to individuals’ understanding of outcomes’ odd. In a previous gambling task (Pelé et al., 2014), we showed that non-human primates used prospective gains and losses to reach their decision even when 18 different lotteries were used. However, the study offered gradually decreasing chances to win from the beginning to the end of the experiment. We suggest that this probably facilitated the extraction of information about odds at each lottery. In the current experiment, we controlled for this factor by presenting those lotteries in a random order. We tested individuals from four ape and two monkey species to better evaluate potential species differences in the cognitive evaluation of outcome odds. We found little species differences. Subjects were not expected value maximizers but used heuristic decision-making. This heuristic called maximax, involves playing as soon as there is at least one chance to win more than what they already possess whatever the risk. Also, most species took into account outcome frequency, playing more as larger reward frequency increased. We showed little effects of learning and reinforcement through repetition of trials. Together, these results are reminiscent of optimistic behaviour, often seen in human managerial decisions and finance where risk is ignored in favour of potential even if low probability gains. Reducing the complex problem of cognitive evaluation to a maximax heuristics can be found in other species than human beings