Otters, but not apes, prepare for mutually exclusive possibilities.

The ability to prepare for mutually exclusive outcomes is often considered uniquely human. Solving such problems requires anticipating alternative futures before acting. In the classic forked-tube task, the optimal strategy is to block both exits to secure a reward: children under four years and great apes typically fail, whereas older children succeed. Using this paradigm, we tested three Asian small-clawed otters (Aonyx cinereus) and one Eurasian otter (Lutra lutra) and compared their performance with chimpanzee data. Otters covered both exits significantly more often than chimpanzees, with all individuals succeeding within their first trials. Initial inconsistency in maintaining the strategy appeared linked to anatomical constraints that limited reward success. When retested two months later with an apparatus better suited to otter morphology, individuals adopted and maintained dual coverage as success increased, indicating that the behavior tracked the payoff structure of the task rather than reflecting low-level mechanisms such as trial-and-error learning. Together, these findings indicate that blocking both exits is an adaptive response to the tasks causal structure, supporting the ecological intelligence hypothesis: cognition evolves in response to ecological demands, particularly foraging challenges that place recurrent pressures on memory, decision-making, and executive control, rather than being driven solely by social complexity.