Cognition is one of the mechanisms underlying behavioral flexibility, but flexibility of cognition itself may vary as a result of trade-offs between the ability to learn new information and the ability to retain old memories. How and when cognitive flexibility is constrained by this trade-off remains poorly understood. We investigated cognitive flexibility in wild food-caching mountain chickadees in the field at different elevations experiencing different levels of environmental harshness, using a spatial learning and memory reversal paradigm. There were no significant differences in sampling strategies between elevations, but high elevation chickadees performed worse than low elevation chickadees on the reversal task, indicating lower cognitive flexibility. Compared to the initial learning task, low elevation chickadees improved their performance during the reversal task, while high elevation chickadees performed worse. High elevation birds inspected previously rewarding locations more frequently than other locations, suggesting that reduced cognitive flexibility is associated with proactive interference. Considering that high elevation chickadees cache more food and are likely more dependent on these caches than their conspecifics from low elevation, and that chickadees from both elevations use similar sampling strategies, our findings suggest that stronger memories of more caches might interfere with acquisition and retention of new memories. Overall, our results suggest that predictably harsh environments might favor stronger memories at the expense of decreased cognitive flexibility, which is likely driven by increased proactive interference.

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Cognitive flexibility allows animals to readily acquire new information even when learning contingencies may rapidly change, as is the case in highly variable, but predictable environments. While cognitive flexibility is broadly thought to be beneficial, animals exhibit inter-and intra-specific variation, with higher levels of flexibility associated with reduced memory retention and vice versa. In this review, we discuss when and why such variation may exist and focus specifically on memory and memory flexibility. We argue that retained memories may negatively affect the acquisition of new information, most likely via proactive interference, and available data suggest that there may be a trade-off between memory retention and acquiring new memories. We discuss neurogenesismediated forgetting as the mechanism reducing memory interference, as new neurons enhance learning new information, but also cause forgetting of older memories. Selection may be expected to favor either end of the continuum between memory retention and memory flexibility depending on life history and environment. More stable environments may favor memory retention over flexibility whereas rapidly changing environments may favor flexibility over retention. Higher memory capacity also seems to be associated with higher memory interference, so higher neurogenesis rates associated with forgetting of unnecessary information may be favored when higher capacity is beneficial such as in food-caching species. More research is necessary to understand if inter-and intra-specific differences in the association between memory retention and flexibility are related to some general ecological patterns, whether this association is heritable, and whether developmental conditions and experience have different effects on this association in different species.