Increased brain growth in escaped rainbow trout

Recent examples of rapid brain size plasticity in response to novel laboratory environments suggest that fish brain size is a flexible trait, allowing growth or shrinkage of brain tissue based on short term needs. Nevertheless, it remains to be seen if plasticity of fish brain size is relevant to natural environmental conditions. Here, using rainbow trout escaped from a farming operation as a natural experiment, we demonstrate that adult fish brain size can change rapidly in response to life in a natural lake environment. Specifically, escaped trout had on average 15% heavier brains relative to body size than captive trout after living for about 7 months in the lake. Because relative brain size of most escaped trout fell above the range of variation seen within the captive trout population, we conclude that increased brain size was achieved by plasticity after escape. Brain morphology analysis showed that the most anterior regions (olfactory bulbs and rest of telencephalon) contributed most to the increase in overall brain size in escaped trout. Relative size of the heart ventricle, another organ which can be subject to plastic changes under variable environmental conditions in fish, did not differ between escaped and captive trout. Massive and selective brain growth under the changed environmental conditions associated with escape from holding pens highlighted the plastic potential of fish brain size and suggests that a shift to increased complexity of life in the wild setting of a lake imposed greatly increased cognitive requirements on escaped trout.