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Yawning is the signature behavior of hippopotamuses living in natural African habitats. Animal studies profile the theoretical role of yawning in cooling the brain to counteract overheating for better brain function. For hippos residing in a pool during the day, the top of the head is exposed to direct heating by the sun. We examined a prediction from the brain-cooling hypothesis for yawning in a pod of approximately 40 hippos resident in a pool off the Luangwa River. We predicted that when dusk approaches and hippos leave for foraging, and where optimal functioning of the brain would be essential, the frequency of yawning would markedly increase and approximate one yawn per hippo. Yawning was expected to be infrequent when hippos returned to the pool in the morning and during the day. Predictions were borne out over 4 consecutive day/night observations. In the 1.5 hours of evening, just before the hippos left the pool, there was a mean of 1.3 yawns per hippo compared with 0.03 yawns per hippo in 1.5 hours in the morning just after returning from foraging. The findings clearly support the concept of brain cooling as one of the functions of yawning in free-living hippos.