Audio and video files showin reactions of hippos to acoustic cues
Four male hippos (H1–H4) at DAK were conditioned to associate a novel acoustic cue (1–2 kHz frequency modulated tone, 2 s duration) with the delivery of carrots on land in their indoor holding areas. Only male hippos could be isolated in the exhibit pools due to management constraints. Carrots were used because they were not a typical food item and were the only food item that sunk to the bottom of pool. Once the hippos demonstrated anticipatory search behaviour in response to the acoustic cue (turning and moving toward the food object), this procedure was transferred to their outdoor exhibit pools (N = 10 pre-experimental sessions) to elicit search behaviour for carrot bundles (4–6 carrots tied together with edible banana leaves) on land (the river banks), in shallow water, and ultimately deeper water (approximately 15 feet deep).
Three of the four male hippos conditioned did not search underwater for carrot bundles in exhibit pools. A fifth male hippo (H5) was included later, but was not pre-conditioned due to time constraints and only tested in the exhibit pools. We recorded the five male hippos during ten baseline sessions (N = 2 sessions with solitary males, N = 7 sessions with paired males, N = 1 session with three hippos) when no food items were present and 68 experimental sessions (N = 46 sessions with solitary males, N = 22 sessions with paired males) when carrots were offered.Video 1 shows search behaviour by a male hippo in an experimental session. One click train is produced underwater which was identified using spectral analysis and noted on the video. Audio 1 is a clip of a broadband click train from Figure 3 and Audio 2 a clip of a narrowband click train from Figure 3.