Interactions Between Sperm Whales and Risso's and Northern Right Whale Dolphins off San Diego, CA

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Aerial surveys provide a valuable platform to record and document behavior of marine mammals above and below the sea surface. This approach is advantageous in avoiding disturbance from the observational platform while circling outside the sound cone of the plane. Since 2008, the U.S. Navy has instituted a marine mammal monitoring program in southern California from several platforms, including aerial surveys, and has successfully captured previously undocumented behaviors and species interactions during the aerial effort. This included focal behavioral interactions between sperm whales, northern right whale dolphins, and Risso's dolphins on 14 May 2011, 24 nm west of San Diego, CA. This -1.5-hour encounter was documented in detail with high- definition digital photographs and video as the group traveled NE along the edge of a steep underwater drop-off. Risso's dolphins initiated charges towards the heads of the sperm whales on multiple occasions, followed by fast retreats. Sperm whale adults responded by displaying an open lower jaw. Risso's dolphins appeared to direct this behavior only toward adult sperm whales that had recently surfaced from long (> 20 min) dives; it was not directed toward the four calves in the group. Northern right whale dolphins intermingled with the Risso's dolphins and sperm whales, although they did not approach sperm whales as closely or abruptly as the Risso's dolphins. While similar apparently aggressive Risso's dolphin behavior has been documented toward other cetacean species, this is the first known occurrence of head-on charging by Risso's dolphins, accompanied by the elicited jaw display response from sperm whales. The interaction may be similar to aggressive pilot whales behavior towards sperm whales, and may function in acquiring regurgitated sperm whale food, or other needs by the Risso's dolphins.