Robin W. Baird
Mammalia 62:129-134. 1998.
Summary
An interaction involving two Pacific white-sided dolphins and a neonatal harbor porpoise was observed in eastern Haro Strait, WA, in 1994, and lasted for five hours. On a number of occasions one of the dolphins was observed holding the porpoise’s flipper in its mouth and dragging the porpoise through the water. Both dolphins were observed to occasionally leap on top of the porpoise as it swam at or near the water's surface. During close approaches of the porpoise to our vessel, one or the other dolphin quickly swam between the porpoise and our boat, and turned away in such a fashion as to force the porpoise away from the boat. Two and one half hours into the observation, a decision was made to capture the porpoise, and the porpoise was captured 2.5 hours later, surviving for five days in captivity. Despite being dragged by its flipper, the porpoise only had superficial skin abrasions on its flippers when captured. Five wounds, consistent with bites, were found on the caudal peduncle. No other trauma was noted, and cause of death was determined to be bacterial pneumonia. Although Pacific white-sided dolphins previously have been reported associating with at least 10 different species of cetaceans, associations with harbor porpoises do not appear to have been previously published. This observation is the first reported of an apparently non-predatory interspecific interaction involving a lone cetacean neonate. Possible causes of the interaction are discussed, including epimeletic behavior, aggressive behavior due to competition, predation or kleptoparasitism, and object-oriented play. In many ways the observations appear similar to object-oriented play (or non-mutual, interspecific play) behavior, with, in this case, the "object" being the harbor porpoise. The function(s) or beneficial effects of "play" in such cases are usually related to physical training or skill development, and this is certainly a possibility in this case.