Abstract

We present the first evidence of a traumatic interspecies interaction between a Risso’s dolphin (Grampus griseus) and coastal bottlenose dolphins (Tursiops truncatus) in the San Francisco (SF) Bay Area. A female Risso’s dolphin calf stranded live on Agate Beach, Marin County, California on October 24th 2017. The dependent calf was euthanized due to poor body condition and maternal separation. Upon further examination, the calf had rake marks on 40% of her body. While rake marks on Risso’s dolphins are mainly conspecific in nature, these rake marks were different. The interdental distance was much shorter and more consistent with that of a bottlenose dolphin (approx. 10 mm).¹ The rake marks were also in parallel arrays of up to 11 full epidermal depth lacerations. Bottlenose dolphins have between 18 and 27 pairs of teeth in their mandibular and maxillary arcades and could inflict such a pattern of rakes, while Risso’s dolphins have only between two and seven pairs of teeth in the mandible only and could not. Similar rake marks are also frequently found on harbor porpoises (Phocoena phocoena) known to have been fatally attacked by bottlenose dolphins along the California coastline.^2,3,4 The incidence of “porpicide” has increased along the central and northern California coast over the past 30 years as bottlenose dolphins have initially migrated north on a seasonal basis and more recently, on an almost annual residency basis^3,4,6 “porpicides” are diagnosed as probable when the lesions include typical rake marks, skeletal fractures, and soft tissue contusion and possible when there are fractures and contusions but no rake marks. Fractures are often multiple including ribs, vertebrae, mandibles, and skull bones. Death often ensues from hemothorax as a result of compound rib fractures or concussion and asphyxia if there is significant cranial trauma. The cause of death for this calf was more to do with maternal separation than bottlenose dolphin aggression as apart from the skin rake marks, there was no significant internal trauma. This Risso’s dolphin calf was very similar in size to the average adult harbor porpoise (Risso’s dolphin: 1.62 m straight length and 58 kg mass; average adult harbor porpoise: 1.4–1.6 m straight length and 45–60 kg mass).⁴ Our findings suggest that bottlenose dolphins within the SF Bay area may target individuals of this size range. In the eastern North Atlantic, bottlenose dolphin aggression against harbor porpoises and Risso’s dolphins has been documented.¹ Future monitoring of these interspecies interactions will help us understand the changing ecological landscape of cetaceans in the SF Bay Area.

Acknowledgements

The authors would like to thank the staff and volunteers of The Marine Mammal Center for their effort and participation within the stranding network.

Literature Cited

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