Energetic savings of bow-riding dolphins

Bow-riding occurs when dolphins swim in the pressure waves at the front of a vessel. Bow-riding is hypothesized to be "fun" for dolphins or to save them energy although the energetics have not been explored. An UAS (Unoccupied Aerial System) was used to follow and video-record adult dusky dolphins (Lagenorhynchus obscurus) bow-riding in front of a research vessel or free-swimming off Kaikoura, New Zealand. Videos of individual dolphins swimming in a linear direction at consistent speeds were analyzed with respiration rate used as a proxy for energy expenditure (bow riding n = 51; free-swimming n = 62). The respiration rates of bow-riding dolphins remained relatively constant across swimming speeds and were 45% lower than free-swimming dolphins at speeds exceeding 4 m/s, indicating substantial energetic savings. The respiration rates of free-swimming dolphins increased exponentially with speed, suggesting that dolphins incur comparatively high energetic expenditures from swimming rapidly. This research advances understanding of the biological function of bow-riding behavior and supports the energy saving hypothesis. Swimming energetics can be used to assess the impacts of anthropogenic disturbances to dolphin energy budgets.