Bottlenose dolphin (Tursiops truncatus) spy-hopping (swimming upright) with half of its body above the water.
Bottlenose dolphins (Tursiops truncatus) are curious creatures. If something is going on at the surface, they want to know what's happening, so they stick their heads vertically out of the water, a manoeuvre known as spy-hopping, while effectively treading water. And in 2018, Frank Fish [West Chester University (WCU), USA] and Terrie Williams (University of California, Santa Cruz, USA) took advantage of this ability to test a technique that allows scientists to measure forces under water. However, while watching the animals protrude their heads out of the water, Fish was intrigued. He wanted to see what was really going on beneath the surface, to find out how the dolphins move their tail fins, known as flukes, as they bob vertically at the surface and how they alter their tail beat if they want to rise further out of the water.
Fish and five keen student helpers headed to the National Aquarium, Baltimore, USA, equipped with movie cameras to film the dolphins through a window in the wall of the 4.9 million litre pool while the six animals beat their tails to swim vertically. ‘Not getting the camera splashed was the hardest part of this study’, chuckles Fish, recalling that the dolphins enjoyed slapping their tails as they swam past, drenching him and his colleagues while they were setting up the cameras on the other side of the wall. ‘This was a rite of passage for any new students’, he laughs. Once the cameras were rigged up, trainers encouraged the dolphins to spy-hop, so that the dolphins protruded their heads, then the top half of their body and finally the top two-thirds of their body out of the water, while Fish filmed their tail beats beneath the surface. David Kramer and Maura Sheehan (both at WCU) then analysed the movements to find out how the animals generated the extraordinary power required to lift their bodies clear of the water.
Sure enough, the higher their bodies protruded, the faster the dolphins beat their tails, sweeping them to and fro at 1.75 tail beats s−1 when only their heads were above the surface, but reaching 2.57 tail beats s−1 when two-thirds of their body was out of the water, which is the rate at which they beat their tails when swimming at 20 km h−1 – but less than the top speed ever recorded for a dolphin (29.9 km h−1). And when the team calculated the amount of thrust produced by the dolphins, which had to be sufficient to carry the weight of their bodies protruding above the water, it ranged from 289 to 1263 N, sufficient to support almost 130 kg.
But were the dolphins adapting how their tails moved to generate sufficient thrust to hold ∼60% of their body mass above the water? This time, Kramer analysed how far the flukes flexed as the dolphins beat their tails to support their weight; however, there was barely any change in the flukes’ flexibility as the dolphins worked harder. They were not adjusting how their tails manoeuvred to hold more of the body above the surface.
So, dolphins simply beat their tails faster to lift themselves further out of the water and Fish adds that the dolphins’ tail beats have a lot in common with the wing beats of hovering hummingbirds; the flukes flip over at the beginning and end of each sweep of the tail, just like the hummingbird's wings flip over at the top and bottom of each wingbeat. He also explains that synchronised swimmers do something similar to dolphins when swimming vertically, but their smaller feet only produce a fraction of the dolphin's thrust.
