When Jim Rohr and Frank Fish decided to get to grips with cetacean swimming, heading out into the ocean wasn't a viable option. But heading down to the aquarium was. Most major aquaria house a few of the large mammals,trained to perform and entertain, and the team knew these animals would be ideally prepared for participating in their swimming tests to record the first comprehensive collection of swimming efficiency parameters for a large number of whales and dolphins (p. 1633).
Rohr explains that a cetacean's efficiency can be related to its tail beat amplitude, frequency, and swimming speed. These parameters can be combined to calculate an index known as the Strouhal number, with efficient swimmers recording Strouhal numbers between 0.2 and 0.4. Cetaceans were thought to fall well within this range, with Strouhal numbers between 0.25–0.35, but would this hold up to the test when Rohr and Fish put seven cetacean species through their paces?
The team began videoing animals ranging from bottlenose and spotted dolphins, up to killer whales and pilot whales, swimming over a range of speeds in their aquaria homes. After capturing over 260 swimming sequences,Rohr and Fish calculated Strouhal numbers for each sequence and plotted their distributions. But instead of falling within the predicted range of values,many of the mammals' Strouhal numbers fell below the predicted range, with 74%recording Strouhal numbers from 0.2–0.3 when swimming most efficiently. Also, the distribution of the animal's Strouhal numbers suggested that vorticity control by their tails is important for the animals' swimming performance.