We tested Madeleine using all eight gaits in the diving well at Vassar’s pool.[160] Using an underwater video camera, we recorded Madeleine’s movements as we accelerated her from rest, raced her to top speed, and then had her break as quickly as she could (Figure 7.8). While she was doing all of this, we measured her accelerations and energy consumption using an on-board three-axis accelerometer and electrical power monitor.[161]
The result? Two flippers are just as good as four in terms of top cruising speed (Figure 7.9). This was not what we’d expected. Another good theory dashed upon the rocky shore of empirical investigation! But this wreck is enlightening. Four flippers, in addition to not moving Madeleine along at any faster top speed, also takes twice the electrical power of two flippers. Two flippers good, four flippers bad—right? Not so fast. Four flippers are really handy if you want to accelerate quickly from rest, and using four gets you off the mark 1.4 times faster than using two flippers.
What we have here is called a performance trade-off. If cruising is your game, then you should use two flippers. If accelerating is important, use four. Trade-offs like this are the stuff of evolution, as the selection environment fiddles with what works best in a population at a given time and place. Change how schools of fish are distributed, for example, and maybe you have to cruise more to find your prey.
FIGURE 7.9. Madeleine’s behavior with two and four flippers. To our surprise, Madeleine reaches the same top cruising speed using only two flippers. To maintain the same cruising speed with four flippers, Madeleine has to use twice the electrical power. Four flippers offer an advantage when Madeleine accelerates from a start. The points represent the means for three trials of each of four gaits. The error bars are the standard errors of the mean. Statistical tests back up all claims of a performance metric being the same or different.
Let’s get back to our motivating question: why do we see living aquatic tetrapods using only two flippers for propulsion? Based on the results from Robot Madeleine, my empirically educated guess is that living species tend to do a lot of cruising, relying most heavily on that aspect of their swimming behavior in the game of life. For example, green turtles, when they aren’t asleep on the ocean floor, are cruising around, moving among beds of sea grass, which they visit for their vegetarian meals. Penguins cruise rapidly from shore to fishing grounds, where they dive and maneuver through schools of prey. Seals and sea lions, too, eat fish, although sea lions, in particular, are no-table for their rapid turns and maneuvers, as we talked about with SeaLioTron. Sea lions may be an exception to the two-flipper-cruiser rule that we are sketching out. Frank Fish, Jenifer Hurle, and Dan Costa have measured centripetal accelerations of up to five times that of gravity in the California sea lion, Zalophus californianus.[162] Although those accelerations are angular rather than linear like those that we measured in Robot Madeleine, they highlight the fact that the sea lions don’t need top cruising speeds but instead rapid accelerations to capture quick and elusive fish.
What about our extinct four-flippered plesiosaurs? From the physical evidence Robot Madeleine produced, we can circumscribe the likely scenarios. For small plesiosaurs the size of Madeleine, I can imagine them feeding as sit-and-wait ambush predators: hang in the water until something tasty swims close by and then—bam! Hit the accelerator and grab some lunch. Although this ambush behavior might work for smaller plesiosaurs, I don’t think that it’s possible for the giant short-necked pliosaurs like Kronosaurus or Predator X.[163] The trouble is that at ten or fifteen meters long, they are simply too massive to accelerate quickly. For the same reason that you never see a tractor-trailer beat a sports car off the line when the light turns green, you would never see Kronosaurus waiting and then lunging at a fish traveling by. Neither truck nor pliosaur can generate the mechanical power needed to launch their massive bodies quickly. When starting from a stop, their performance is constrained by (1) the amount of power that either internal combustion or skeletal muscle can produce and (2) their massiveness.
So what’s a poor giant sea monster to do with four flippers? For the Predator X documentary that aired on the History Channel, I did some very crude calculations. The team of paleontologists that discovered Predator X, led by Dr. Jorn Hurum, estimates that Predator X was fifteen meters long. Using data that are available on the length and mass of great whales, I estimate that Predator X had a mass of about thirty-nine thousand kilograms, or thirty-nine metric tonnes. If Predator X accelerated at Madeleine’s peak acceleration from rest, about 0.085 m s–2 (see Figure 7.9), it would move 8.5 centimeters in one second, a tiny fraction of its 1,500-centimeter total length. If you happened upon Predator X sitting still in the water, you’d have nothing to worry about, unless you swam right into its mouth!
If, however, you ran into Predator X while she was already moving, you might be in trouble. I’m guessing that Predator X cruised around and used its large flippers, like those on a humpback whale, to maneuver, redirecting its forward-cruising momentum into a feeding lunge of the type seen in blue whales, accelerating to one or two m s–2 to hit speeds perhaps as high as two to three m s–1.[164] But what’s Predator X predating? Unlike humpback or blue whales, which use their baleen plates to filter whole schools of small fish or krill out of sea water, Predator X, with its large teeth, was probably grabbing onto other large and relatively sluggish animals. The long-necked plesiosaurs may have been this short-necked plesiosaur’s target.
Here’s the best part: if Predator X cruised around looking for an unwary plesiosaur, then perhaps it only needed to use two of its flippers at any one time. Why waste the energy needed to flap all four if Robot Madeleine tells us that you won’t swim any faster for the additional effort? Imagine if you could run a marathon on your legs or your arms. You could race until your legs were sore and then switch to your arms. This is straight out of the Department of Crazy Ideas. But based on what we now know about the physics of four-flippered swimming, switching between front and back propulsive systems makes sense.
For the truth police: we can never know for sure how plesiosaurs swam. Because they are extinct, their behavior is lost. No matter how accurate we make Robot Madeleine’s limb anatomy, no matter how big or small we scale her, no matter how exhaustively we search the neural control space, the very best we can do using an ET is to talk about what is more or less possible. ETs help us circumscribe the plausible; what guides our judgment is the physical reality of behavior, that interaction of an embodied agent and its physical environment.
160
The coaches of Vassar’s swim teams, Lisl Prater-Lee, Tom Albright, and Jesup Szatkowski, were kind enough to allow Robot Madeleine both training and experiment time in the pool.
161
You can find all of the details of this set of experiments in the following paper: J. H. Long Jr., J. Schumacher, N. Livingston, and M. Kemp, “Four Flippers or Two? Tetrapodal Swimming with an Aquatic Robot,”
162
F. E. Fish, J. Hurle, and D. P. Costa, “Maneuverability by the Sea Lion
163
Predator X is the stage name of a heretofore undescribed species of pliosaur unearthed in the Norwegian Arctic. The History Channel aired an eponymous special on Predator X, and clips of the documentary are available at www.history.com/videos/predator-x-revealed#predator-x-revealed. Robot Madeleine, by the way, was featured!
164
The accelerations of twenty-two-meter-to twenty-seven-meter-long blue whales have been measured in the wild: J. A. Goldbogen, J. Calambodkidis, E. Oleson, J. Potvin, N. D. Pyenson, G. Schorr, and R. E. Shadwick, “Mechanics, Hydrodynamics and Energetics of Blue Whale Lunge Feeding: Efficiency Dependence on Krill Density,”