When we returned to Vassar bearing the bad news about model 1, Kira resolved to find a better way to build model 2. Rather than linking with the stiff coffee stirrers, she designed a method to hang the vertebrae, like a string of pearls, into a mold and pour gelatin around the whole construct. The gelatin encased the vertebral column, forming a sort of pig-in-a-blanket. I loved this idea because the blanket encasement was the beginning of a body, a structure that we had blithely ignored in our previous attempts to build a vertebrated tail. With a bit more work, we thought that this new vertebral column + body design would work.
When Kira, Keon, Kurt, Gianna, Virginia, and I contemplated honing the new pig-in-a-blanket design, we had an epiphany: stop! We were spending all of our time making biomimetic vertebral columns and none of it evolving robots. The lack of a working Tadro4 system was particularly poignant because we had just welcomed into the lab a talented robot engineer, Nicole Doorly, a major in cognitive science. In order not to lose her and to move our research program along, we had to settle quickly on a vertebral column design that, in our unsteady hands, we could scale up in order to produce reliable and custom columns at the rate that we needed.
Compromise. Everybody hates that word. A compromise, it is rumored, is guaranteed to satisfy no one. No one on Team Tadro was going to be satisfied with a compromise because we’d put so much time into the design and prototyping of the fancy models 1 and 2. We were scared of the bête noire of compromise under the bed, knowing all the work that we were about to discard. Enter sandman: “Take my hand. We’re off to Never-Never Land.”[123]
In the new predator-prey land of Tadro4, the biomimetic-vertebral-column compromise consists of an artificial notochord, borrowed from Tadro3, outfitted with a series of rings forming vertebrae (Figure 6.6). This model 3, which we call, surprisingly, the ring centra vertebral column, has a number of advantages, he says bravely, over the previous vertebral column models: (1) it doesn’t need to be held together by ligaments or neural arches, (2) it has fewer parts than either model 1 or 2, and (3) it can be assembled more quickly, in about five minutes (compared to thirty minutes) once all of the parts are present.
What we lose with model 3, though, is the ability to evolve the shape of the centra because the ring centra have no cup-like joint faces.[124] With that simplification, we also lose the close approximation of shape to the vertebral columns of sharks. And we’re not done yet. We further simplified manufacturing, keeping the overall length of the vertebral column constant.
Simplification begets simplification. A constant column length plus unchanging centra meant that only the length of the vertebral joint could change as the number of vertebrae did. Simple! Increase the number of vertebra, and the amount of intervertebral joint available for bending decreases, thereby stiffening the vertebral column.
FIGURE 6.6. Biomimetic vertebral column, model 3. Selected for use with the Tadro4 PreyRo, this model was created by the team working on model 2 and newcomers to the lab, including Hannah Rosenblum, Elise Stickles, Hassan Sakhtah, and Andres Gutierrez. Featuring ring centra slipped on and glued to a continuous hydrogel (top image), this model is a compromise between having vertebrae and being able to build many columns quickly and in a repeatable manner.
The full manufacturing process for model 3 involved making a slew of hydrogels, cross-linking them all in the same way so as to produce artificial notochords of similar material properties, and then gluing ring centra to each notochord to create an artificial vertebral column (Figure 6.6). This process was scaled up to production level for the game of life with Tadro4, under Gianna’s supervision, with Hannah Rosenblum, Hassan Sakhtah, Elise Stickles, and Andres Gutierrez operating our assembly line.
Because we distilled the evolution of the vertebral column down to a single trait—number of vertebrae—we had a system that allowed us to test our next biological hypothesis: selection for enhanced feeding behavior and predator avoidance drove the evolution of vertebrae. Testing this hypothesis also relies on the design of the stuff attached to the vertebral column, which I haven’t told you about yet. We need the body with sensors to track light and predators, a microcontroller to compute the two-layer subsumption neural system, and motors to flap and turn the tail. All of this comes together in Tadro4 (Figure 6.7).
Tadro4 is really two different kinds of robot: an Evolvabot that we call “PreyRo” (Prey and Robot) and a nonevolving robot predator, “Tadiator” (Tadpole and Gladiator). When PreyRo and Tadiator interact in a water world with a light source, this is the Tadro4 predator-prey world (Figure 6.8). The fact that Tadiator doesn’t evolve doesn’t worry us, by the way. In biological predator-prey systems, predators are often much longer lived than their prey. Not that an evolving predator wouldn’t be interesting! But we’ve got to leave something to do in the Tadro5 world.
FIGURE 6.7. PreyRo is a Tadro4 Evolvabot. PreyRo is modeled after the early vertebrate fish, Drepanaspis, here shown in a photograph I took of Louis Ferragalio’s 1953 model at the American Museum of Natural History (specimen 8462). Both target and model share the following features: rigid, nearly circular body; body flattened in the dorso-ventral direction, like a pancake; a short propulsive tail; and paired eyes. Other features of PreyRo are based on what we know about living fishes: a lateral line for predator detection (IR proximity detectors); a two-layer subsumption neural architecture; a vertebral column with shark-like vertebral centra.
FIGURE 6.8. PreyRo versus Tadiator. When PreyRo detects Tadiator, the nonevolving predator robot, it initiates an escape response (top image). During a three-minute trial PreyRo and Tadiator have multiple encounters, as shown by the overlapping paths. At time 1 PreyRo and Tadiator start from opposite sides of the tank. Time 2: PreyRo heads for the light and Tadiator heads for PreyRo. Time 3: PreyRo detects Tadiator as Tadiator closes the gap. Time 4: before Tadiator gets to PreyRo, PreyRo turns rapidly away and escapes. That first close encounter took just twenty seconds. Each point on the paths represents the position of PreyRo or Tadiator each second.
PreyRo is modeled after a species of Paleozoic fish called Drepanaspis gemeundenensis (Figure 6.7). Drepanaspis, a jawless marine fish living 400 million years ago, swam using a short flexible tail and a rigid, flattened disk of a body, which lacked paired fins of any sort. Preserved in the bones that form its rigid body disk is the evidence of its sensory systems: a pair of small and widely spaced eyes and a lateral line system.[125] The flattened shape of the body of Drepanaspis is similar to that of living skates, stingrays, and electric rays, most of whom spend time on and in the ocean floor, feeding, burrowing, and resting. As Marianne pointed out, electric rays may be the most similar living species to Drepanaspis in terms of locomotor function if not ancestry because neither has or had the ability to use its body disk for propulsion. They generate thrust with a short tail that makes the whole animal look like a pancake propelled by a headless fish pushing, tugboat-like, from the rear. What we know about electric rays is that they are able to swim up in the water column. We suspect that Drepanaspis had the same ability.
124
For a detailed comparison of model 3 and model 1 versions, see the following paper: J. H. Long Jr., T. Koob, J. Schaefer, A. Summers, K. Bantilan, S. Grotmol, and M. E. Porter, “Inspired by Sharks: A Biomimetic Skeleton for the Flapping, Propulsive Tail of an Aquatic Robot,”
125
The presence of a lateral line is debated, but examination of the different specimens of