In the streams and rivers of South America, the 2-cm-long glass knifefish lives among a high population of predators, and prefers to stay hidden, biding its time until it must emerge to find food or flee.
While hiding, the knifefish may move its fins to counteract the force of the current: “hovering” in the water. But close observation of the fish’s fins reveals contrasting activities: starting at the front, the fin creates a wave that pushes the fish forward, against the current. But the back of the fin creates a wave that moves in the opposite direction, pushing with the current. It appears as though the fish is spending extra energy fighting its own movement.
New research appearing in the Proceedings of the National Academy of Sciences offers an evidence for why this might be advantageous for an animal or insect. The research, led by Noah Cowan at Johns Hopkins University, demonstrates how the opposing forces give the fish a boost in stability and in maneuverability—traits which engineers generally consider dichotomous.
The “mutually opposing forces” observed in the fins of the glass knifefish are not unique to this species. Similar antagonistic forces have been observed in at least a dozen other animals (no complete count yet exists), including cockroaches, humming birds, some lizards, hawkmoths and electric fish. As early as 1991, scientists have hypothesized that these forces might improve the animal’s locomotive abilities.
Cowan and his group appear to be the first scientists to put that hypothesis to the test. The team studied the knifefish under high-speed cameras to map the kinematics of the fin as it created the two opposing waves. One wave begins and the front of the fin while the other starts at the back. The two waves meet at a nodal point, which the fish can adjust depending on how much energy it must exert to fight the current (the same muscle groups are responsible for generating the waves in both directions).
“As the fish is able to actively move that [nodal point] back and forth, it’s able to generate a rapid change in thrust direction,” Cowan says. “So you get both the self stabilization and the maneuverability at the same time.” By contrast, a fish generating a single, unidirectional wave must initiate a new motion each time it wants to change its thrust direction.
In addition to direct observation, the Johns Hopkins team created a mathematical model to describe the motion, which they then tested using a fish-like robot built by engineers at Northwestern University. Looking forward, Cowan and his group are interested in understanding the neural processes that change the position of the nodal point on the knifefish’s fin.
While there is no equation that establishes stability and maneuverability as inversely proportional, Cowan says engineering books frequently state that an increase in one requires a decrease in the other.
“The idea of generating a vehicle that orients its thrusters [to] cancel each other out on purpose, seems very wasteful and inefficient,” Cowan says. But if nature is willing to sacrifice a little energy for a better balance of these two elements, then perhaps humans should consider it as well.