An article at Live Science, “Seahorse’s Amazing Tail Could Inspire Better Robots,” describes a paper in Science on the unique shape of the tail of the seahorse. Researchers propose that its shape could provide improved architecture for robotic appendages:

Seahorses are of special interest to robot researchers because of their unusual skeletal structure, which scientists say could help them design bots that are hardy and strong yet also flexible enough to carry out tasks in real-world settings.

“Human engineers tend to build things that are stiff so they can be controlled easily,” study co-author Ross Hatton, an assistant professor in the College of Engineering at Oregon State University, said in a statement. “But nature makes things just strong enough not to break, and then flexible enough to do a wide range of tasks. That’s why we can learn a lot from animals that will inspire the next generations of robotics.”

Seahorses are true fish, classified as ray finned fishes, a subclass of the bony fishes. But they have a unique skeletal architecture in their tails, which in cross-section are square rather than round:

In particular, seahorses have square (rather than round) bony plates that surround the “backbone” of their tails. These odd features help the fishes bend, twist and get a stronger grip on their surroundings. But, the square structures also make them more resistant to being crushed by predators, the researchers said.

A summary article in Science explains the mechanical advantages of this unique and robust design:

Most animals and plants approximate a cylinder in shape, and where junctions occur (as with branches of trees or limbs on animals), those corners are “faired,” meaning smoothly curved so that one surface grades into the next (1). When living organisms deviate from the norm, there’s usually a good biomechanical reason: a clue to some specific problem that needs to be solved. Among their suite of unusual characteristics, seahorses possess a true oddity: a prehensile tail with a square, rather than round or elliptical, crosssectional shape. On page 46 of this issue, Porter et al. (2) report that there are distinct mechanical advantages to being square. Using threedimensional (3D) printing to construct physical models, the team demonstrates that the multiplated anatomy of the square seahorse tail shows greater resistance to mechanical deformation than a similar model that has a round cross section. … Porter et al. suggest that a square tail provides superior resistance against compressive injury (i.e., a bite from a predator)

The technical paper in Science elaborates:

[T]he square prism is more resilient when crushed and provides a mechanism for preserving articulatory organization upon extensive bending and twisting, as compared with its cylindrical counterpart. Thus, the square architecture is better than the circular one in the context of two integrated functions: grasping ability and crushing resistance.

So why is this shape more robust than a cylindrical or circular one? A square ring made of multiple plates on each side can retain its shape while being compressed under stress, and then return to its normal shape because the plates can slide past one-another. A circular ring, however, cannot do so without losing its shape and becoming permanently deformed. As the paper explains, “a square ring changes size but not shape when biaxially compressed because of the ability of the corner plates to slide over one another,” whereas “a round ring cannot retain its shape when biaxially compressed, because the plates interfere with one another.” This is a keen design that allows the tail to both bend and accommodate stresses without losing its shape.

The paper explains how the design could be put to use by human engineers:

Beyond their intended practical applications, engineering designs are convenient means to answer elusive biological questions when live animal data are unavailable (for example, seahorses do not have cylindrical tails). Understanding the role of mechanics in these prototypes may help engineers to develop future seahorse-inspired technologies that mimic the prehensile and armored functions of the natural appendage for a variety of applications in robotics, defense systems, or biomedicine.

Here’s how the authors conclude their paper after studying the engineering design of the seahorses tail:

The highly articulated bony plates that surround the central vertebral axis of a seahorse tail actively facilitate bending and twisting as well as resist vertebral fracture from impact and crushing. To explore why the bony plates are arranged into cross-sectional squares rather than circles, we analyzed themechanics of 3D-printed models that mimic the natural (square prism) and hypothetical (cylindrical) architectures of a seahorse tail skeleton. Physical manipulation of the two prototypes revealed that the square architecture possesses several mechanical advantages over its circular counterpart in bending, twisting, and resistance to crushing. The enhanced performance realized in the square architecture provides insight into the way in which seahorses may benefit from having prehensile tails composed of armored plates organized into square prisms, rather than cylinders. This study demonstrates that engineering designs are convenient means to answer elusive biological questions when biological data are nonexistent or difficult to obtain.

These papers are peppered with claims that the square shape “evolved” because of the mechanical advantages it provided. But simply identifying the advantages a system provides — and asserting they evolved — does not constitute a Darwinian evolutionary explanation. It seems that the best way to study the operation of these structures is to treat them as engineered systems that are designed with some kind of a purpose. Scientists can pay lip service to Darwinian evolution, but it isn’t helping them understand nature.

Image credit: Florin DUMITRESCU (Own work) [CC BY-SA 3.0 or GFDL], via Wikimedia Commons.