CHAMPAIGN, Ill. — Nature has invented countless types of pointy appendages, and scientists have long sought to explain what makes these structures so effective at puncturing other things. A new study models the key physical characteristics of puncturing tools to reflect their diversity in nature, finding that the shape of a biological tool is driven in part by tradeoffs between its puncture efficiency and its ability to resist bending or buckling.
The findings are described in the journal Science Advances.
"There's a vast diversity of puncture tools in nature, like fangs and stingers and spines and thorns," said Philip Anderson , a professor of evolution, ecology and behavior at the University of Illinois Urbana-Champaign who led the new research. "It's ubiquitous across the entire tree of life, including plants, animals, fungi, bacteria and viruses."
Anderson has spent more than two decades studying how the laws of physics and biomechanics influence evolutionary processes, focusing primarily on the predatory or defensive structures that plants and animals use to damage, impale, grasp, impede or inject defensive compounds into other organisms.
For the new study, he and his colleagues looked at tools across plant and animal kingdoms to try to identify common physical attributes.
"Scientists are interested in finding underlying physical laws that all this diversity has to adhere to," Anderson said. Studies often focus on a single species at a time, examining things like the shape of its tool, the speed at which it is deployed and the efficiency with which it punctures a given material. The findings are sometimes reported as if they reflect universal physical laws.
"But when it comes to biology, I think we need to embrace the diversity of it," Anderson said. "If there was a universal law, then I would expect all puncture tools to look more similar to each other, but they don't. There's great variety in how these puncture tools work."
To better understand the principles driving such diversity, Anderson and his colleagues digitally modeled the primary characteristics of puncture tools in nature.
"We took two very basic measurements, one of which is its taper," he said. "If you look at a puncture tool from the side, is it a big broad triangle, like a shark's tooth? Or is it a thin, elongated triangle, like a fang? And then we also looked at its cross-section. Is it more round, like an elephant's tusk? Or is it flattened, like a stingray barb?"
Years of studies of the puncture performance of differently shaped tools have shown that while pointed objects that are rounder in cross-section may do a good job of initiating a fracture, flatter tools are likely to penetrate a material more deeply because they do not have to displace as much of the target material, Anderson said.
"The flatter it is, the easier time it should have inserting itself, because it has to push the material apart less," he said. "You're making a very thin wound versus a wide, circular one."
But flatness comes with other concerns, he said. Flatter materials may be more susceptible to bending or buckling, which could be detrimental to the organism. So, the researchers also calculated each tool's ability to resist buckling.
In simulations, the team compared the puncture performance of 25 cone shapes that varied in both taper and cross-sectional shape — equivalent to the variation seen across the more than 140 biological puncture tools they measured. For each of the 25 cone models, they calculated "how much energy it takes for a tool to create a fracture and insert itself" to a specific depth, Anderson said.
The long list of species that guided the dimensions of the cones reflects the diversity of tools found in vertebrates, invertebrates and plants.
The analysis revealed that some cones performed better than others across both puncture efficiency and buckling resistance.
"We could see a combined performance, where maybe you've got a tool that's decently resistant to buckling and also does a good job of puncturing. But if you tried to make it better at resisting buckling, you would lose puncture performance, and vice versa," Anderson said. "So, you're almost looking for a middle ground where both of these types of performance are being as optimized as they can be."
The cones with the highest performance on both measures include those whose taper and roundness were most like a scorpion's stinger, a king cobra's fangs, a rose prickle, a shark tooth, the talons of a red-tailed hawk, the mandibles of an army ant and the love dart of at least one land snail, Anderson said.
Just as interesting, he said, was a look at cone types that punctured efficiently but were more susceptible to buckling. This includes cones shaped more like cactus spines, which are more disposable, say, than something like a carnivore's canines "so it doesn't matter if they break."
Tools like a carnivore's canines appear to be more optimized to resist buckling, but puncture less efficiently. This may reflect their function, Anderson said. Perhaps for some species it is more important to be able to grasp their prey with their teeth than to pierce the flesh of their targets and risk breaking their teeth.
"A mammal doesn't want to break its tooth because it only gets two: the baby tooth and the adult tooth," he said. "So, evolutionarily, it's more important. You get better survivorship if you prevent that tool from breaking."
Similarly, fish spines may do more to protect the animals from being eaten than to pierce the flesh of potential prey.
"It may not be that the fish need to be puncturing other animals," Anderson said. "Maybe they're just making themselves too big to swallow."
Anderson said the findings should be useful to the field of bioinspiration, where new tools are designed to reproduce the form and functionality of something found in nature.
"Rather than looking at just one organism at a time and saying, 'We're going to mimic that,' I think we're finding that it would be more useful to look at overall trends, to see what a range of biological puncture tools are doing, and draw inspiration from that," he said.
Anderson also is an affiliate of the Beckman Institute for Advanced Science and Technology at the U. of I.
The National Science Foundation supported this research.
