Researchers at Northwestern University have identified a single mechanical ratio that dozens of unrelated sea creatures, from jellyfish to squid to bony fish, appear to share when they swim. The finding ties the physics of aquatic locomotion directly to the evolution of animal body shapes, offering a quantitative explanation for why species separated by hundreds of millions of years of evolution ended up moving through water in strikingly similar ways.
One Number Across Eight Lineages
The central discovery is a ratio known as the “specific wavelength,” which compares the wavelength of an animal’s undulating fin or body to its overall length. Across a wide range of swimmers that use median or paired fins, that ratio converges on roughly 20. The number held steady whether the animal was a knifefish, a cuttlefish, or a stingray, and it persisted across different swimming speeds. Supporting datasets published alongside the study confirm that predicted and observed fin undulation counts track closely as velocity changes, reinforcing the idea that animals actively maintain this mechanical sweet spot rather than drifting toward it by accident.
The pattern showed up in at least eight independent groups of swimmers, meaning these organisms did not inherit the trait from a single common ancestor specialized for this motion. Instead, each lineage arrived at the same engineering solution on its own. A separate account from Northwestern University notes that very different species converged on this shared strategy, underscoring that the similarity is not superficial but rooted in the underlying mechanics of how fins push against water.
Why Physics Narrows the Options
The logic behind the ratio is grounded in fluid dynamics. When an animal undulates a fin to push itself through water, the wavelength of that undulation determines how efficiently thrust is generated relative to drag. A general framework for separating thrust and drag in undulatory swimmers, developed for both living animals and bio-inspired robots, shows that deviations from the optimal ratio sharply reduce propulsive efficiency. Animals that wander too far from the sweet spot waste energy, and in the ocean, wasted energy can mean the difference between catching prey and being caught.
This constraint acts like a filter on body design. If an animal evolves a longer fin, it must also adjust the wavelength of its undulation to stay near the ratio of 20. If it evolves a shorter body, the same recalibration applies. The result is that body shape and locomotion become tightly coupled: change one, and physics demands a corresponding change in the other. That coupling helps explain why animals with radically different anatomies, a flat stingray versus a ribbon-shaped knifefish, can still converge on the same movement pattern and maintain similar swimming performance.
Locomotion as a Sculptor of Shape
The swimming ratio is one piece of a broader picture in which motion sculpts form. Separate research on teleost fishes by Sarah T. Friedman and colleagues found strong links between swimming mode and body shape. Among the species studied, pufferfishes and filefishes contributed most to variation within their locomotion category, suggesting that even under shared mechanical constraints, some lineages explore a wider range of forms than others.
That tension between constraint and variation is central to understanding biodiversity. Physics sets boundaries, but within those boundaries, evolution still has space to experiment. Vertebral shapes, fin placements, and body cross-sections can all shift as long as the overall mechanics of thrust and drag remain efficient enough. In some cases, small changes in stiffness or muscle arrangement may allow a species to tweak how it uses the same basic body outline, opening new ecological niches without abandoning the underlying mechanical template.
The principle extends beyond water. A comparative biomechanics study in Nature Communications documented repeated convergence in movement patterns across both swimming and flying animals, drawing an analogy between aquatic undulation and aerial flapping. Whether an animal is beating wings through air or oscillating fins through water, it must negotiate similar trade-offs between lift, thrust, and drag. The implication is that fluid mechanics, regardless of the medium, imposes recurring design pressures on animal bodies and channels evolution toward a limited set of workable solutions.
From Shallow Reefs to the Deep Ocean
If physics constrains shape, then different physical environments should produce different shapes. That prediction holds up when comparing habitats. Fish that live near the seafloor experience different flows and obstacles than those cruising through open water, and their bodies reflect those demands. Bottom-dwelling species often adopt flattened or elongated forms that let them hug substrates and resist currents, while midwater swimmers tend toward streamlined, torpedo-like outlines optimized for sustained cruising.
Depth itself also matters. Work on morphological disparity across ocean zones has shown that body shape diversity in teleosts increases with depth, with deep-sea fish displaying more extreme and varied silhouettes than many of their shallow-water relatives. Rather than enforcing uniformity, the deep ocean’s combination of low light, high pressure, and patchy resources appears to relax some constraints while intensifying others, allowing unusual mechanical solutions to thrive. Oddly shaped heads that host oversized sensory organs, elongated bodies that minimize energy expenditure, and specialized fins for hovering or sudden bursts of speed all emerge as viable responses to the distinctive physics of the deep.
These environmental contrasts highlight that mechanical rules do not dictate a single “best” shape. Instead, they define a landscape of possibilities, with different peaks corresponding to different combinations of habitat, behavior, and locomotion style. Evolution then explores that landscape, sometimes settling repeatedly on the same peak—such as the specific wavelength ratio in undulatory swimmers—and sometimes discovering new ones as conditions change.
Mechanotypes and the Bigger Question
Researchers at EMBL and the University of Geneva have pushed this line of thinking further with the concept of “mechanotypes,” categories of body shape defined by shared mechanical demands rather than ancestry. In this view, a starfish, an earthworm, a mouse, and a human can all be grouped according to the physical problems their bodies solve—such as bearing weight, transmitting forces, or moving through a particular medium—rather than solely by their positions on the tree of life. The same mechanical category might include animals that look very different but rely on comparable structural strategies.
Even bilateral symmetry, the left-right mirroring seen in most animals, may owe its persistence to physics. Modeling work has suggested that locomotion in three-dimensional space is sufficient to favor symmetric designs because asymmetries tend to introduce unwanted torques and inefficient motion. Over evolutionary time, bodies that are more balanced move more predictably and waste less energy. This gives symmetry a mechanical advantage independent of specific genes or developmental pathways.
Taken together, these lines of evidence paint a picture in which evolution and physics are tightly intertwined. Natural selection does not start with a blank canvas. It works within the constraints of fluids, forces, and materials. Yet those constraints are not purely limiting. By narrowing the range of viable options, they also create robust, repeatable solutions that can be discovered again and again, from reef fish to deep-sea oddities, from gliding birds to hovering squid. The specific wavelength ratio identified in undulatory swimmers is one such solution, a numerical fingerprint of how water, muscle, and motion coevolve to shape the diversity of life.
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*This article was researched with the help of AI, with human editors creating the final content.
