The physics behind a water bear's lumbering gait

Tardigrades, known colloquially as water bears or moss piglets, are eight-legged segmented micro-animals. There are about 1,300 known species in the phylum Tardigrada, with the earliest known specimens having been found in amber from 145 to 66 million years ago, but it’s likely they have a significantly earlier origin, maybe from over 500 million years ago.

These ‘slow steppers’ are one of the toughest and most resilient form of life on earth, with some species able to survive for up to 30 years without food or water and endure temperature extremes of up to 150 degrees Celsius. Yet they can be found almost anywhere: Tardigrades are prevalent in mosses and lichens and feed on plant cells, algae, and small invertebrates. When collected, they may be viewed under a low-power microscope, making them great for research by students and amateur scientists. 

'Tardigrades can survive the deepest seas, artic temperatures and even the frozen vacuum of space'. Dr Jasmine Nirody

Tardigrades are usually about 0.5 mm long when fully grown. Animals as small and soft as tardigrades seldom have legs and almost never bother walking - round worms of similar size and body type thrash about, slithering their doughy forms over unpredictable substrates. 

Yet the water bear uses eight stubby legs to improbably propel itself through marine and freshwater sediment, across desert dunes, and beneath the soil. Their dumpy plod raises the question of why tardigrades evolved to walk at all. 

They are so small that they generate and feel miniscule inertial forces, so walking in water for them is like a human walking through a much more viscous fluids, like honey. Yet they’re able to plod along like animals over 100,000 times their size.

Recent research, joint between the University of Oxford, Rockefeller University, and Princeton University, found that their coordination mirrors the stepping patterns of larger panarthropods such as stick insects and spiders. Can there be any ‘universal’ locomotive strategies in a set of animals that includes cicadas, lobsters, tardigrades, & centipedes? This diverse group spans an overwhelmingly wide range of sizes, body plans, and habitats.

Unlike vertebrates, which have distinct gaits for each speed - picture a horse's hooves as it transitions from a walk to a gallop - tardigrades run more like insects, scurrying at increasing speeds without ever changing their basic stepping patterns. These stepping patterns seem to follow a small set of 'coordination rules' that pop out when we look at the phase offsets between individual leg pairs - so, for example how soon will the front right leg be lifted after the leg directly behind it. 

'Perhaps the best way to navigate unpredictable terrain with a microscopic body is to plod like a water bear.'  Dr Jasmine Nirody

Tardigrades also respond to changes in substrate stiffness. As the ground under their feet gets softer and gives way, they switch to a ‘bounding’ coordination pattern. The same switch - from out-of-phase to in-phase movement of leg pairs - has also been observed in desert beetles, which are significantly larger and have rigid bodies - moving along shifting sands.

These observed commonalities may suggest the same low-dimensional control architecture is used across insect taxa, or more generally across panarthropods. This discovery implies the existence of either a common ancestor or an evolutionary advantage that explains why one of the smallest and softest creatures evolved to walk just like larger, hard-bodied panarthropods.

These similarities open up several interesting evolutionary questions. One possible explanation is that tardigrades, long assumed to fit neatly into no existing taxonomy, may share common ancestors - and even a common neural circuit -  with arthropods such as fruit flies, ants, spiders, and other segmented scurrying creatures. 

One hypothesis for a shared neural architecture is rooted in the structure of the insect ventral nerve cord (VNC), a part of the nervous system that sends out signals to the legs. This structure is well conserved across panarthropods. 

Another possibility is that there is no ancestral connection between tardigrades and arthropods, but that the unrelated groups of organisms independently arrived at the same walking and running strategies because they were evolutionarily advantageous. 

Of course, in a group as diverse as panarthropods, there are going to be exceptions to even the most 'universal' rules. There are desert beetles that gallop instead of walking. Some centipedes send ‘backwards’ traveling waves instead of forward. But such divergences from a core set are exciting, not discouraging. They can give us hints into what features of a strategy are important for specialized performance in particular environments.

'We don't know much about what happens at the extremes of locomotion—how to make an efficient small walker, or how soft-bodied things should move.' Dr Jasmine Nirody

Beyond the implications for evolutionary biology and the study of animal locomotion, the findings may have ramifications for the burgeoning fields of soft and microscale robotics.

By studying how small animals evolved to move across challenging environments, scientists may be able to design robots that can more efficiently squeeze into small spaces or operate at the microscale.

'Tardigrades exhibit robust interlimb coordination across walking speeds and terrains' published by PNAS