Animals use different types of gaits when moving around in their habitats. A gait is described by the pattern and timing of footfalls during locomotion, and in quadrupeds these gaits are usually classified as “symmetrical” or “asymmetrical.” In a symmetrical gait (walk, trot), the left and right limbs of a pair alternate and the timing of footfalls is evenly spread, while in an asymmetrical gait (bound, gallop), the limbs may move together and the timing of footfalls is uneven.
We tend to associate galloping with large mammals, such as horses, camels, cheetahs and giraffes, while bounding is typical of hares and other smaller mammals. In fact, mammals were long thought to be the only lineage capable of using asymmetrical gaits and because of this, scientists had suggested that the ability to move in this way probably emerged only after mammals first appeared on the planet some 210 million years ago.
Recently, however, research has shown that crocodiles can also gallop when moving at their fastest, and bounding locomotion has been observed in swimming turtles and hopping frogs. Furthermore, some amphibious fish move by crutching (dragging themselves by their fins across land) while other fish species move along the sea- or riverbed by punting (pushing themselves with their pelvic fins); these are also forms of asymmetrical locomotion.
These unusual examples led experts to wonder whether animals may have evolved the ability to coordinate their limbs in an asymmetrical manner much earlier than previously thought.
To look into the evolutionary history of asymmetrical locomotion, Eric McElroy from the College of Charleston and Michael Granatosky from the New York Institute of Technology searched published literature for animals that are known to use asymmetric “foot falls” when moving along surfaces with their feet or fins. They searched among species of mammals, marsupials, monotremes, reptiles, frogs, toads and fish, and drew up a family tree that mapped the evolutionary history of this ability.
“In total we compiled data from 308 species,” said McElroy. The researchers constructed the family tree by allocating a score of 0 to species that only used evenly timed walks, trots and runs, and a score of 1 to species that showed any sign of moving asymmetrically by bounding, crutching, punting or galloping. The experts then ran a series of simulations to find out how likely it was that asymmetric gaits appeared earlier or later in the evolutionary tree.
“It took months to work out all the kinks in the analysis,” said McElroy. The results indicate that 472 million years ago, the earliest ancestors of almost all modern animals – including fish – were capable of moving with some kind of proto-asymmetric gait. Whether they were punting, crutching or bounding along the seabed isn’t known, but the animals were capable of coordinating their limbs in an asymmetrical manner to propel themselves along.
Furthermore, the analysis reveals that some creatures, such as lizards, salamanders, frogs and even elephants, have undergone a secondary loss of this ability to bound or gallop, even though they have ancestors in their family trees that were capable of coordinating asymmetric movements. It is possible that they became too big or too slow to get airborne during the gait, or perhaps they lost the nerves necessary for coordinating the limbs independently.
It remains, however, that the ability to bound or gallop is not just a mammalian attribute. Almost all animals that are alive today have ancestors that were capable of moving asymmetrically, even though some subsequently lost this ability. In all likelihood, mammals inherited their ability to coordinate asymmetrical movements from some ancient fishy ancestor that propelled itself along the seabed on its fins, long before any species hauled itself out onto dry land for the very first time.
The study is published in the Journal of Experimental Biology.