by Francesca Giammona, PhD Candidate at Wake Forest University, Studying biomechanics of terrestrial locomotion of fishes
This year at the Society for Integrative & Comparative Biology 2022 Annual Meeting, there were, as always, a variety of symposia showcasing a diversity of research fields. Symposium s2, entitled “Evolutionary conservation and diversity in a key vertebrate behavior: “walking” as a model system,” highlighted how different organisms walk, and how individual species may vary their walking behavior under changing conditions. These different animals spanned ancient creatures such as dinosaurs, to more modern, atypical walkers such as fishes.
Concerning dinosaurs, Dr. John Hutchinson of the Royal Veterinary College described how dinosaurs shifted from moving on all fours to an upright, bipedal position over time. As this occurred, many parts of their anatomy also changed. Imagine a crocodile moving across a surface versus a bird – when moving one foot, the crocodile pushes off by flexing the entire foot, while the bird instead utilizes uses its toes to push off the ground. As dinosaurs began walking on two legs, they made a shift from a crocodilian to a more avian walking form. They also began to move their hips more when walking, and developed longer legs and feet to better transmit force to the ground and vault over their legs. These changes allowed them to move much more swiftly, and likely allowed many dinosaur species to out-compete other predators, such as ancient crocodilians, for food.
Somewhat related to dinosaurs are guinea fowl, one of the oldest species of poultry and game bird in the world. Dr. Peter Falkingham from Liverpool John Moores University conducted a series of experiments examining how guinea fowl walk under different substrate conditions. Picture yourself running across concrete – the ground is solid, and as you push off with each foot, you are propelled forward. Now, picture yourself running on clay. It is a bit harder to push off the surface, because as you push, you sink down a little bit and your foot gets stuck to the material. Now try to imagine yourself running on progressively wetter and wetter clay – it gets harder and harder to move. These very same scenarios are what Dr. Falkingham had guinea fowls experience, and he filmed their feet in each condition. Walking was separated into a touch down phase for each foot (where one foot comes into contact with the ground) and a kick off phase (where the same foot lifts off the ground).
Together, the time it takes for these two events to occur is called the stance phase. When running across a hard surface, both touch down and kick off happen nearly instantaneously in these animals, creating a very short stance phase. Both feet are generally not on the ground at the same time. However, when guinea fowl are running across more liquid clay surfaces, touch down and kick off take much longer, meaning there are times when both feet are touching the surface at the same time. For very liquid-y substrates, videos of guinea fowl running look almost as if they are swimming – they sink down so much that nearly their entire legs are submerged in the clay. This work makes an interesting point about where running ends and swimming begins under the right conditions.
Moving away from birds and reptiles, Dr. Chen Li of Johns Hopkins University studied a smaller, less loved creature – the cockroach. As it turns out, a cockroach moving through different types of terrain requires many different locomotor behaviors. Moving through tall blades of grass, up obstacles of different heights, over gaps, or through holes between small, rigid columns of trees calls for a variety of movement types. To get through grasses, cockroaches will actually rotate their body sideways, performing a roll behavior in order to better fit through small holes. To cross a gap or climb small obstacles, they will lift their heads and increase speed in order to achieve the momentum needed to climb or clear a gap. In getting around columns, cockroaches can rotate their legs more outward and upward for better turning. While these behaviors are all technically part of cockroach walking, they are clearly also very specialized for use in specific situations.
A final example of walking is one that many people would never consider “normal” animal behavior – fish walking on land. Dr. Emily Naylor of The George Washington University studies how mudskippers, amphibious fishes that spend large portions of their lives in the mud, move on surfaces made of different materials and inclines.
Mudskippers move on land with their pectoral fins, which are located on either side of the body. They will use these fins to push off the ground and “crutch” forward, vaulting the body over the fins. On very soft, gelatinous substrates, a mudskipper will sprawl its pectoral fins out, holding them further away from the body. It will also spend less time with its fins in the air as it pushes itself off the ground – this is similar to the pattern seen in the guinea fowl, as one could imagine it is a lot harder to lift a body off mud compared to concrete. When going uphill, these fish will incorporate their tail during movement to brace themselves and prevent slipping, and will again spend less time with their pectoral fins off of the ground. By learning the specifics of this walking behavior, it could potentially allow scientists to predict where these fish can travel on land, and how successful they may be in getting to new environments. If you would like to see an example of mudskipper crutching behavior, click here for a video of Dr. Naylor’s work!
It is clear from these examples that walking is a bit more complex than most people would imagine. As animals first came onto land, they needed to find ways to navigate their environments, and using limbs was a worthwhile solution. As different animals have evolved, new methods of walking have come about, and each organism has developed certain locomotive behaviors to best fit their habitat and life history strategy. By learning about them all, scientists can compare and contrast the methods used by different animals, and piece together interesting patterns about the evolution of locomotive behaviors.
s2 papers already in advanced
Integrative and Comparative Biology, icac008, https://doi.org/10.1093/icb/icac008
Integrative and Comparative Biology, icac009, https://doi.org/10.1093/icb/icac009
Integrative and Comparative Biology, icac023, https://doi.org/10.1093/icb/icac023
Connect with Blogger Francesca Giammona