One of the most important evolutionary events in animal’s history was the transition from water to land, about 360 million years ago. To make this transition possible, several physiologic and morphologic adaptations were necessary, and to understand them is not an easy topic in Biology. When we face big problems, one of the best ways to solve them is to look for other’s help and knowledge, right? That’s why, recently, biologists are uniting efforts with mathematicians, physicists and robotic engineers to better understand which characteristics were essential to the land conquest by vertebrates. A group of researchers, led by Professor Daniel Goldman, developed a study based on three complementary approaches: a biological model for early terrestrial locomotors, a robot developed to replicate the locomotion of these organisms in a simpler way, and also mathematical models.
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The biological model used was the mudskipper (Periophthalmus barbarus), a small fish capable of moving terrestrially using synchronous motions of the pectoral fins and also capable of using their tails for rapid jumping. This species was chosen as a model because it’s believed to have a similar body plan to first tetrapods to move on terrestrial environments, like Ichthyostega. Benjamin McInroe, one of the researchers of the group, explains that “the fish provided a morphological, functional model of these early walkers”.
Figure 1: Target and model systems for understanding early tetrapod locomotion on granular media:(A) A reconstruction of Ichthyostega (∼360million years ago), an example of an early tetrapod body plan, by RaulMartin. (B) Skeletal reconstruction of Ichthyostega, an example of an early tetrapod body plan [from(20)],highlighting the pectoral limbs (green) and tail (blue). (C) The mudskipper (Periophthalmus barbarus), a biological model for early terrestrial locomotors. (D) Amicro–computed tomography scan reconstruction of a mudskipper skeleton, highlighting the pectoral fin (green) and tail (blue). (E) The MuddyBot, a 3D printed robot developed tomodel the locomotion of crutching early tetrapods. Limbsare in green and the tail is in blue.
Complementarily, McInroe says that the robot allowed them to “simplify the complexity of the mudskipper and, by varying the parameters, understand the physical mechanisms of what was happening. With the mathematical model and its simulations, we were able to understand the physics behind what was going on.” In the evolutionary transition from an aquatic to a terrestrial environment, early tetrapods faced the challenges of terrestrial locomotion on flowable substrates, such as sand and mud of variable stiffness and incline. Researchers have realized that when mudskippers were moving on substrates with higher incline angles, crutching with only the pectoral fins became less effective than it was on flat grounds. At these conditions, animals started to use “steps” for which the tail was used to propel themselves forward. The robotic model was useful to systematically testing how locomotor performance in realistic granular environments was affected by variations in foot placement, limb adduction, and tail use, at various substrate inclines. With this experiment, the researches confirmed the importance of tail propulsion for locomotion success or failure, especially at high substrate angle. To climb 20-degree sandy slopes, both the mudskippers and the robot often slid backward if they didn’t use their tails.
Fig. 2: Mudskipper locomotion on granular media at different substrate inclines (θ).
One of the study’s most significant impacts is to provide new insights into how vertebrates made the transition from water to land. The focus on tail relevance for locomotion of early tetrapods is relatively new because, for a long time, scientists have studied this theme based on fossil records and focusing on limbs. This recent research has opened a way to biologists to better know how natural selection can act to modify structures already present in organisms, and allow them to move in a completely different environment – like swimming or walking. Besides the biological importance of this study, it also represents an advance in robotic engineering. The knowledge developed with the observation and mathematical modeling will help designers to create robots able to move across granular surfaces with various inclinations more efficiently – and with less likelihood of getting stuck in the mud.
Maybe more fascinating than the discoveries on animal’s locomotion on land and the advances in robotics, this research represents a good approach for biologists and engineers to work together. Uniting the knowledge and the methods used by diverse fields of expertise can only bring great advances and novel perspectives on Science.
By Bruna de Oliveira Cassettari
Reference: McInroe, B. et al. Tail use improves performance on soft substrates in models of early vertebrate land locomotors. Science 53, 154-158 (2016)