# Framework for Developing Bio-Inspired Morphologies for Walking Robots
## 1 Introduction  
The morphology of walking robots often resembles animals at a high abstraction level with the same number of limbs and placement of body parts. Bio-inspired robots that recreate animal mobility at a deeper level require close interdisciplinary collaboration between biologists and engineers. Potential benefits of developing such bio-inspired robots include improved movement versatility, dexterity, and durability, as well as other useful features such as using the same limbs for locomotion, grasping, burrowing and object manipulation. The improved understanding from studying animals' embodied features and testing the differences they enable in robots can benefit research in both biology and engineering. The ability to recreate an animal's embodied features in a robotic system is also a powerful demonstration of the feasibility of the current understanding of the animal's functional principles. In this spirit, [PERSON], [PERSON] et al. [1] describes biologists' tasks as reverse-engineering studies: asking both how an animal traitworks and why it is needed from functional and biological viewpoints. Subsequently, engineers can apply such insights to robots. Thus, biologists need to know what information is useful for the engineers, and engineers need to know what questions the biologists seek to answer. Bio-inspiration can be an even stronger design approach in engineering, enabling the use of structures and subjects from nature as proof of concept and inspirational guides for creating novel robotic systems and devices [2]. In robotics, bio-inspiration has been used to develop machines that fly like bees [3], swim like salamanders [4; 5], walk like stick insects [6], climb like geckos [7] and manipulate objects like dung beetles [8]. The domain of bio-inspired walking robots can be divided into two main categories: _template_ and _anchor_ models [9]. A template model is a simplified replica of the animal's morphology, which can help speed up development and highlight core principles of the morphology. Quadruped robots such as _Mini Cheetah_[10] and _Hubbardog2_[11] are examples of template models with a morphology supporting specific control principles and providing stable mechanics (Figure 1a). An anchor model recreates the animal's morphology more precisely to highlight its unique embodied features. For example, _Pleurobot_ is presented in [4] as a quadruped anchor model inspired by salamanders, capable of mimicking both swim and gait patterns. Most robots are somewhere on a spectrum between template or anchor, starting from prescriptive and simple template models and moving towards a more elaborate representation of the animals' embodied behaviour [9]. In [12], the quadruped robot _Serval_ is presented as being in between the two model types with its template-like design and control that also has slight bio-mimic influences. Hexapod template models such as AMOS [13] and MORF [14] often have similar kinematics, e.g., placement of motors, identical legs and leg joints that are angled to move the leg either vertically or horizontally (Figure 1b). Having joint angles aligned with the yaw, roll, or pitch axes can simplify the development of the controller, as well as lessen the torque on the motor for the first degree-of-freedom (DOF1) during locomotion (Figure 1b), which is quite different from insect morphologies, where the movement of one joint will influence the limb in all axes [15]. The hexapod robots Lauron V [16] and Hector [6] are described as having legs angled in a similar way to stick insects. These examples of anchor models are demonstrating increased mobility due to having higher speed and kinematic flexibility. In [15], the morphology of the dung beetle is measured and used as a model for a simulated robot, demonstrating that its anchor morphology exceeds both in standing on and manipulating ball-shaped objects. Thereby investigating bio-inspired robot models can develop both biological and engineering hypotheses. However, this does require a formal interdisciplinary exchange of knowledge.