<s>[INST] Why don't animals have wheels? Answer as briefly as possible. [/INST] Several organisms are capable of rolling locomotion. However, true wheels and propellers—despite their utility in human vehicles—do not seem to play a significant role in the movement of living things. First, there are several developmental and evolutionary obstacles to the advent of a wheel by natural selection. Secondly, wheels are often at a competitive disadvantage when compared with other means of propulsion.</s>\ <s>[INST] For the organisms that are capable of rolling locomotion, how do they move that way without wheels? [/INST] Organisms capable of rolling locomotion typically achieve this by using their body shape and muscle contractions to roll themselves along a surface. For example, a pill bug, also known as a rollie-pollie, can roll into a tight ball as a defence mechanism. The pill bug's exoskeleton provides a hard exterior to protect its body while it rolls. Similarly, some caterpillars use their flexible bodies to roll themselves into a tight ball as a defence mechanism. Other organisms, such as certain snakes, can use their flexible bodies and muscle contractions to "inch" themselves along in a rolling motion.</s>