Facultative bipedalism

A facultative biped is an animal that is capable of walking or running on two legs (bipedal), as a response to exceptional circumstances (facultative), while normally walking or running on four limbs or more.^[1] In contrast, obligate bipedalism is where walking or running on two legs is the primary method of locomotion. Facultative bipedalism has been observed in several families of lizards and multiple species of primates, including sifakas, capuchin monkeys, baboons, gibbons, gorillas, bonobos and chimpanzees. Several dinosaur and other prehistoric archosaur species are facultative bipeds, most notably ornithopods and marginocephalians, with some recorded examples within sauropodomorpha. Different facultatively bipedal species employ different types of bipedalism corresponding to the varying reasons they have for engaging in facultative bipedalism. In primates, bipedalism is often associated with food gathering and transport.^[2] In lizards, it has been debated whether bipedal locomotion is an advantage for speed and energy conservation or whether it is governed solely by the mechanics of the acceleration and lizard's center of mass.^[3] Facultative bipedalism is often divided into high-speed (lizards)^[4] and low-speed (gibbons),^[5] but some species cannot be easily categorized into one of these two. Facultative bipedalism has also been observed in cockroaches^[6] and some desert rodents.^[7]