A facultative

Facultative

Facultative means "optional" or "discretionary" , used mainly in biology in phrases such as:* Facultative anaerobe, an organism that can use oxygen but also has anaerobic methods of energy production...

 biped is an animal that is capable of walking or running on two legs, often for only a limited period of time, in spite of normally walking or running on four limbs or more. Well-known examples include many lizard

Lizard

Lizards are a widespread group of squamate reptiles, with nearly 3800 species, ranging across all continents except Antarctica as well as most oceanic island chains...

s such as the Basilisk lizard

Basiliscus (genus)

Basiliscus is a genus of large corytophanid lizards, commonly known as basilisks, which are endemic to southern Mexico, Central America, and northern South America.-Taxonomy and etymology:...

, and even some cockroach

Cockroach

Cockroaches are insects of the order Blattaria or Blattodea, of which about 30 species out of 4,500 total are associated with human habitations...

es when running at top speed. In order to be considered a true facultative biped, an animal must be capable of sustained movement over many strides while bipedal – simply adopting a static bipedal posture while resting or looking around is not sufficient.

Species

Facultative bipedality is most common in lizards, but also occurs in primate

Primate

A primate is a mammal of the order Primates , which contains prosimians and simians. Primates arose from ancestors that lived in the trees of tropical forests; many primate characteristics represent adaptations to life in this challenging three-dimensional environment...

s, bear

Bear

Bears are mammals of the family Ursidae. Bears are classified as caniforms, or doglike carnivorans, with the pinnipeds being their closest living relatives. Although there are only eight living species of bear, they are widespread, appearing in a wide variety of habitats throughout the Northern...

s, insect

Insect

Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...

s, crab

Crab

True crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" , or where the reduced abdomen is entirely hidden under the thorax...

s and even octopus

Octopus

The octopus is a cephalopod mollusc of the order Octopoda. Octopuses have two eyes and four pairs of arms, and like other cephalopods they are bilaterally symmetric. An octopus has a hard beak, with its mouth at the center point of the arms...

ses. It is commonly suggested that many extinct basal archosaur

Archosaur

Archosaurs are a group of diapsid amniotes whose living representatives consist of modern birds and crocodilians. This group also includes all extinct non-avian dinosaurs, many extinct crocodilian relatives, and pterosaurs. Archosauria, the archosaur clade, is a crown group that includes the most...

s were facultative bipeds, as well as hadrosaurs.

Cause

In many cases, facultative bipedality is a function of speed. Many lizard species, as well as cockroaches and crabs, will switch to a bipedal gait at very high speeds. Reasons for this are unclear — it may be that a bipedal gait allows greater stride length, without the forelimbs interfering with the swinging and placement of the hind limbs, or it may simply be that at high speeds, the forces in the muscles which retract and extend the hind limbs are so great that animal's body rises into the air, similar to a "wheelie

Wheelie

In vehicle acrobatics, a wheelie is a vehicle maneuver in which the front wheel or wheels come off the ground due to extreme torque being applied to the rear wheel or wheels. Wheelies are usually associated with bicycles and motorcycles, but can be done with other vehicles such as cars, especially...

" in bikes.

Low-speed bipedality is less common, as is most commonly associated with threat displays (bear

Bear

Bears are mammals of the family Ursidae. Bears are classified as caniforms, or doglike carnivorans, with the pinnipeds being their closest living relatives. Although there are only eight living species of bear, they are widespread, appearing in a wide variety of habitats throughout the Northern...

s, goanna

Goanna

Goanna is the name used to refer to any number of Australian monitor lizards of the genus Varanus, as well as to certain species from Southeast Asia.There are around 30 species of goanna, 25 of which are found in Australia...

s, frilled lizards

Frill-necked Lizard

The frill-necked lizard , also known as the frilled lizard or frilled dragon, is found mainly in northern Australia and southern New Guinea. Its name comes from the large frill around its neck, which usually stays folded against the lizard's body. It is largely arboreal, spending the majority of...

), camouflage (octopus), or possessing an anatomy that is highly specialized for arboreal locomotion

Arboreal locomotion

Arboreal locomotion is the locomotion of animals in trees. In every habitat in which trees are present, animals have evolved to move in them. Some animals may only scale trees occasionally, while others are exclusively arboreal. These habitats pose numerous mechanical challenges to animals...

 and makes terrestrial locomotion difficult (gibbon

Gibbon

Gibbons are apes in the family Hylobatidae . The family is divided into four genera based on their diploid chromosome number: Hylobates , Hoolock , Nomascus , and Symphalangus . The extinct Bunopithecus sericus is a gibbon or gibbon-like ape which, until recently, was thought to be closely related...

s).