Upright posture

An upright posture or erect posture is the state of an animal's body where its craniocaudal axis being perpendicular (vertical) to the ground and/or the direction it is facing. Examples of such postures include some tetrapods with bipedal gait^[1] and the swimming posture of seahorses.^[2]

The different orientation of the neuraxis between an upright biped (human) and a quadruped (dog)

The seemingly upright bipedalism of a common ostrich (Struthio camelus) is in fact not a true upright posture

Bipedal tetrapods

Although bipedal tetrapods, such as apes (hominids and gibbons), birds, macropods, springhares and even the frilled lizard, are capable of (at least temporarily) weight-bearing and moving around using only hindlimbs (lower limbs), only the apes (when on the ground) and kangaroos (who routinely use their stiff tail as a "third leg" to support the body^[3]) are capable of a true upright posture with the spinal column axis vertical to the ground, the ventral surface of the torso (chest and abdomen) facing frontwards and the head and directly above the feet. Other bipedal animals such as birds, kangaroo rats and mice and hopping mice actually have a "bowing" posture with the torso facing directly or obliquely downwards, and their "upright" appearance is largely due to the lower neck being flexed dorsally, which makes the head and upper neck upright but forward of the feet, and their craniocaudal axis resembling an exponential function (or "hockey stick") line rather than a vertical line.

Some quadruped animals, such as meerkats and ground squirrels, can also temporarily assume an upright posture on their hindlimbs (typically aided by their tails), often to stay sentry against predators, or as a ritualistic dominance display for territoriality or to compete for mates.

Maintaining an upright posture requires anatomical and biomechanical adaptations that can compensate for not only the burden of weight upon the hindlimbs, but also the added challenge of balance because the line of weight now needs to be maintained between only two supporting columns instead of four, thus a much smaller base area of support. Examples of such adaptations include significant strengthening of the lower appendicular skeleton and musculature, evolutionary changes of the hip joint and femoral neck, lengthening of the digits (in digitigrades) and/or metatarsals (in plantigrades), and recruitment of the tail as an additional tripodic support (e.g. kangaroos). In contrast, pseudo-upright bipeds (e.g. birds and tetanuran dinosaurs^[4][5]) tend to use the tail as a counterweight against the head and thorax, which are usually front of their feet.

Humans

Two humans displaying upright posture when running (front) and standing (back)

Main article: Human skeletal changes due to bipedalism

Humans, a genus of hominid apes with only one extant species, are the only clade with an obligately upright posture, of which the forelimbs (upper limbs) rarely (if ever) partake in weight-bearing and locomotion. Other hominids (chimpanzees, bonobos, gorillas and orangutans) rely at least partially on knuckle-walking when moving on the ground, while humans are bipedal full-time except some rare occasions that require crawling.

With the need to persistently maintain a truly upright posture, humans have evolved more specialized adaptations, such as significantly strengthened legs (which are not only much stronger but also longer than the arms), enlarged knees with meniscus compartmentalization, elongated, arched and fat padded feet with proportionally shortened toes, and a shorter but wider and deeper pelvis with larger acetabula.^[6] Humans have also evolved a secondary lordotic curvature of the lumbar spine that shifts the upper body's center of mass more dorsally within the base of support formed between the two feet. The physiological curvatures of the human spine, aided by the flaval and longitudinal ligaments as well as various core muscles, also function kinematically like a spring that absorbs axial shock when running, jumping and lifting weights.^[7]

Seahorses

A seahorse displaying its upright swimming posture

Unlike other fish, seahorses swim upright using the dorsal fin for propulsion and the pectoral fins for steering. Their prehensile tails lack the caudal fin and instead are used to grasp onto seaweed, coral or any object that can anchor them against sea currents.^[8] Once anchored, the seahorses hold themselves up into the current to passively ambush any food (typically small crustaceans such as mysids) that floats or swims nearby.^[9]

Gallery

• 
  A western lowland gorilla walking upright through water, using a branch as a walking stick
• 
  A lar gibbon running upright on ground
• 
  Two red kangaroos fighting upright, with their tails as additional support
• 
  A red junglefowl standing on a tree with a near-upright posture
• 
  The frill-necked lizard is known to run near-upright on its hindlimbs to escape predators
• 
  Meerkats often stand upright to spot for potential predators
• 
  Two marmots fighting upright via a shoving contest

See also

• Orthograde posture, an upright walking posture
• Bipedalism
• Handstand, an inverted erect posture using only the upper limbs for weight-bearing
• Headstand, an inverted erect posture using the vertex of the head for weight-bearing
• Prone position
• Razorfish, a fish known for swimming in an inverted vertical posture