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=== Bipedalism in Dinosaurs ===
=== Bipedalism in Dinosaurs ===
Bipedalism was common in all major groups of [[Dinosaur|dinosaurs]].<ref name=pmid28254476/> [[Phylogenetics|Phylogenetic]] studies indicate that bipedalism in dinosaurs arose from one [[Common descent|common ancestor]], while quadrupedalism arose in multiple lines, coinciding with an increase in body size.<ref name=pmid28254476/> To understand how bipedalism arose in dinosaurs, scientists studied extant facultatively bipedal lizards, especially of the [[clade]] [[squamata]].<ref name=pmid28254476/> The proposed explanation for the evolution of bipedalism in dinosaurs is that it arose in smaller [[Carnivore|carnivores]] that were competing with larger carnivores.<ref name=pmid28254476/> The need for speed and agility prompted the adaptation of a larger hind-limb muscle, which in turn prompted the shift to facultative bipedalism, where the weaker front legs would not slow them down.<ref name=pmid28254476/> Facultatively bipedal dinosaurs encountered ecological pressures for longer periods of high speed and agility, and so longer periods of bipedalism, until eventually they became continually bipedal.<ref name=pmid28254476/> This explanation implies that facultative bipedalism leads to obligate bipedalism.<ref name=pmid28254476/>
Bipedalism was common in all major groups of [[Dinosaur|dinosaurs]].<ref name=pmid28254476/> [[Phylogenetics|Phylogenetic]] studies indicate that bipedalism in dinosaurs arose from one [[Common descent|common ancestor]], while quadrupedalism arose in multiple lines, coinciding with an increase in body size.<ref name=pmid28254476/> To understand how bipedalism arose in dinosaurs, scientists studied extant facultatively bipedal lizards, especially of the [[clade]] [[squamata]].<ref name=pmid28254476/> The proposed explanation for the evolution of bipedalism in dinosaurs is that it arose in smaller [[Carnivore|carnivores]] that were competing with larger carnivores.<ref name=pmid28254476/> The need for speed and agility prompted the adaptation of a larger hind-limb muscle, which in turn prompted the shift to facultative bipedalism, where the weaker front legs would not slow them down.<ref name=pmid28254476/> Facultatively bipedal dinosaurs encountered ecological pressures for longer periods of high speed and agility, and so longer periods of bipedalism, until eventually they became continually bipedal.<ref name=pmid28254476/> This explanation implies that facultative bipedalism leads to obligate bipedalism.<ref name=pmid28254476/>

Studying the biomechanics of motion directly contributes to understanding the morphology of both modern primates and fossil records. Bipedal locomotion appears to have evolved separately in different primates including humans, bonobos, and gibbons.<ref name=":2">{{Cite journal|last=Vereecke|first=Evie|last2=D'Août|first2=Kristiaan|last3=Van Elsacker|first3=Linda|last4=De Clercq|first4=Dirk|last5=Aerts|first5=Peter|date=2005|title=Functional analysis of the gibbon foot during terrestrial bipedal walking: Plantar pressure distributions and three-dimensional ground reaction forces|url=https://onlinelibrary.wiley.com/doi/epdf/10.1002/ajpa.20158|journal=American Journal of Physical Anthropology|language=en|volume=128|issue=3|pages=659–669|doi=10.1002/ajpa.20158|issn=0002-9483|via=}}</ref> The evolutionary explanation for the development of this behavior is often linked to load-carrying in chimpanzees, bonobos, macaques, capuchin monkeys, and baboons.<ref name=":1">{{Cite journal|last=Hanna|first=J. B.|last2=Schmitt|first2=D.|last3=Wright|first3=K.|last4=Eshchar|first4=Y.|last5=Visalberghi|first5=E.|last6=Fragaszy|first6=D.|date=2015-08-01|title=Kinetics of bipedal locomotion during load carrying in capuchin monkeys|url=https://www.sciencedirect.com/science/article/pii/S0047248415001323|journal=Journal of Human Evolution|language=en|volume=85|pages=149–156|doi=10.1016/j.jhevol.2015.05.006|issn=0047-2484|via=}}</ref> The ability to carry more materials may be either a selective pressure or a significant advantage, especially in uncertain environments where commodities must be collected when found as they can potentially become unavailable.<ref name=":7">{{cite journal|last1=Carvalho|first1=S.|last2=Biro|first2=D.|date=March 2012|title=Chimpanzee carrying behaviour and the origins of human bipedality|url=https://doi.org/10.1016/j.cub.2012.01.052|journal=Current Biology|volume=22|issue=6|pages=R180-R181|doi=10.1016/j.cub.2012.01.052}}</ref> Load carrying affects limb mechanics by increasing the force on the lower limbs, which may affect the evolution of anatomy in facultatively bipedal primates.<ref name=":1" />

Various arboreal adaptations may have affected the evolution of bipedalism as well. Vertical climbing and brachiation changes the structure and purpose of the forelimbs, making quadrupedal walking more difficult and contributing to the shift to bipedal locomotion. Gibbons and sifakas are examples of this: their movement through trees makes quadrupedal walking difficult, resulting in bipedal walking and galloping, respectively.<ref name=":02">{{Cite journal|last=Wunderlich|first=R. E.|last2=Schaum|first2=J. C.|date=2007|title=Kinematics of bipedalism in Propithecus verreauxi|url=https://zslpublications.onlinelibrary.wiley.com/doi/abs/10.1111/j.1469-7998.2006.00253.x|journal=Journal of Zoology|language=en|volume=272|issue=2|pages=165–175|doi=10.1111/j.1469-7998.2006.00253.x|issn=0952-8369|via=}}</ref><ref>{{Cite journal|last=Preuschoft|first=Holger|date=2004|title=Mechanisms for the acquisition of habitual bipedality: are there biomechanical reasons for the acquisition of upright bipedal posture?|url=https://onlinelibrary.wiley.com/doi/abs/10.1111/j.0021-8782.2004.00303.x|journal=Journal of Anatomy|language=en|volume=204|issue=5|pages=363–384|doi=10.1111/j.0021-8782.2004.00303.x|issn=0021-8782|pmc=PMC1571303|pmid=15198701|via=}}</ref> Arboreal adaptations making bipedalism advantageous is supported by research that shows that hip and thigh muscles involved in the bipedal walking often most resemble those used in climbing.<ref name=":12">{{Cite journal|last=Fleagle|first=John G.|last2=Stern|first2=Jack T.|last3=Jungers|first3=William L.|last4=Susman|first4=Randall L.|last5=Vangor|first5=Andrea K.|last6=Wells|first6=James P.|date=January 1981|title=Climbing: A biomechanical link with brachiation and with bipedalism|url=https://www.researchgate.net/profile/John_Fleagle/publication/285387305_Climbing_A_biomechanical_link_with_brachiation_and_with_bipedalism/links/569d41e208ae950bd7a670d4.pdf|journal=Symposia of the Zoological Society of London|volume=48|pages=359-375|via=ResearchGate}}</ref>

In lizards, bipedal running developed fairly early in their evolutionary history. Fossils suggest this behavior began approximately 110 million years ago.<ref name=":9">{{cite journal|last1=Lee|first1=H. J.|last2=Lee|first2=Y. N.|date=Feb 2018|title=Lizards ran bipedally 110 million years ago|url=https://www.nature.com/articles/s41598-018-20809-z|journal=Scientific Reports|volume=8|issue=1|pages=2617|doi=10.1038/s41598-018-20809-z}}</ref> Facultative bipedalism may also promote adaptive radiation.<ref name=":112">{{Cite journal|last=Clemente|first=Christofer J.|date=2014|title=The evolution of bipedal running in lizards suggests a consequential origin may be exploited in later lineages|url=https://www.ncbi.nlm.nih.gov/pubmed/24820255|journal=Evolution; International Journal of Organic Evolution|volume=68|issue=8|pages=2171–2183|doi=10.1111/evo.12447|issn=1558-5646|pmid=24820255|via=}}</ref>


==References==
==References==

Revision as of 15:14, 1 November 2018

A facultative biped is a animal that is capable of walking or running on two legs (bipedal), often for only a limited period (facultative), in spite of normally walking or running on four limbs or more.[1] It differs from obligate bipedalism in that facultative bipeds use four limbs as their primary method of locomotion, only occasionally becoming bipedal, often for a specific purpose. Primates and lizards are the most common animals that engage in facultative bipedalism–this behavior has been observed in several families of lizards and multiple species of primates, including sifakas, capuchin monkeys, baboons, gibbons, and chimpanzees. There are multiple different types of bipedal motion, with different facultative bipeds using different types. This corresponds to the different reasons various species have for engaging in facultative bipedalism. In primates, bipedalism is often associated with food gathering and transport.[2] In lizards, 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 has been debated.[3] Facultative bipedalism is often divided into high-speed (lizards)[4] and low-speed (gibbons),[5] but some species can not be easily categorized into one of these two. Facultative bipedalism has also been observed in cockroaches[6] and some desert rodents.[7]

Types of Bipedal Locomotion

Within the category of bipedal locomotion, there are a four main techniques: walking, running, skipping, and galloping.[8]Walking is when the footfalls have an evenly spaced gait, and one foot is always on the ground.[8] When both feet are off the ground at the same time, called the aerial phase, this is running.[8] Skipping is when there is an aerial phase, but the two feet hit the ground immediately after each other, and the trailing foot changes after each step.[8] Galloping is similar to skipping, but the trail foot does not change after each step.[8] This is not an exhaustive list of the forms of bipedalism, but most bipedal species utilize one or more of these techniques.[8]

Facultative Bipedal Species

Facultative bipedalism occurs in primates, cockroaches, desert rodents, and lizards; specific lizard families known as facultative bipeds are the Agamidae, Crotaphytidae, Iguanidae, and Phrynosomatidae.[4][6] Facultative bipedalism evolved in the common ancestor of most major dinosaur groups, and it arose independently within lizards and mammals.[1][4]

Primates

Bipedalism is found commonly throughout the primate order. Among apes[2][8] it is found in humans, chimpanzees,[9][10][11] orangutans, gorillas, and gibbons.[12][13][11][14] Among monkeys it is found in capuchins[15][16] and baboons.[17][2] Among strepsirrhines it is found in sifakas[16][8] and ring-tailed lemurs.[18]

A sifaka skipping bipedally.
A sifaka galloping bipedally.

Lemurs

The Sifaka, Propithecus, which is a type of lemur native to the island of Madagascar, is one of the primary examples of facultative bipedalism. While moving through the trees, they locomote using a vertical clinging and leaping strategy.[8] On the ground, they can walk on their back two legs, as a way to conserve energy.[8] Sifakas can locomoting bipedally in two separate ways: walking and galloping.[8] First, they can walk slowly, as described in the definition of walking above.[8] Second, they can use what is called a bipedal gallop, wherein the leading and trailing foot switch every 5-7 steps.[8] Propithecus and humans are the only species known to utilize a skipping/galloping type of locomotion.[8]

A capuchin monkey standing on two legs.

Ring-tailed lemurs, Lemur catta, can be arboreal or terrestrial.[18] While terrestrial, they move quadrupedal 70% of the time and bipedally 18% of the time, which is more than any other species in their genus.[18] While bipedal, they can locomote by hopping or walking.[18]

Monkeys

Capuchin monkeys are arboreal quadrupeds, but can lomote bipedally on the ground.[15] They use a spring-like walk that is not a run because there is no aerial phase.[15] While humans employ a pendulum-like gait which allows for the interchange of kinetic and potential energy, capuchins do not.[15] This means the energy costs of bipedalism in capuchins is very high.[15] It is thought that the reduced energy costs that a pendulum-like gait affords humans is what led to the evolution of obligate bipedalism.[15]

Olive baboons are described as a quadrupel primate, but bipedalism is observed occasionally and spontaneously in captivity and in the wild. Bipedal walking is rarely used, but most often occurs in instances where the infant loses its grip on the mother while she's walking quadrupedally.[19] Immature baboons seem to be more bipedal than adults. These bipedal postures and locomotion in infants, although infrequent, seem to clearly distinguish them from adult baboons.[17] In the wild, locomotor behavior of these baboons vary as a result of their need to find food and to avoid predators.

In gelada baboons, they sit while traveling bipedally in more of a "shuffling" motion, and also use bipedal locomotion when traveling short distances. [20]

Apes

A chimpanzee standing on two legs.

Studies have shown that apes in closed forest habitats are more bipedal than chimpanzees and baboons, stationary or moving[2], and that the proportions of the foot in the gorilla are better adapted to bipedal standing than other monkey species. In specific circumstances, some ape feet even do better than human feet in terms of bipedal standing, having a larger RPL (ratio of the power arm to the load arm) and reducing the muscle force when the foot contacts the ground. [21]

Gibbons (of the genus Hylobates) are low-speed obligate bipeds when on the ground.[16][22] Because they usually move through trees, their anatomy has become specialized for vertical clinging and leaping, which uses hip and knee joint extensions that are similar to those used in bipedal motion.[12][13][14] They also utilize three back muscles that are key to bipedal motion in chimpanzees as well as humans.[11]

Chimpanzees exhibit bipedalism most often when carrying valuable resources (such as food gathering/transporting purposes) because chimps can carry more than twice as much when walking bipedally as opposed to walking quadrupedally.[19] Bipedalism is practiced both on the ground and aloft when feeding from fruit trees. Foraging in short trees with both feet on the ground allows for individuals to reach higher into trees.[2]

The pelvis and lower body morphology in Australopithecus indicate bipedalism: the lumbar vertebrae curve inward, the pelvis has a human-like shape, and the feet have well developed transverse and longitudinal arches that indicate walking. However, other features indicate reduced locomotor competence. The pelvis is also broad, which requires greater energy to be used during walking. Australopithecines also have short hind-limbs for their weight and height, which also shows a higher energy expenditure. This indicates that this species practiced bipedal locomotion more infrequently, which meant that the costs did not impact them.[2]

In orangutans, bipedalism is more often considered an extension of "orthograde clamber", where the majority of the body mass is held up by the forelimbs, rather than its own independent form of locomotion. However, there are few instances where the hindlimbs carry most of the body weight, only using forelimbs for support. Bipedal posture and motion are most often seen during feeding.[23]

Lizards

Many families of lizards, including Agamidae, Crotaphytidae, Iguanidae and Phrynosomatidae, have been observed to engage in facultative bipedalism. In lizards, this behavior occurs as a result of rapid acceleration caused by the location of the lizards’ hind legs which induces a friction from the ground to produce a reaction force on the rear legs.[4] When the hind limbs build enough power, a strike from the hind limb opens the lizard’s trunk angle and shifts its center of mass; this, in turn, increases front limb elevation, allowing bipedal locomotion over short distances.[24][25] When modeled, an exact number of steps and rate of acceleration leads to an exact shift in the center of mass that allows the elevation of the front limbs: too fast and the center of mass moves too far back and the lizard falls over backward, too slow and the front limbs never elevate. However, this model does not account for the fact that lizards may adjust their movements using their forelimbs and tail to increase the range of acceleration in which bipedal locomotion is possible.[24]

There is some debate over whether bipedalism in lizards confers any advantage. Advantages could include faster speeds to evade predators, or less energy consumption, and could explain why this behavior has evolved. Research has shown that bipedal locomotion does not increase speed but can increase acceleration.[26][24] It is also possible that the behavior developed as a physical property of the lizards' movement only because of the acceleration of the center of mass, and passive engagement with this physical motion minimizes the energetic cost that it would take to keep the forelimbs on the ground.[24] Recent research has shown that the actual acceleration at which lizards begin to run bipedally is lower than the previous model predicted, suggesting that lizards actively rather than passively engage in bipedalism and thus, the behavior may confer some advantage that has not yet been identified.[26] Alternatively, while the origin of the behavior may have been due to physical motion, it may have conferred an advantage such as easier maneuvering that was then exploited.[25]

Evolution of Bipedalism

Bipedalism in Dinosaurs

Bipedalism was common in all major groups of dinosaurs.[1] Phylogenetic studies indicate that bipedalism in dinosaurs arose from one common ancestor, while quadrupedalism arose in multiple lines, coinciding with an increase in body size.[1] To understand how bipedalism arose in dinosaurs, scientists studied extant facultatively bipedal lizards, especially of the clade squamata.[1] The proposed explanation for the evolution of bipedalism in dinosaurs is that it arose in smaller carnivores that were competing with larger carnivores.[1] The need for speed and agility prompted the adaptation of a larger hind-limb muscle, which in turn prompted the shift to facultative bipedalism, where the weaker front legs would not slow them down.[1] Facultatively bipedal dinosaurs encountered ecological pressures for longer periods of high speed and agility, and so longer periods of bipedalism, until eventually they became continually bipedal.[1] This explanation implies that facultative bipedalism leads to obligate bipedalism.[1]

Studying the biomechanics of motion directly contributes to understanding the morphology of both modern primates and fossil records. Bipedal locomotion appears to have evolved separately in different primates including humans, bonobos, and gibbons.[27] The evolutionary explanation for the development of this behavior is often linked to load-carrying in chimpanzees, bonobos, macaques, capuchin monkeys, and baboons.[28] The ability to carry more materials may be either a selective pressure or a significant advantage, especially in uncertain environments where commodities must be collected when found as they can potentially become unavailable.[29] Load carrying affects limb mechanics by increasing the force on the lower limbs, which may affect the evolution of anatomy in facultatively bipedal primates.[28]

Various arboreal adaptations may have affected the evolution of bipedalism as well. Vertical climbing and brachiation changes the structure and purpose of the forelimbs, making quadrupedal walking more difficult and contributing to the shift to bipedal locomotion. Gibbons and sifakas are examples of this: their movement through trees makes quadrupedal walking difficult, resulting in bipedal walking and galloping, respectively.[30][31] Arboreal adaptations making bipedalism advantageous is supported by research that shows that hip and thigh muscles involved in the bipedal walking often most resemble those used in climbing.[32]

In lizards, bipedal running developed fairly early in their evolutionary history. Fossils suggest this behavior began approximately 110 million years ago.[33] Facultative bipedalism may also promote adaptive radiation.[34]

References

  1. ^ a b c d e f g h i Persons, W. Scott; Currie, Philip J. (2017). "The functional origin of dinosaur bipedalism: Cumulative evidence from bipedally inclined reptiles and disinclined mammals". Journal of Theoretical Biology. 420: 1–7. doi:10.1016/j.jtbi.2017.02.032. PMID 28254476.
  2. ^ a b c d e f Hunt, Kevin D. (1996-02-01). "The postural feeding hypothesis: an ecological model for the evolution of bipedalism". South African Journal of Science. 92 (2): 77–90. hdl:10520/AJA00382353_7777.
  3. ^ Clemente, C. J.; Withers, P. C.; Thompson, G.; Lloyd, D. (2008). "Why go bipedal? Locomotion and morphology in Australian agamid lizards". Journal of Experimental Biology. 211 (13): 2058–2065. doi:10.1242/jeb.018044. PMID 18552294.
  4. ^ a b c d Schuett, Gordon W.; Reiserer, Randall S.; Earley, Ryan L. (2009). "The evolution of bipedal postures in varanoid lizards". Biological Journal of the Linnean Society. 97 (3): 652–663. doi:10.1111/j.1095-8312.2009.01227.x.
  5. ^ Preuschoft, Holger (2004). "Mechanisms for the acquisition of habitual bipedality: Are there biomechanical reasons for the acquisition of upright bipedal posture?". Journal of Anatomy. 204 (5): 363–384. doi:10.1111/j.0021-8782.2004.00303.x. PMC 1571303. PMID 15198701.
  6. ^ a b Alexander, R. McN. (2004). "Bipedal animals, and their differences from humans". Journal of Anatomy. 204 (5): 321–330. doi:10.1111/j.0021-8782.2004.00289.x. PMC 1571302. PMID 15198697.
  7. ^ Russo, Gabrielle A.; Kirk, E. Christopher (2013). "Foramen magnum position in bipedal mammals". Journal of Human Evolution. 65 (5): 656–670. CiteSeerX 10.1.1.591.2458. doi:10.1016/j.jhevol.2013.07.007. PMID 24055116.
  8. ^ a b c d e f g h i j k l m n Wunderlich, R. E.; Schaum, J. C. (2007). "Kinematics of bipedalism in Propithecus verreauxi". Journal of Zoology. 272 (2): 165–175. doi:10.1111/j.1469-7998.2006.00253.x.
  9. ^ O'Neill, Matthew C.; Lee, Leng-Feng; Demes, Brigitte; Thompson, Nathan E.; Larson, Susan G.; Stern, Jack T.; Umberger, Brian R. (2015). "Three-dimensional kinematics of the pelvis and hind limbs in chimpanzee (Pan troglodytes) and human bipedal walking". Journal of Human Evolution. 86: 32–42. doi:10.1016/j.jhevol.2015.05.012. PMID 26194031.
  10. ^ Carvalho, Susana; Biro, Dora; Cunha, Eugénia; Hockings, Kimberley; McGrew, William C.; Richmond, Brian G.; Matsuzawa, Tetsuro (2012). "Chimpanzee carrying behaviour and the origins of human bipedality". Current Biology. 22 (6): R180–R181. doi:10.1016/j.cub.2012.01.052. PMID 22440797.
  11. ^ a b c Shapiro, Liza J.; Jungers, William L. (1988). "Back muscle function during bipedal walking in chimpanzee and gibbon: Implications for the evolution of human locomotion". American Journal of Physical Anthropology. 77 (2): 201–212. doi:10.1002/ajpa.1330770208. PMID 3207169.
  12. ^ a b Vereecke, Evie; d'Août, Kristiaan; Van Elsacker, Linda; De Clercq, Dirk; Aerts, Peter (2005). "Functional analysis of the gibbon foot during terrestrial bipedal walking: Plantar pressure distributions and three-dimensional ground reaction forces". American Journal of Physical Anthropology. 128 (3): 659–669. doi:10.1002/ajpa.20158. PMID 15861422.
  13. ^ a b Yamazaki, Nobutoshi; Ishida, Hidemi (1984). "A biomechanical study of vertical climbing and bipedal walking in gibbons". Journal of Human Evolution. 13 (7): 563–571. doi:10.1016/S0047-2484(84)80028-7.
  14. ^ a b Stern, Jack T.; Susman, Randall L. (1981). "Electromyography of the gluteal muscles in Hylobates, Pongo, andpan: Implications for the evolution of hominid bipedality". American Journal of Physical Anthropology. 55 (2): 153–166. doi:10.1002/ajpa.1330550203.
  15. ^ a b c d e f Demes, Brigitte; O'Neill, Matthew C. (2013). "Ground reaction forces and center of mass mechanics of bipedal capuchin monkeys: Implications for the evolution of human bipedalism". American Journal of Physical Anthropology. 150 (1): 76–86. doi:10.1002/ajpa.22176. PMID 23124531.
  16. ^ a b c Hanna, Jandy B.; Schmitt, Daniel; Wright, Kristin; Eshchar, Yonat; Visalberghi, Elisabetta; Fragaszy, Dorothy (2015). "Kinetics of bipedal locomotion during load carrying in capuchin monkeys". Journal of Human Evolution. 85: 149–156. doi:10.1016/j.jhevol.2015.05.006. PMID 26073073.
  17. ^ a b Druelle, François; Berillon, Gilles (2013). "Bipedal Behaviour in Olive Baboons: Infants versus Adults in a Captive Environment". Folia Primatologica. 84 (6): 347–361. doi:10.1159/000353115. PMID 23969888.
  18. ^ a b c d Gebo, Daniel L. (1987). "Locomotor diversity in prosimian primates". American Journal of Primatology. 13 (3): 271–281. doi:10.1002/ajp.1350130305.
  19. ^ a b Rose, M. D. (1977). "Positional behaviour of olive baboons (Papio anubis) and its relationship to maintenance and social activities". Primates. 18: 59–116. doi:10.1007/BF02382953.
  20. ^ Wrangham, R.W. (1980–2005). "Bipedal locomotion as a feeding adaptation in gelada baboons, and its implications for hominid evolution". Journal of Human Evolution. 9 (4): 329–331. doi:10.1016/0047-2484(80)90059-7. ISSN 0047-2484.{{cite journal}}: CS1 maint: date format (link)
  21. ^ Wang, W.J.; Crompton, R.H. (2004–2012). "Analysis of the human and ape foot during bipedal standing with implications for the evolution of the foot". Journal of Biomechanics. 37 (12): 1831–1836. doi:10.1016/j.jbiomech.2004.02.036. ISSN 0021-9290.{{cite journal}}: CS1 maint: date format (link)
  22. ^ Vereecke, Evie E.; D'Août, Kristiaan; Aerts, Peter (2006). "The dynamics of hylobatid bipedalism: evidence for an energy-saving mechanism?". The Journal of Experimental Biology. 209 (Pt 15): 2829–2838. doi:10.1242/jeb.02316. ISSN 0022-0949. PMID 16857866.
  23. ^ Thorpe, Susannah K.S.; Crompton, Robin H. (2005). "Locomotor ecology of wild orangutans (Pongo pygmaeus abelii) in the Gunung Leuser Ecosystem, Sumatra, Indonesia: A multivariate analysis using log-linear modelling". American Journal of Physical Anthropology (in French). 127 (1): 58–78. doi:10.1002/ajpa.20151. ISSN 0002-9483.
  24. ^ a b c d Aerts, P.; Van Damme, R. (2003). "Bidpedalism in lizards: whole-body modelling reveals a possible spandrel". Philosophical Transactions of the Royal Society B: Biological Sciences. 358 (1437): 1525–1533. doi:10.1098/rstb.2003.1342.
  25. ^ a b Clemente, Christofer J. (2014). "The evolution of bipedal running in lizards suggests a consequential origin may be exploited in later lineages". Evolution; International Journal of Organic Evolution. 68 (8): 2171–2183. doi:10.1111/evo.12447. ISSN 1558-5646. PMID 24820255.
  26. ^ a b Clemente, Christofer J.; Withers, Philip C.; Thompson, Graham; Lloyd, David (2008-07-01). "Why go bipedal? Locomotion and morphology in Australian agamid lizards". Journal of Experimental Biology. 211 (13): 2058–2065. doi:10.1242/jeb.018044. ISSN 0022-0949. PMID 18552294.
  27. ^ Vereecke, Evie; D'Août, Kristiaan; Van Elsacker, Linda; De Clercq, Dirk; Aerts, Peter (2005). "Functional analysis of the gibbon foot during terrestrial bipedal walking: Plantar pressure distributions and three-dimensional ground reaction forces". American Journal of Physical Anthropology. 128 (3): 659–669. doi:10.1002/ajpa.20158. ISSN 0002-9483.
  28. ^ a b Hanna, J. B.; Schmitt, D.; Wright, K.; Eshchar, Y.; Visalberghi, E.; Fragaszy, D. (2015-08-01). "Kinetics of bipedal locomotion during load carrying in capuchin monkeys". Journal of Human Evolution. 85: 149–156. doi:10.1016/j.jhevol.2015.05.006. ISSN 0047-2484.
  29. ^ Carvalho, S.; Biro, D. (March 2012). "Chimpanzee carrying behaviour and the origins of human bipedality". Current Biology. 22 (6): R180–R181. doi:10.1016/j.cub.2012.01.052.
  30. ^ Wunderlich, R. E.; Schaum, J. C. (2007). "Kinematics of bipedalism in Propithecus verreauxi". Journal of Zoology. 272 (2): 165–175. doi:10.1111/j.1469-7998.2006.00253.x. ISSN 0952-8369.
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