Abstract
This paper illustrates how simple mechanical models based on morphological, ethological, ecological and phylogenetic data can add to discussions in evolutionary biology. Bipedal locomotion has evolved on numerous occasions in lizards. Traits that appear repeatedly in independent evolutionary lines are often considered adaptive, but the exact advantages of bipedal locomotion in lizards remain debated. Earlier claims that bipedalism would increase maximal running speed or would be energetically advantageous have been questioned. Here, we use 'whole body' mechanical modelling to provide an alternative solution to the riddle. The starting point is the intermittent running style combined with the need for a high manoeuvrability characterizing many small lizard species. Manoeuvrability benefits from a caudal shift of the centre of mass of the body (body-COM), because forces to change the heading and to align the body to this new heading do not conflict with each other. The caudally situated body-COM, however, might result in a lift of the front part of the body when accelerating (intermittent style), thus resulting in bipedal running bouts. Based on a momentum-impulse approach the effect of acceleration is quantified for a mechanical model, a virtual lizard (three segments) based on the morphometrics of Acanthodactylus erythrurus (a small lacertid lizard). Biologically relevant input (dimensions, inertial properties, step cycle information, etc.) results in an important lift of the front part of the body and observable distances passively covered bipedally as a consequence of the acceleration. In this way, no functional explanation of the phenomenon of lizard bipedalism is required and bipedalism can probably be considered non-adaptive in many cases. This does not exclude, however, some species that may have turned this consequence to their benefit. For instance, instantaneous manipulation of the position of the centre of the body-COM allows stable, persisting bipedal running. Once this was achieved, the bipedal spandrel could be exploited further.
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Selected References
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- Farley C. T., Ko T. C. Mechanics of locomotion in lizards. J Exp Biol. 1997 Aug;200(Pt 16):2177–2188. doi: 10.1242/jeb.200.16.2177. [DOI] [PubMed] [Google Scholar]
- Fedak M. A., Seeherman H. J. Reappraisal of energetics of locomotion shows identical cost in bipeds and quadrupeds including ostrich and horse. Nature. 1979 Dec 13;282(5740):713–716. doi: 10.1038/282713a0. [DOI] [PubMed] [Google Scholar]
- Gould S. J., Lewontin R. C. The spandrels of San Marco and the Panglossian paradigm: a critique of the adaptationist programme. Proc R Soc Lond B Biol Sci. 1979 Sep 21;205(1161):581–598. doi: 10.1098/rspb.1979.0086. [DOI] [PubMed] [Google Scholar]
- Irschick D. J., Jayne B. C. Effects of incline on speed, acceleration, body posture and hindlimb kinematics in two species of lizard Callisaurus draconoides and Uma scoparia. J Exp Biol. 1998 Jan;201(Pt 2):273–287. doi: 10.1242/jeb.201.2.273. [DOI] [PubMed] [Google Scholar]
- Irschick DJ, Jayne BC. Comparative three-dimensional kinematics of the hindlimb for high-speed bipedal and quadrupedal locomotion of lizards . J Exp Biol. 1999 May;202(Pt 9):1047–1065. doi: 10.1242/jeb.202.9.1047. [DOI] [PubMed] [Google Scholar]
- Jindrich DL, Full RJ. Many-legged maneuverability: dynamics of turning in hexapods . J Exp Biol. 1999 Jun;202(Pt 12):1603–1623. doi: 10.1242/jeb.202.12.1603. [DOI] [PubMed] [Google Scholar]
- Roberts T. J., Kram R., Weyand P. G., Taylor C. R. Energetics of bipedal running. I. Metabolic cost of generating force. J Exp Biol. 1998 Oct;201(Pt 19):2745–2751. doi: 10.1242/jeb.201.19.2745. [DOI] [PubMed] [Google Scholar]
- SNYDER R. C. The anatomy and function of the pelvic girdle and hindlimb in lizard locomotion. Am J Anat. 1954 Jul;95(1):1–45. doi: 10.1002/aja.1000950102. [DOI] [PubMed] [Google Scholar]
- Taylor C. R., Rowntree V. J. Running on two or on four legs: which consumes more energy? Science. 1973 Jan 12;179(4069):186–187. doi: 10.1126/science.179.4069.186. [DOI] [PubMed] [Google Scholar]