Bipedal and quadrupedal locomotion in lizards
The ability of lizards to run bipedally has long intrigued both laypeople and biologists, but with the exception of a few studies, little is understood of the detailed mechanism by which lizards run bipedally? Do lizards use the same hindlimb movements in quadrupedal and bipedal running? Is bipedal locomotion faster than for quadrupedal locomotion? Do lizards that typically run bipedally have different morphological characteristics than quadrupedal species? These and other questions are poorly understood.
Several of our studies have addressed these questions, both in the laboratory and in the field. In the laboratory, we compared the bipedal and quadrupedal locomotion of five morphologically different lizard species (Fig. 1).
Figure 1. A phylogenetic tree of five morphologically and behaviorally distinct desert lizards used in studies of high-speed kinematics and bipedal and quadrupedal locomotion.
The lizard species shown in Fig. 1 are the desert iguana (Dipsosaurus dorsalis), the zebra-tailed lizard (Callisaurus draconoides), the Mojave fringe-toed lizard (Uma scoparia), the desert horned lizard (Phrynosoma platyrhinos), and the western whiptail (Cnemidophorus tigris tigris). These five species span a wide range of different terrestrial lizard phenotypes, and their phylogenetic relationships are well resolved. These species also are similar in size, and occur sympatrically in large regions of the Southwestern Mojave desert. These five species have differing degrees of specialization for bipedality and maximal sprinting performance. On the one hand, C. draconoides runs very quickly, frequently runs bipedally in nature, and has many of the morphological characteristics of a bipedal specialist. On the other hand, P. platyrhinos is a relatively slow runner, has never been observed to run bipedally, and is morphologically adapted for a cryptic existence. All five lizard species were induced to run at maximum speeds on a high-speed treadmill, and a total of 23 two- and three-dimensional kinematic variables were measured for each species, as well as maximum speeds.
Figure 2. Quadrupedal locomotion in the zebra-tailed lizard (Callisaurus draconoides) when running at about 5 m/s.
One of the most striking findings was the nearly horizontal posture of the body during bipedal locomotion (Figs. 2, 3), and the large amount of forward extension of the hindlimb. Most biologists and laypeople envision bipedal locomotion as occurring with a high trunk angle, but this may be true only for certain specialized species and slow locomotion. Second, bipedal locomotion was rarely, if ever, faster than quadrupedal locomotion. Thus, there appears to be no obvious performance advantage for bipedal locomotion. The kinematics of bipedal and quadrupedal locomotion differed significantly, but these differences were more quantitative than qualitative.
Figure 3. A comparison of quadrupedal (left) and bipedal (right) locomotion in the desert Iguana (Dipsosaurus dorsalis) when running at about 4 m/s.
Another striking feature of high-speed locomotion was the highly extended hindlimb and the very large knee angles, which reached as high as 130 degrees. Thus, high-speed locomotion in these lizards does not conform to the traditionally held view of a "sprawling" lizard. These studies indicate that classification of lizards as "sprawling" is misleading, and studies of other terrestrial and arboreal lizards may shed light on the generality of these findings.
We have also taken great pains to expand studies of quadrupedal and bipedal locomotion beyond simple laboratory settings, and have conducted extensive fied studies on how often lizards use bipedal locomotion in nature, and whether the use of such locomotion provides a performance advantage. The field research to date shows about an 11% advantage (in terms of stride length, see image at top of page) for bipedal locomotion based on tracks left in the sand by zebra-tailed lizards.
Irschick DJ, Jayne BC. 1999. Comparative three-dimensional kinematics of the hindlimb for high-speed bipedal and quadrupedal locomotion of lizards. Journal of Experimental Biology. 202:1047-1065.
Irschick DJ, Jayne BC. 1999. A field study of effects of incline on the escape locomotion of a bipedal lizard, Callisaurus draconoides Physiological and Biochemical Zoology. 72:44-56