Quantitative study of locomotion in teiid lizards

Abstract

Methods are proposed whereby locomotive patterns of lizards can be compared quantitatively from motion pictures. Several species of the family Teiidae and one iguanid, Callisaurus, are analyzed by these methods. In the animals studied, no major differences exist in vertical movements of head for quadrupedal walk gaits; however, a correlation exists between amount of up-and-down movement and quadrupedal fast and bipedal gaits: the faster the animal, the greater the amount of vertical head movement. As a teiid increases its speed, total vertical distance of head displacement increases. The faster a teiid runs, the more erectly it holds its head. Angles of the trunk and head from the horizontal are less in teiids than in iguanids and agamids. The faster the teiid, the greater the angle; regardless of the gait, fast teiids hold their trunks more erect than do slow-moving forms; the latter in effect drag their abdomens. The acetabulum and abdomen in any gait are held higher from the ground in teiids that demonstrate both quadrupedal walk and quadrupedal fast gaits than in those showing only quadrupedal walk gait. In fast teiids, the plane of motion of the femur has changed slightly from a horizontal toward a vertical position. Whenever teiids employ quadrupedal walk gaits, the fore foot leaves and touches the ground ahead of the contralateral hind foot; in quadrupedal fast gaits, the hind foot leaves and touches the ground ahead of the contralateral fore foot. Fast teiids must run through at least one cycle of quadrupedal fast gait before becoming bipedal; quadrupedalism rather than bipedalism is predominantly utilized by fast teiids. In quadrupedal walk gaits, hind feet of Tupinambis are put in front of ipselateral fore feet, while in all other teiids studied they are placed behind the fore feet. In quadrupedal fast gaits, only Ameiva, Cnemidophorus tigris, and Tupinambis place hind in front of fore feet. In fast-running teiids, the distance of stride lengthens as speed increases; rates of stride increase more with change of gait in teiids than in other fast-moving lizards thus far studied. Summation of all relative values of stride and associated characters for quadrupedal gaits results in the following progression: Pholidopolus -Teius-Crocodilurus-Ameiva-Tupinambis-Cnemi-dophorus-Callisaurus. Pholidopolus is the poorest locomotive performer; Callisaurus is the best. This progression in motion picture data are suggestive of possible steps in the evolution of rapid terrestrial locomotion of lizards. Similarity in locomotive patterns of Teius, Ameiva, Cnemidophorus, and Tupinambis occur even though they seem superficially to run differently; this similarity reflects the close taxonomic relationships of these animals. Despite differences in morphological and behavioural characteristics of fast running in the families Teiidae, Iguanidae, and Agamidae, all seem to run efficiently and at fast speeds.