Duncan J. Irschick

Do Lizards Avoid Habitats in Which Performance Is Submaximal? The Relationship between Sprinting Capabilities and Structural Habitat Use in Caribbean Anoles

Recent years have seen an increased emphasis on measuring ecologically relevant performance capabilities to understand associations between morphology and habitat use. Such studies presume that performance is invariant, but in eight Caribbean Anolis lizard species, we found that maximal sprinting ability depends on surface diameter. Moreover, these species differ in the degree to which sprint speed declines with decreasing surface diameter, defined as “sprint sensitivity” (high sprint \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

A Field Study of the Effects of Incline on the Escape Locomotion of a Bipedal Lizard, Callisaurus draconoides

\mathrm{sensitivity}\,=\mathrm{substantial}\,

A comparative analysis of clinging ability among pad-bearing lizards

\end{document} declines in speed between broad and narrow dowels). The habitat constraint hypothesis postulates that Anolis lizards will avoid structural habitats in which their maximal sprinting capabilities are impaired. The habitat breadth hypothesis postulates that species whose performance is less affected by substrate will use a greater variety of habitats than species whose performance varies to a greater extent on surfaces of different diameters. Field observations quantified the proportion of time that lizards spent on different perch diameters. Both hypotheses were confirmed: species with high values of sprint sensitivity avoided using perches on which their maximal sprinting abilities are impaired, whereas species with low sprint sensitivity used such “submaximal” surfaces more frequently. Species with low sprint sensitivity used a broader range of structural habitats than species with high sprint sensitivity.

COMPARATIVE THREE-DIMENSIONAL KINEMATICS OF THE HINDLIMB FOR HIGH-SPEED BIPEDAL AND QUADRUPEDAL LOCOMOTION OF LIZARDS

Comparisons between closely related radiations in different environments provide a unique window into understanding how abiotic and biotic factors shape evolu- tionary pathways. Anolis lizards have radiated extensively in the West Indies, as well as mainland Central and South America. In the Caribbean, similar communities of anole species specialized for different habitats (ecomorphs) have evolved independently on each Greater Antillean island. We examined ecological and morphological data on 49 Anolis species (33 Caribbean, 16 mainland) to investigate whether the same set of ecomorphs has arisen in mainland regions. More generally, we investigated whether the relationship between ecology and morphology was similar among anoles in the two regions. Radiations in the two regions are very different. The majority of mainland anole species exhibit morphological characteristics unlike any Caribbean ecomorph. Furthermore, rela- tionships between ecology and morphology are very different between the two sets of anole species. Among mainland anole species, toepad size is positively correlated with perch height, whereas tail length is negatively related to perch diameter. In contrast, among Caribbean anole species, both forelimb length and body mass are positively associated with perch diameter, and both tail length and hindlimb length are negatively related to perch diameter. Biomechanical considerations provide a functional basis for some of these cor- relations, but much variation remains to be explained. These findings demonstrate that factors that caused anole species to converge repeatedly in the West Indies are not present in mainland regions, and that environmental factors can strongly influence the shape of evolutionary radiations.

EFFECTS OF INCLINE ON SPEED, ACCELERATION, BODY POSTURE AND HINDLIMB KINEMATICS IN TWO SPECIES OF LIZARD CALLISAURUS DRACONOIDES AND UMA SCOPARIA

We analyzed footprints on the surface of a sand dune to estimate maximal running speeds and the incidence of bipedality in nature, as well as to investigate the effects of incline on the escape locomotion of the lizard Callisaurus draconoides. Previous laboratory tests predicted that inclines would negatively affect sprinting performance in C. draconoides. Although physiologists commonly assume that escape locomotion will be near maximal capacity, we found that only 11% of all strides were greater than 90% of maximal speed of C. draconoides. Escape paths averaged 10 m in length and were generally straight. Approximately 30% of the strides taken by C. draconoides were bipedal, and this value was three times greater than previously found for the closely related species Uma scoparia. The modal value of bipedal stride lengths was greater than that for quadrupedal strides. Inclines negatively affected velocity of only the first meter of C. draconoides escape paths. The location of nearest cover had better predictive value for the initial orientation of C. draconoides escapes than incline. On steep slopes (>15°), C. draconoides avoided running directly downhill and uphill and primarily ran horizontally, whereas on shallow slopes, lizards exhibited approximately equal amounts of horizontal, direct uphill, and direct downhill running.

Effects of incline and speed on the three-dimensional hindlimb kinematics of a generalized iguanian lizard (Dipsosaurus dorsalis)

The maximal running speeds of lizards have commonly been measured using level racetracks in a laboratory, but the normal speeds of locomotion of lizards and most animals in their natural habitats are not well documented. Thus we quantified the locomotor activity of the lizard Uma scoparia in its natural sand-dune habitat to determine how commonly animals run near their maximal speed and how inclines in the environment might affect habitat use and the speed of locomotion. We used leaf blowers to erase footprints in the sand in three 40 m x 100 m plots differing in surface topography and vegetation and then measured attributes of the paths and tracks left by U. scoparia that were not disturbed (by observers) during their period of peak activity. Laboratory observations allowed us to estimate speed for each field measurement of stride length (n = 5993). The frequency distribution of stride speed had two distinct modes (mean values 0.25 and 2.00 m/s), for which only the lower one is likely to be aerobically sustainable. Bipedal locomotion was extremely rare (<0.5% of all strides), but much undisturbed locomotion was unex- pectedly fast. The inclines of the surfaces upon which locomotion occurred conformed to random habitat use. In contrast, the inclines of paths made by lizards had lower than expected amounts of nearly horizontal locomotion and a propensity for traveling almost directly uphill, especially when surfaces were steep. Habitat structure varied significantly among plots and affected several aspects of locomotion. For example, path length, average speed per path, and average magnitudes of path and surface inclines per path varied significantly among the three plots. Multiple regression analysis also revealed that the average speed per 1-m interval was maximized for paths with relatively shallow downhill slopes (-6% grade) and decreased significantly both with increased amounts of vegetative cover and increased turning between successive intervals. Unexpectedly, during undisturbed and es- cape locomotion, lizards used inclines similarly.