Field experiments and observations on terrestrial desert lizards

A central ecological issue is how environmental heterogeneity affects how species modulate locomotion and space use. Sand-dwelling desert lizards are excellent for addressing this issue because their entire locomotion is on the ground, and their footprints are faithfully preserved in the sand. Thus, it is poosible to accurately reconstruct both the speed of locomotion (based on stride length), and their movements (based on their paths). Footprints of two species of lizard (Callisaurus draconoides, Uma scoparia) were studied in the soft sand of the Kelso dune system in southern California to examine how incline, vegetative cover, and other factors affected locomotion when lizards were escaping a threat (approach of a human), and when they moved undisturbed through the habitat. These two lizards are closely related, are morphologically and behaviorally different, yet occur sympatrically in various sand dune systems. From tracks left in the sand, one can gain accurate values of stride length, as well as determine the orientation of paths relative to landmarks in the environment. Studies in the laboratory on sand show that both lizards exhibit a linear relationship between stride length and speed, which can be used to estimate speeds of movement in nature.

The fringe-toed lizard, U. scoparia has several specializations for movement on sandy surfaces, such as laterally oriented toe fringes, a countersunk jaw, and a smooth skin that facilitates burrowing into sand. This lizard has a relatively stout body, short limbs, and a short tail, and moves predominantly by quadrupedal locomotion. By contrast, the closely related zebra-tailed lizard (C. draconoides) is more specialized for movement on firm substrates, and is considered a bipedal specialist. Consequently, Callisaurus has several specializations for high-speed bipedal locomotion, including long hindlimbs, a long tail, and long distal elements.

Because moving up inclines is energetically more expensive than moving on level, or near-level surfaces, one might predict that lizards will avoid moving on steep inclines, which on sand dunes reach as high as 32°. On the other hand, because incline does not affect maximal speed or acceleration as greatly in small animals as large animals, one might expect that lizards will preferentially flee uphill when escaping from a predator. Jayne & Ellis (1998) elicited escape locomotion of fringe-toed lizards by approaching the animals and measuring how speed (based on a stride-by-stride basis) and the orientation of escape paths were related to incline, angle of turning, and the location of nearby landmarks (vegetative cover, burrows, the steepest available incline). By comparing the frequency distributions of inclines used during escape with the inclines available in the habitat, they determined that Uma escaped randomly with respect to incline, but that maximal speeds were negatively affected both by running up steep hills and by large turn angles. Thus, running in a straight line on a level surface enhances maximal speed. Further, most Uma appeared to use a planned escape route, by which they ran towards and then down burrows

Bipedal locomotion has been cited as an important behavior that allows some lizards to move at faster speeds than strictly quadrupedal lizards, but few studies have examined how often lizards run bipedally in nature, and whether speeds during bipedal locomotion are actually faster than for quadrupedal locomotion. Irschick & Jayne (1999) examined the effects of incline and other habitat variables on the escape locomotion of Callisaurus draconoides, and also measuring whether each stride was bipedal or quadrupedal. Bipedal strides were, on average, 12% longer than quadrupedal strides, so if one assumes a similar stride frequency between the two modes, bipedal locomotion is significantly. Laboratory studies in Callisaurus for locomotion on a high-speed treadmill show a similar ratio of longer stride lengths for bipedal locomotion, suggesting that this result is robust. Another result in common between U. scoparia and C. draconoides was that maximal stride lengths were typically not achieved until several meters into the escape path, some of which were 30 m long. Therefore, racetrack estimates of speed likely underestimate the maximal speeds of which both Uma and Callisaurus are capable, a problem that may be overcome by use of high-speed treadmills.

Callisaurus lizards exhibited an interesting threshold effect in regards to incline use. On shallow slopes (<15°), over which most of the locomotion occurred, lizards moved randomly with respect to the incline. By contrast, lizards avoided moving directly up or down steep hills (>15°), and preferred to run horizontally across the hill (this cut-off value was found by evaluating movements on a variety of inclines). Thus, animals may have complex and often unpredictable behaviors in regards to usage of habitats that pose functional challenges.

A field experiment was used to examine the effects of incline and vegetative cover on the undisturbed locomotion of U. scoparia. Three 40x100 m plots that differed in both incline and amount of vegetative cover were established. Before lizards were active each morning, all tracks were erased on the sand dunes, and paths were examined several hours later. Similar to escape locomotion, U. scoparia moved around the habitat randomly with respect to incline, but at two preferred speeds. Slow locomotion occurred near burrows and mounds of vegetation, which Uma used as retreats from both predators and high temperatures, whereas the high-speed movements (typically > 2 ms-1) generally occurred in open areas of the dune (undisturbed desert iguanas, Dipsosaurus dorsalis, have been observed to engage in a similar bimodality of speeds near Dale Dry Lake, San Bernardino County, California)

Uma also tended to move more slowly on steep inclines, and near vegetation, resulting in different locomotor behavior in different parts of the sand dune. On steep slopes with little vegetation, the average speed per meter was only 1.54 ms-1, whereas on shallow slopes with more vegetation, the average speed per meter was 1.76 ms-1. In addition, incline, speed, and orientation interacted for the undisturbed locomotion of Uma. On shallow inclines, locomotion tended to be relatively fast, and was distributed approximately equally on different inclines. On steep surfaces, lizards favored direct uphill over downhill locomotion, and most locomotion was relatively slow. This research underscores several themes. (1) Environmental effects on locomotor performance are complex and interactive. (2) Racetrack speeds may underestimate the true maximal speeds of lizards. (3) Species may exhibit threshold effects in terms of usage of habitats.

Sand-dune two-step

Relevant literature:

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

Jayne BC, Irschick DJ.  2000.  A field study of incline use and preferred speeds for the locomotion of lizards.  Ecology. 81:2969-2983.

Irschick DJ, Garland T. Jr.  2001.  Integrating function and ecology in studies of adaptation: Studies of locomotor capacity as a model system.  Annual Review of Ecology and Systematics.  32:367-396.