Predation behaviors and anti-predation behaviors of the lizards, Gambelia wislizenii and Aspidoscelis tigris in an open desert scrub

Kathryn Bajema, Frazier Coe, Brandon Harris, and Kimberley Wolfley 

Biol 417a,b, Summer Session 2004

Department of Biology Western Washington University

 

Abstract

The causes and consequences of spatiotemporal patterns of distribution and abundances of animals are profoundly and fundamentally important endeavors in evolutionary ecology.  Lizards are useful model terrestrial vertebrates for which to study spatiotemporal patterns because lizards can be found, observed, and captured with relative ease. The Alvord Basin of southeastern Oregon is an excellent locale for observational-descriptive and experimental research on the behavioral ecology of two easy-to-study terrestrial lizards, Gambelia wislizenii and Aspidoscelis tigris.   The research locale largely comprised three mesohabitats: sandy flats, dune, and hardpan.  We conducted our study on these mesohabitats, and principally on a 100 x 100 meter plot, wherein most perennials had been mapped in 2001 by previous students enrolled in Biol 417a,b.  In each mesohabitat, by direct visual observations, we documented the distribution and abundance of grasshoppers, the principal prey of G. wislizenii.  And as an indicator of microhabitat quality for foraging A. tigris, we used pit traps to measure distribution and abundance of terrestrial arthropods under perennial plants and in the open in each mesohabitat.  The maps allowed us to track foraging locations, distances and time moved by each lizard in each mesohabitat, and under plants and in the open in that mesohabitat.  Thus, we could compare species, and sexes within each species, for 1) the locations of potential prey of each lizard species and 2) the anti-predator behaviors (as measured by the lizards’ evasion behaviors when pursued by a human) of each lizard species, using, the foraging times and locations of each lizard species. We found trends in the spatial and temporal distribution of G.wislizenii with respect to the distribution, abundance, encounter, and capture of their prey, A.tigris and grasshoppers.  The patterns of A. tigris foraging behaviors appeared to be related to prey distributions, whereas the evasion behavior of A. tigris appeared to be related to the reluctance of G. wislizenii to run straight through a shrub.  

 

 

Introduction

            Vertebrates, with their large brains and complex behaviors, offer intriguing challenges to the behavioral ecologist. Understanding the causes and consequences of spatiotemporal patterns of distribution and abundances of vertebrates arguably are important endeavors in evolutionary ecology and conservation biology.  It is difficult to study the behavior of animals, in particular vertebrates, compared to smaller organisms.  It is a rational choice to use lizards when studying behavior because they can be found, observed, and captured with relative ease.  The open habitat of the Alvord desert scrubs facilitate these processes.  The lizards that naturally live in this habitat are species Gambelia wislizenii and Aspidoscelis tigris and are the subjects of this study.   

There are four basic autecological tasks of animals.  Three of the four are necessary for studying behavior in Gambelia wislizenii and Aspidoscelis tigris.  Food acquisition is the first that will be discussed.  Distribution of arthropods may contribute to understanding of the spatiotemporal patterns of the lizard that consume them.  Grasshoppers in particular are worthy Grasshoppers, especially those moving on plant perimeters and on open ground are a primary prey of G. wislizenii (Steffen 2002), but so is A. tigris (Parker and Pianka 1976).  Prey of A. tigris, however, tend to be the slower moving or sedentary prey that live in the litter environments under plants (Anderson 1986). 

Prior observational-descriptive research comparing male and female G. wislizenii (Rose 2004) revealed anecdotal trends for males to use short-wait ambush predation exclusively for grasshoppers, whereas the female G. wislizenii mix a similar short-wait ambushing (for grasshoppers) with a long-wait ambush predation, with A. tigris as the principal intended prey.  Therefore the second autecological task is avoiding predators.  One would expect the behavior of the two lizard species while foraging to accommodate the needs for antipredation behavior, and thermo regulation. 

            The third basic task of animals is coping with abiotic stresses and avoiding abiotic extremes.  Hence thermoregulation is another primary behavioral task of lizards.  Understanding the habitat will be necessary in assessing the spatiotemporal patterns of the lizards.  

 

·      Hypothesis 1:  Gambelia wislizenii will be most commonly observed foraging in mesohabitats and microhabitats in which their primary prey, grasshoppers, are most abundant.

·      Hypothesis 2:  Female G. wislizenii will be observed spending less time in locomotion than male G. wislizenii because females use a long-wait ambush predation behavior to facilitate capture of A. tigris.

·      Hypothesis 3:  Aspidoscelis tigris will be most commonly observed foraging in mesohabitats and microhabitats in which arthropods are most abundant.

·      Hypothesis 4:  A. tigris will run more evasively (get out of view faster, as indicated by distance until out of view) from a pursuer than the G. wislizenii because of more intense selection for effective anti-predation behavior.  G. wislizenii, will behave in a way that will pertain more to the purpose of avoiding getting stepped on than to evade capture.

·         Hypothesis 5:  Sarcobatus vermiculatus will be the perennial plant most commonly chosen as a refugium from predators, because this shrub tends to be larger, more densely branched, and thornier than the other common plant Artemesia tridentata.

 

 

 

Study Site

The study site is in Harney County, Oregon, in the north end of the Pueblo Valley of the Alvord Basin of the Great Basin Desert.   Because the site has only 20% total plant cover, and modest-sized perennials are widely spaced, we can easily encounter and observe lizards and some of their primary prey.  The two larger and more abundant shrubs are greasewood (Sarcobatus vermiculatus) and Great Basin Sage (Artemisia tridentata); about a dozen other plant species, including four grasses also inhabit the two ha core study area.  The three major mesohabitats on the study site are: sandy flats, dunes, and hardpan; these site are easily identifiable. 

 

 

 

 

Methods

·         Pairs of researchers searched for Gambelia wislizenii and Aspidoscelis tigris  on a 100-meter by 100-meter plot during three different time periods of the day: 0730-0930, 0930-1200, and 1630-1830. 

·         If a lizard encountered appeared unwary, then the lizard’s behaviors were recorded with an audio tape recorder by one observer, while the other observed drew the lizard’s pathway on the map. Primarily noted was the lizard’s body position, lighting on the lizard, type of movement , and location in relation to nearby perennial plants. 

  • Gambelia. wislizenii behavior was recorded for approximately 20 minutes, and Aspidoscelis tigris behavior was recorded  approximately 8 minutes
  • Directly following the researchers’ observation bout, the male observer would chase the lizard, whereas the female observer would map the chase and record the length of time of the chase. 
  • The tapes and maps were analyzed in laboratory at WWU, and exact times were recorded for specific movements and positions.  Distances were measured from the maps. 
  • A 40 m x 10 m plot was surveyed on each of three dunes, three hardpan areas, and three sandy flat areas for distribution and abundance of grasshoppers. Each plot was surveyed during three time periods (0730-0930, 0930-1200, 1630-1830).  Each grasshopper’s coloring, location, and body orientation were noted, as well as adult and nymph status. 
  • Standard pit trapping techniques were used to compare distribution and abundance of arthropods under Sarcobatus vermiculatus,  Artemesia tridentata, and in the open in each of the three mesohabitats.   

 

 

 

 

Results:

 

Figure 1:  Comparison of grasshopper abundance across the three different mesohabitats of the Alvord: dune, sandy flat, and hardpan.  Data was collected from three separate locations from each mesohabitat.  Data was collected three times a day for the equivalent of three days.  The nine plots were 10 x 40 meters split into sixteen 5 x 5 m quadrants, in which we used eight for our study.  The sample size used was 786 grasshoppers of which 285 were adults 474 were nymphs and 27 were unidentified.

 

 

Figure 2:  Display of the pit trap data in average number of organisms by species per trap in small A. tridentada.  The purple lines show plants having one pit trap per plant and the red lines show plants having two pit traps per plant.  n = 11.

 

 

 

Figure 3:  The abundance and distribution of arthropods determined using pit traps in these various locations; under plants vs. in the open and hardpan mesohabitat vs. sandy flat mesohabitat.  n = 329.


 

Figure 4:  The mean distance traveled while foraging per minute of observation time for Gambelia wislizenii (Gw) and Aspidoscelis tigris (At).  These foraging distances are compared across sandy flats, hardpan, and dune mesohabitats.   Sample size consisted of 35 G. wislizenii and 10 A. tigris.

 

 

 

 

 

Average G. wislizenii and A. tigris Pause Times by Location

 

 

Figure 6:  Comparisons of average percent observation time between Gambelia wislizenii and Aspidoscelis tigris for three behavior types:  Foraging in locomotion, foraging while not in locomotion and thermoregulating.  Sample size consisted of 35 G. wislizenii and 10 A. tigris.

 

 

 

 

 

 

 

 

G. wislizenii Behavior Comparisons of Males vs. Females

 

Figure 8:  The distribution of “predator” evasion distances run by A. tigris.  An evasion distance is how far A. tigris runs until it enters a shrub to seek shelter from a human pursuer.  Sample size is one trial for each of 11 individuals.

 

 

 

 

Figure 9:  The mean distances for predator evasion by G. wislizenii.  This is the distance that they run from a predator prior to finding shelter in a plant.  The comparisions seen in this chart are for the mean distance ran for males versus females.  The sample size consisted of 17 females and 9 males. 

 

Figure 10:  The plant preference of G. wislizenii when evading a human pursuer.  This is the final plant that the lizard comes to rest within.  The sample size was 26 pursuits.

 

 

 

 

 

 

 

 

 

 

 

 
An evasion pathway during antipredation by Gambelia wislizenii
 

 

 

Figure 11:  The anti-predation behavior of G. wislizenii when pursued by human.  The X signifies the point at which the pursuit of the lizard begins and the dotted line shows the path of the pursuer. The circle is the point where the lizard has stopped once it had evaded the pursuer.  The sample size for pursued lizards was 26 G. wislizenii.

 

 

 

Table 1:  The anti-predation behavior as seen in figure 9 is quantified in tables like, this using Evasion Decision Number (ED#), Movement Number (M#), Direction (DR), Distance (DS), and Evasion Endpoint (EEND).

Chased Lizard

M/F

Time Period

0930-1200

ED#

MV#

DR

DS

EEND

GW, ( - - G)

M

Coordinates

73NS 124 WE

1

1

S

0.3

 

 

 

Lizard Facing

SW

1

2

E

0.93

 

 

 

Lizard Position

E P LS

1

3

N

1.13

OP

 

 

Chaser Direction

SW

2

4

NNE

0.75

 

 

 

Total Distance

9.94

2

5

NE

1.01

 

 

 

     Along Perimeters

6.19

2

6

E

1.44

 

 

 

     Though Plants

0

2

7

E

0.63

LA (180, 0)

 

 

     In Open

3.75

3

8

SE

0.75

 

 

 

End Plant

LS

3

9

E

1.32

 

 

 

 

 

3

10

S

0.63

 

 

 

 

 

3

11

SW

0.75

LA, 115

 

 

 

 

 

 

 

 

Results Summary

 

  • Reject Null Hypothesis 1: Our research did not show that Gambelia wislizenii was found on the dunes more frequently where Othoptera were most abundant. 

 

  • Do Not Reject Null Hypothesis 2:  Female G. wislizenii were observed spending less time in locomotion than male G. wislizenii.

 

  • Not enough information to test Hypothesis 3

 

  • Do Not Reject Null Hypothesis 4:  A. tigris ran farther in response to a chaser than G. wislizenii (Figure 2). 

 

  • Do Not Reject Null Hypothesis 5:  S. vermiculatus was the most common plant chosen for refuge at the end of a chase.  It is a thick, spiny shrub, allowing for the most favorable protection from predators. (Figure 8)             

 

 

 

Discussion

 

 

More grasshoppers were found in the dune mesohabitat than in the sandy flats and the hardpan combined (figure 1).  The dunes had the largest variety of plants as well as amount of cover.  These factors indicate that the more vegetation present the better the mesohabitat is for sustaining Orthoptera, providing them food and shelter.  Further data looking at the distribution of arthropods using pit traps (figure 2 and 3) shows that they predominate in and around vegetation where they can seek cover and food.

 

            A. tigris had a farther mean foraging distance than G. wislizenii in the dune and sandy flat mesohabitats, and an overall greater foraging distance as a mean of the three mesohabitats (figure 4).  This data correlates well with previous data, which showed that the two lizard species incorporate different foraging techniques due to different prey choice.  Further A. tigris must avoid predation by G. wislizenii, while G. wislizenii has no major predators in the Alvord Basin.

 

The mean pause time in the open for G. wislizenii was more than seven times greater than that of A. tigris (figure 5).  All observations seemed to indicate that G. wislizenii has no fear of being in the open probably because they have no natural predators to worry about and their primary foraging technique is to look into plants from the outside.  Conversely, A. tigris avoids being in the open, foraging more consistently inside of plants.  Along with this the mean number of seconds for pause time under a plant for G. wislizenii was more than twice as long as that of A. tigris (figure 6).

 

The mean percent of time male G. wislizenii spent moving was nearly twice as long as that of the female G. wislizenii, in the 0730-1130 time period (figure 7).  This data suggests, once again, that prey choice is involved in this behavior, indicating that females are larger and more capable of feeding on A. tigris, therefore utilizing long wait ambush feeding techniques more often. 

 

When pursued, A. tigris tended to find shelter in a plant three to four meters from the start of pursuit, often fleeing through bushes before coming to rest (figure 8).  The male G. wislizenii had a mean evasion distance of nearly twice that of the female G. wislizenii (figure 9).  These data may be related to smaller size and, perhaps, greater agility in males relative to females.

 

            In 54% of all evasion decisions, G. wislizenii chose to retreat to Sarcobatus vermiculatus (figure 10).  S. vermiculatus is not only common on the plot, it is a relatively large shrub, and is more densely branched and spinier than Artemisia. Hence, it is expected that this plant provides more protection from larger predators than most other plants.

 

 The anti-predation response that A. tigris exhibited seemed more random, running through and then into bushes in an attempt to escape the chaser.  G. wislizenii generally went around bushes mainly avoiding the chaser’s feet, and circumventing the perimeter of bushes until stopping in the terminal bush (figure11, table 1). 

 

 

Conclusion

 

We found trends in the spatial and temporal distribution of G. wislizenii with respect to the distribution, abundance, encounter, and capture of their prey, A.tigris and grasshoppers.  The patterns of A. tigris foraging behaviors appeared to be related to prey distributions, and primarily involved techniques to avoid encounters with G. wislizenii.  Whereas the evasion behavior of A. tigris appeared to be related to the reluctance of G. wislizenii to run straight through a shrub.  Further it can be noted that G. wislizenii foraging techniques, especially males were geared much more toward the capture of grasshoppers in and around plants than in capturing A.tigris.  However female G. wislizenii, during morning hours, used an ambush foraging technique that would allow for increased probability for capturing A.tigris and or grasshoppers.