Prey Choice by the Desert Horned Lizard Phrynosoma platyrhinos Relative to Spatiotemporal Distribution of its Potential Prey

Monie Jones, Kalin Huddleston, Tera Matson

Bio 417a & 417b, Summer 2002

Western Washington University

Bellingham, WA

Introduction

In the Alvord Basin of the Great Basin Desert the Desert Horned Lizard Phrynosoma platyrhinos, is abundant, easy to capture, and its diet, principally ants, is easily discerned by counting the ant heads in the fecal pellets. Because the colonies of most ants are easy to find and observe for surface activity throughout the day in the desert, the relative abundance of ants in the lizard’s diet can be compared with the spatiotemporal distribution and abundance of ants. Such precise comparisons of prey availability and predator choice are rare in ecology, hence the Phrynosoma-ant system is a rare opportunity to examine a variety of hypotheses in foraging theory.

Methods

Ants

· Conduct a survey of a 120m by 130m plot and locate and flag all ant colonies.

· Determine number of colonies for each ant species.

· Quantify the distance and compass direction from each ant colony entrance to the nearest large perennial plant or patch of perennials.

· Randomly choose a subset of 62 ant colonies to further quantify.

· Determine number of plants and species, and volume of the nearest patch of perennials.

· Quantify soil types at ant colony entrance and under nearest plant patch

· Measure air and soil temperatures at the beginning and end of each ant activity survey.

· Document activity of ant colony by counting the number of ants leaving or entering the nest during 1-minute counts at each nest. This count was done simultaneously for leaving and entering at 62 nests at set time intervals six times a day for three days.

· Analysis was conducted using Chi² tests to determine species preference for substrate type and vegetation proximity

Lizards

· Conduct repeated field searches of a 150m by 150m plot (includes the ant plot) to locate all P. platyrhinos and record behavior of the lizard and the substratum, microhabitat, and mesohabitat of the lizard when it was first seen.

· Mark, record size (weight, length) and sex distributions of P. platyrhinos.

· Document encounters of eight adult P. platyrhinos a minimum of 3 times each, as an attempt to gain preliminary information on home range size and pattern of individual use of mesohabitats.

· Collect fecal samples to measure relative abundances among ant species; use fecal pellets defecated in bag, or opportunistically express full fecal pellets samples from P. platyrhinos.

· Compare the proportion of all ants represented by each ant species in P. platyrhinos fecal pellets with the relative abundances among ant species on the plot.

Results, page 1

· Ant colonies were most common on the two sandy mesohabitats, and were largely absent on the hardpan (larger open spaces) on the 120 x 130 m study plot (Figure 1).

· Relative abundances of ant species (colonies) on the study plot are shown in Figure 2.

· The relationship of substratum and ants, in general, is shown in Figure 3. Ant species A and D were analyzed for preference in substrata, as indicated by pit trap data. Both species were found to show a significant preference for sandy substrata (Species A: X²_cal=4.59, X²_exp=3.84 @ p=0.05, df =1; species D: X²_cal=11.72, X²_exp=3.84 @ p=0.05, df=1).

· For those ant species whose colonies were found, there were no significant preferences for proximity to any species of shrubs because distribution of ant colonies near species of shrubs closely resembled the natural abundance of shrub species (Figure 4).

· Nearest neighbor distance between colonies of ant species A (for which we have the most data) is 6-10 meters (Figure 5).

· Measures of distance from each colony entrance of species A (ant species for which we have the most data) on the plot to the nearest shrub (Figure 6) appears to show a tendency for colonies to be nearer shrubs than if the colony entrances were randomly distributed; a statistical analysis of these distributions were not performed, however.

· Anecdotal evidence is strong for colonies of ant species D, the honey ants to be entirely associated with shrubs (ants on plants, no colonies in the open, and one colony was discovered in the ground under a shrub).

· Activity during the day varied among ant species. Six 130-minute periods were compared throughout the day for entry and exit activity at colony entrances; totals are shown in Figure 7. Ant species A decreased activity during midday, whereas ant species B showed almost no activity change throughout the time periods, and species C fluctuated with no apparent relationship to time of day.

Results, page 2

· Ant species D (honey ants) activity, as measured by the number of honey ants seen per shrub known to have had honey ant activity on it on days immediately before the counts were made, was obviously less during the mid-hours of the day (Figure 8).

· Phrynosoma platyrhinos were spotted most frequently on the western sandy flat area of plot (Figure 9), and corresponded with one of the areas more densely occupied by ant colonies (Figure 1).

· Diets of P. platyrhinos (totals for first fecal pellet of all individuals) were analyzed. Lizards ate more of ant species B (mean=15.375, t test=2.758, df=23, P=0.011) than one would infer from the apparent abundance of observed colonies. No significant preference or aversion by P. platyrhinos was apparent for other common ant species when compared to the abundances of their colonies (sp. A: mean=32.458, t test=1.296, df=23, P=0.208; sp. C: mean=3.958, t test=1.669, df=23, P=0.109; sp. F: mean=2.667, t test=1.750, df=23, P=0.093). Although ant species G was found in Phrynosoma fecal pellets, no colonies of G were found on the study plot. Ant species E was not found in any lizard fecal pellet analyzed.

· Individuals of P. platyrhinos, for which we had three fecal pellets, were examined for tendencies of individuals to specialize their feeding among ant species. No obvious non-random pattern exists, that is, it may be that with repeated samples individuals represent the population (Figure 10), although only 3 repeated samples is minimal.

· As the warmth and dryness of summer progressed, we had the impression that the abundance of ant species D (honey ants) found on the plot was decreasing (counts were made only late in the research period), so we compared the number of honey ants eaten near the start of the 3 week study period versus near the end were compared to determine if there was a significant change in the eating habits of P.p. over the study period that reflected our visual impressions. The difference did not meet statistical significance, but the trend was apparent, despite small sample sizes (diff. of means=0.064, pooled variance t=1.634, df=18, p=0.120).

Figure 1. Locations of ant colonies on a 120m by 130m plot within the 150m x 150m plot.

Figure 2. Total number of colonies found for each ant species on the study plot.

Figure 3. Distribution of ant colonies among substrata

(Sd = sand, Hp = hardpan, P = pebbles, Gr = gravel).

Figure 4. Distribution of woody perennial plants (shrubs) that are nearest to

ant colony entrances on the study plot. The proportions are virtually identical to abundances of shrubs on the plot.

Figure 5. Distance to nearest colony of species A for each colony of species A on the plot.

Figure 6. Distance from the colony entrance of ant species A to the nearest shrub.

Figure 7. Temporal pattern of ant activity during the activity period of

Phrynosoma platyrhinos, compared among ant species. Values are the

total number of entries and exits by all individuals of a species totaled

for the same number of colonies entrances at soil surface, and same

lengths of time observed per colony for each of three species, A, B, and C.

Figure 8. The temporal pattern of honey ant activity (species D), as measured

by the number of ants seen per each shrub known to have honey ants on it.

Figure 9. Positions of individual Phrynosoma platyrhinos when first sighted on each day;

dotted lines link consecutive sightings.

Figure 10. Distribution of ants eaten by Phrynosoma platyrhinos:

mean number of each ant species among three fecal pellets in each of eight lizards.

Discussion

Our data on distribution and abundance of ant colonies can be considered as preliminary and somewhat inconclusive. For example, colonies of ant species B (Veromessor) were difficult to find, thus our estimate of their colony abundance could not have been representative of the population on site. In contrast, the diet of Phrynosoma platyrhinos was relatively high in abundance of ant species B, whereas other ants that were in the diets of P. platyrhinos were eaten in proportion to their apparent abundances. Honey ants were measured by frequency of sightings on shrubs, and we do not know their availability to horned lizards.

Although ant activity at the colony level appeared to be relatively unpredictable, a definite depression in activity during midday was seen in species A, B, and D, presumably due to high ambient midday temperatures. Species C had low activity overall, but sample size for species C was small, so activity depression in species C during midday could neither be supported nor discounted. We observed anecdotally that ant activity increased when light intensity or temperature around the colony entrance decreased. It would be of scientific interest to determine if the Phrynosoma fed more heavily during periods of higher ant activity. We know too little about the distribution and abundance of several of the species of ants eaten by P. platyrhinos on the study plot. And even for the more prevalent species in P. platyrhinos diets, we do not know whether the lizards tend to eat those ants near the colonies or whether the ants are abundant enough and mobile enough for a stationary, ambushing P. platyrhinos to eat its daily fill. Observation of several neighboring colonies of each ant species and documenting the spatiotemporal patterns of individual ants as they forage would help develop hypotheses to predict the use and preference among ant species by P. platyrhinos.

Many more sightings of P. platyrhinos are needed to document mesohabitat choice, microhabitat choice, and spatiotemporal patterns of individual lizards as they forage. With more sightings of each individual, then more fecal pellets could be obtained from each individual lizard. Many more field workers (perhaps 12) than the three students in 2002 (albeit with intermittent help by others) are needed; perhaps radiotransmitters could be attached to lizards, thereby permitting the encounter frequency of researchers and lizards to be improved markedly. The methods used and data obtained in 2001 and 2002 should provide students in future classes excellent perspectives on which new methods to apply to the study of the behavioral ecology of ant prey and lizard predator.

Appendix A:

Key for Data Categories obtained by Ants & Phrynosoma Research Team, 2002

Ant Nest Data Sheets

1. Nest#

2. Date (Date first found)

3. Time (Time first found, military time)

4. Ant Soil (Soil type around mound, see bottom for codes)

5. Species of ants (for codes, see bottom of page)

6. Location(N) (Location – north of south)

7. Location(W) (Location – west of east)

8. Observations

Plant /Ant Data Sheets

1. Nest #

2. Distance from mound (in cm)

3. Direction from mound (in compass direction: N, NW, W, SW, S, etc.)

4. Plant species represented in clump (four letter codes: see bottom)

5. Plant clump volume (in cm, height marked with an H adjacent to the number)

6. Soil type at base of Plant (soil codes: see bottom)

Ant Activity Data Sheets

1. Date (When Obs. was recorded)

2. Time (Bout time, see bottom for intervals)

3. Nest # (ID # of nest studied)

4. Presence/Absence (Appearance or not of ants)

5. Enter # (# entering in one minute)

6. Exit # (# exiting in one minute)

7. HD # (# of honeydew ants found in one minute on shrub checked)

8. Start Air Temp (Temperature, in Celsius, at start of bout, in sun)

9. Start Soil Temp (Temperature, in Celsius, at start of bout, in sun)

10. Stop Air Temp (Temperature, in Celsius, at end of bout, in sun)

11. Stop Soil Temp (Temperature, in Celsius, at end of bout, in sun)

Phrynosoma Data Sheet

1. Date (Date first processed)

2. Bag # (Bag # of initial bag)

3. Toe Clip & Color (Toe Clip and Color assigned to individual)

4. Sex (M(male) or F(female))

5. SVL (Length, in cm, from tip of snout to vent)

6. Tail (Length, in cm, of tail)

7. Mass (in grams, of individual)

8. Fecal Bottle # (ID #’s of fecal bottles for this individual)

Fecal Pellet Data Sheet

1. Vial # (ID # of fecal bottle)

2. Toe Clip & Color (of individual that produces the fecal material)

3. Expd/Bag (Whether fecal material was found in bag or expressed)

4. # A (# of ant species A found in fecal material)

5. # B (# of ant species B found in fecal material)

6. # C (# of ant species C found in fecal material)

7. # D (# of ant species D found in fecal material)

8. # F (# of ant species F found in fecal material)

9. # G (# of ant species G found in fecal material)

10. Misc. (Other prey items found in fecal material)

Phrynosoma platyrhinos Sighting Data Sheet

1. Date (Date of spotting)

2. Time (Time of spotting)

3. Bag # (Bag # if P.p. was caught)

4. Color and Toe (Color and toe clip assigned to that P.p.)

5. Loc(N) (Location north of south)

6. Loc(W) (Location west of east)

7. Substrate (Substrate type P.p. was found on)

8. Move (Movement of P.p. at time of observation)

9. Sun (Solar cover of P.p. at time of initial observation, see bottom for abbr.)

10. Plant (Plant cover P.p. was found under, if found under a plant)

11. Misc. (Observations, etc.)

Soil ID Codes

sd	=	sand substrate
hp	=	hardpan substrate
gr	=	gravel substrate
p sd	=	packed sand
gr-sd	=	gravelly sand
gr-hp	=	gravelly hardpan
sd-hp	=	sand over hardpan
dt-sd	=	dusty sand

Ant species ID Codes (identifications are tentative)

A	=	Pogonomyrex californicus (large-headed red ants)
B	=	Myrmecocystus kennedyi (alien-headed, black-butted ants)
C	=	(bulldog-headed, black-butted ants)
D	=	(honeydew ants)
E	=	(tiny brown line ants)
F	=	(tiny black ants, possible honeydews)
G	=	Unknown ant species (found in P.p. fecal material)

Plant ID Codes

ATCO	=	Atriplex confertifola (Shadscale)
TEGL	=	Tetradymia glabrata (Littleleaf Horsebrush)
SAVE	=	Sarcobatus vermiculatus (Greasewood)
ARSP	=	Artemisia spinescens (Budsage)
ARTR	=	Artemisia tridentata (Basin Big Sagebrush)
GRSP	=	Grayia spinosa (Spiny Hopsage)
ACHY	=	Achnatherum hymenoides (Indian Rice Grass)
DISP	=	Distichlis spicata (Saltgrass)
BRTE	=	Bromus tectorum (Cheatgrass)
ATCA	=	Atriplex canescens (Fourwing Saltbrush)
rabbit	=	Unknown Ericamera species (either E. nauseosa or E. viscidiflora)
dead *	=	Dead plant of *

Bout Timetable for Ant Activity Survey

0730-0940	=	1^st bout of day
0940-1150	=	2^nd bout of day
1150-1400	=	3^rd bout of day
1400-1610	=	4^th bout of day
1610-1820	=	5^th bout of day
1820-2030	=	6^th bout of day

Sunlight Abbreviations

Op Su	=	Open Sun
Fi Su	=	Filtered Sun
Da Sh	=	Dappled Shade
Fu Sh	=	Full Shade

Movement Abbr. codes

B	=	Basking
W	=	Walking (indeterminate purpose)
R	=	Running (indeterminate purpose)
RH	=	Running in response to humans
lunch	=	eating