Effects of intraspecific competition on sheltering behavior in the desert sand scorpion
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Effects of intraspecific competition
on sheltering behavior in the desert sand scorpion
Yousef Awad1, Anthony Ebraham1, Jake Hernandez2, Mariam Moazed2
1
University of California, Riverside; 2University of California, Santa Cruz
ABSTRACT
Competitive exclusion can take place with abiotic resources such as heat or shelter.
Because ectotherms’ internal temperatures are contingent upon the environment, if
warmth is a limited resource, ectotherms may compete for warmer areas in their
environment. In this study, our aim was to understand the importance of temperature
in the selection of a shelter for the desert sand scorpion, Smeringurus mesaenis. We
also aimed to understand how desert sand scorpions compete for limited warmth, and
the role of competitive exclusion in shelter choice. In order to understand the effect of
competition on sheltering behavior, we conducted choice trials between warm and cool
rock shelters with solitary versus paired scorpions of different sizes. Observational
results showed a strong correlation between ground surface temperature and scorpion
body temperature. Scorpions did not show strong preference for warmer shelters when
alone. However, we found evidence for competitive exclusion for warm rocks by bigger
scorpions when paired with smaller scorpions. The competitive exclusion for warm
shelters by larger desert sand scorpions has implications in scorpion population
regulation.
Keywords: Smeringurus mesaensis, competitive exclusion, scorpion behavior, desert,
intraspecific competition
INTRODUCTION where two species of chipmunks coexist,
the larger chipmunk competitively excludes
Food, water, and shelter are essential for the smaller chipmunk from the large
animal survival. When such resources are chipmunk’s tree by barking at it and chasing
limited, organisms with overlapping needs it away, thus excluding the smaller
must compete to obtain them. One way chipmunk from food sources (Brown 1971).
organisms compete is by removing the Abiotic factors can also affect
competitor’s access to a resource competition. Beyond food and mating,
altogether through competitive exclusion competitive exclusion can take place over
(Hardin 1960). Mechanisms of competitive resources such as sunlight or space. For
exclusion take many forms, ranging from instance, the effect of temperature on
displays of aggression and auditory or visual organisms differs depending on whether
warning cues to cannibalism (Polis and they are ectotherms or endotherms.
McCormick 1987). For example, in the Ectotherms have increased metabolic rates
Great Basin mountain ranges of California
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CEC Research https://doi.org/10.21973/N3237M Fall 2019 1/7
at higher temperatures (Schulte 2015). dynamics on top of competitive behaviors.
Higher metabolism means increased In this competitive, heat-limited
mobility to search for food and mates, environment, it is unclear whether desert
which corresponds to increased fitness sand scorpions in this system compete with
(Knies and Kingsolver 2010). Because one another for warmer shelters.
ectotherms’ internal temperatures are We sought to test the importance of
contingent upon the environment, if temperature in shelter selection for desert
warmth is a limited resource, ectotherms sand scorpions. Specifically, we tested the
may compete for warmer areas in their effect of ground temperature on scorpion
environment to increase their fitness. body temperature and size, predicting that
Examples of ectotherms competing over ground surface temperature would closely
thermal refugia include West Virginian match body temperature because scorpions
brook trout and brown trout, dace and use the surfaces they directly contact for
sculpin fishes in Tehama County, and warmth in the absence of solar radiation
western fence lizards in Northern California (Kearney and Predavec 2000). We also
(Hitt et. al. 2017, Baltz et. al. 1982, Sabo tested how desert sand scorpions compete
2003). One example of an ectotherm that for limited warmth, and the role of
may compete for thermal resources is the competitive exclusion in shelter choice. We
scorpion. hypothesized that desert sand scorpions
Scorpions are ectothermic and often hunt would have a preference for warmer rocks
for prey at night (Williams 1987). Scorpion at night, based on the assumption that
behavior in deserts such as the Sonoran warmer rocks will temporarily provide more
desert in the Southwestern United States warmth to sustain metabolism (Schulte
and Northwestern Mexico is driven by 2015). Thus, we also predicted that larger
competition for several reasons. First, scorpions would competitively exclude
because they are nocturnal ectotherms, smaller scorpions from the warmer rocks in
scorpions must compete for limited heat at manipulative experiments due to the
night when incoming heat from solar limitation in warm shelters, which we
radiation is lowest and air and soil believe to be the preferred shelter across all
temperatures fall drastically in the desert sizes of scorpions.
(van Gestel 2013) . Second, high density can
increase competition for resources (Miller METHODS
and Spoolman 2009). Desert sand
scorpions (Smeringurus mesaenis) in 2.1 Study System
particular are in high abundance at low
We conducted an observational study and
elevations in the north west of Anza-
experimental study on the nights of
Borrego Desert State Park, a protected
November 1–5 2019 at the Steele/Burnand
region of the Sonoran desert, which could
Anza-Borrego Desert Research Center in
increase instances of direct competition for
Borrego Springs, California. Desert sand
limited resources within the species
scorpions were collected from 18:30 to
(Korntheuer et. al. 2018). Finally, because
20:30 on the west side of the reserve in a
scorpions are cannibalistic, their
desert shrubland, which ranged in elevation
interactions take on predator-prey
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CEC Research https://doi.org/10.21973/N3237M Fall 2019 2/7from 210 to 240 meters. The average air set to 76.7°C and then transferred to one
temperature during collection was 24.1°C side of the arena. The other was kept in a
(standard deviation of 1.7). The flora tempered classroom to mimic a cooler
consisted mainly of creosote bush (Larrea outdoor temperature and transferred to the
tridentata), cholla cactus (Cholla spp.), and opposite side of the arena. Rocks were
ocotillo (Fouquieria splendens). We placed roughly 16 mm apart. To control for
observed scorpions over a total area of air temperature and light, all trials were
roughly 1 km. To ensure we never collected conducted in a dark room with the
or measured the same scorpion twice, each thermostat set to 18.3°C. Using a
night we flagged our study site and moved temperature gun, we measured the
to an adjacent site with similar gradient in temperature of both rocks to ensure a 10°C
elevation the following night. difference. The average temperature of our
cool rocks was 18.7°C (standard deviation of
2.2 Observational Methods 4.6°C) and the average temperature of our
warm rocks was 29.8°C (standard deviation
We located and collected scorpions in a of 5.1°C). We placed each scorpion in the
wash and along the base of the ridge center of one arena and observed its
extending out of the wash using UV flash location after 15 minutes. Location was
lights. At the time of collection, we categorized as either under warm rock,
measured air temperature with an under cool rock, or in the open.
anemometer, scorpion abdomen To test scorpion behavior when in
temperature using a temperature gun, and competition, we collected scorpions and
the temperature of the surface on which conducted competition trials for three
the scorpion was found using a consecutive nights. We placed two
temperature gun. We then flagged the scorpions of the same sex in 33 cm by 33
location where the scorpion was found. We cm rectangular rubber bin arenas with
measured scorpion size using calipers from heated rocks and cool rocks in a tempered
chelicerae to anus, and recorded their sex classroom. The average temperature of cool
based on pectine size and comb number. rocks was 21.1°C (standard deviation of
2.7°C) and the average temperature of
2.3 Manipulation Methods
warm rocks was 31.3°C (standard deviation
To understand desert sand scorpion of 0.73°C). In each trial, one scorpion of the
behavior when not in a competitive two was relatively larger. The average
environment, we tested 24 scorpions on difference in size between paired scorpions
two nights in arenas made with rectangular was 4.98 mm (standard deviation of 2.9
plastic or rubber bins roughly 1100 cm2 in mm). We released the two scorpions
the area of the base. Each arena was simultaneously in the center of the arena,
carpeted to 1 cm with sand and gravel from equidistant from warm and cool rocks. We
the wash west of the desert research center then observed each scorpion’s location
and two evenly spaced rocks of similar size after 15 minutes. Location was categorized
of roughly 15 mm by 13 mm by 10 mm. One as either under warm rock, under cool rock,
rock was warmed for 10 minutes in an oven or in the open.
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CEC Research https://doi.org/10.21973/N3237M Fall 2019 3/72.4 Statistical Analysis strong effect on rock choice when two
scorpions were competing (N = 36, χ2 =
We used linear regressions to test the 4.01, P = 0.045: Figure 3). Larger scorpions
effects of ground surface temperature on were found more frequently under warm
scorpion body temperature and scorpion rocks than smaller scorpions: of the
size. We used a logistic regression to test scorpions that chose a rock after fifteen
the effects of scorpion size on rock minutes, 13 out of 18 of the larger
preference between warm and cool rocks in scorpions were under warm rocks while
our competition-free trials. Finally, we used only 6 out of 18 of the smaller scorpions
a chi-square test to test the effects of were under warm rocks.
scorpion size on rock preference between
warm and cool rocks in our competition
trials. All statistical analyses were
conducted using JMP Pro 14 and Vassar
Stats chi-square test.
RESULTS
3.1 Observation Results
In total we measured 101 desert sand
scorpions over four consecutive nights.
Scorpion body temperature was closely
correlated with the surface temperature on
which they were found (N = 101, R2 = 0.94,
P = 0.0001: Figure 1A). The surface
Figure 1. Correlation between ground surface
temperature did not correlate to scorpion temperatures and desert sand scorpion
size (N = 101, R2 = 0.03, P < 0.001: Figure 1B). (Smeringurus mesaensis) body temperature (A)
and scorpion size (B). Each point represents a desert
3.2 Responses with and without sand scorpion found during observational studies at
Competition Steele/Burnand Anza-Borrego Desert Research
Center. There was a positive correlation between
In general, temperature of rock shelter surface temperature and scorpion body temperature
(A). There was no correlation between surface
had an effect on scorpion rock choice, temperature and scorpion size (B). Desert sand
especially when in competition with other scorpions of all sizes were found at similar surface
scorpions. In our competition-free trials, temperatures.
after fifteen minutes 18 scorpions were
under warm rocks, 6 were under cool rocks,
and 16 were out in the open. Among the
scorpions that chose a rock, scorpion size
did not have an effect on their choice of
warm or cool rock in competition-free
choice trials (N = 24, χ2 = 0.42, P = 0.51:
Figure 2). However, scorpion size had a
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CEC Research https://doi.org/10.21973/N3237M Fall 2019 4/7Figure 2. Rock choice based on desert sand scorpion Figure 3. Effect of desert sand scorpion
(Smeringurus mesaensis) size (competition-free). (Smeringurus mesaensis) size on rock choice
Bars represent average size of scorpions under cool (competition). Bars represent proportion of small
and warm rocks. Scorpion size was measured in mm and large scorpions under warm rock (dark bar)
from the chelicerae to the anus using a caliper. Cool versus cool rock (light bar). Small versus large
rocks were on average 18.7°C (SD = + 4.6°C), while scorpion pairs had an average size difference of
warm rocks were on average 29.8°C (SD = + 5.1°C). 4.9 mm (SD = +0.02 mm). Cool rocks were on
Scorpion choice was recorded after 15 minutes from average 21.1°C (SD = + 2.7°C), while warm rocks
release into the arena as either under warm rock, were on average 31.3°C (SD = + 0.73°C). Scorpion
under cool rock, or in the open. Scorpions were choice was recorded after 15 minutes from their
collected near the Steele/Burnand Anza-Borrego release into the arena as either under warm rock,
Desert Research Center. Error bars represent + 1 S.E.M. under cold rock, or in the open. Scorpions were
collected near the Steele/Burnand Anza-Borrego
Desert Research Center.
DISCUSSION a preference for warmer rocks. Because
larger scorpions were generally not in
Overall, shelter temperature played a role warmer places in nature, it seems that
in shelter selection of desert sand scorpions may not be limited in warm rocks.
scorpions, and this effect was strongest in However it may also be that a pattern of
trials with two competing scorpions. As one preference for warmth is difficult to detect
would expect for an ectotherm, scorpion in natural settings and is more clearly
body temperature was closely correlated observed in laboratory settings. This
with the surface temperature, since the difference in behavior between natural and
surface of the ground was a source of their laboratory settings has been documented in
warmth. From this correlation, we know other scorpion studies (Becker and Brown
that the temperature of the environment 2016). In competition trials, smaller
has a strong effect on scorpion scorpions were competitively excluded
temperature. Based on this correlation, we from the warmer rocks. This supports our
expected the scorpions to show preference hypothesis that bigger scorpions
depending on shelter temperature as seen competitively exclude smaller scorpions for
in previous research (Becker and Brown warmth. Scorpions may prefer warmer
2016). However, we observed weak locations to facilitate regulating metabolic
evidence that all desert sand scorpions had processes which accelerate digestion,
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CEC Research https://doi.org/10.21973/N3237M Fall 2019 5/7growth and development, and defensive Future studies could look into the
behaviors (Becker and Brown 2016). We intersection between intraspecific
found stronger preference for warmer rocks competition and predation in scorpions.
in competition trials than in competition- Furthermore, the effect of cannibalism on
free trials. Therefore, we believe that when populations could be explored through the
alone in an arena, scorpions may have been desert sand scorpion, and through
more concerned with escaping the cannibalistic species more broadly. Beyond
enclosure than choosing a shelter, but our focus on sheltering behavior,
when in a competitive or even predatory intraspecific competition and predation
environment, scorpions sought shelter and have huge roles in population dynamics,
were forced into a choice between warm since intraspecific competition is the main
and cool rocks. Based on these findings, we limiting factor on population regulation
believe that competition may be a stronger (Schoener 1973). From our evidence for
driving force for taking shelter than competitive exclusion in desert sand
warmth. This assumption mirrors findings in scorpions comes new insights on how these
other ectotherms such as rock-dwelling scorpions coexist and regulate their
geckos, who prioritize avoiding predators population size through competition. We
over thermal regulation (Downes 1998). observed competitive exclusion by larger
Scorpions often cannibalize and 8–22% of scorpions. Since larger species of scorpions
their diet can be based on cannibalism such as the giant desert hairy scorpion
(Polis 1987). We observed that when two (Hadrurus arizonensis) coexist with desert
scorpions with a size difference of 21.5 mm sand scorpions, we wonder if the larger
were paired in an arena, the smaller one species could exhibit similar patterns of
was eaten. Thus, we believe that there are exclusion on the smaller species. Such
not only competition dynamics at play, but competition between scorpion species
predatory ones too. In other words, could further affect scorpion population
scorpion sheltering behavior in the dynamics.
competition trials may be driven by a fear
of being eaten rather than by a desire to ACKNOWLEDGMENTS
acquire favorable shelter. Therefore, in
addition to competition, we also know that We wish to thank the Steele/Burnand
direct predation via cannibalism has an Anza-Borrego Desert Research Center
effect on sheltering behavior. If fear of doi:10.21973/N3Q94F for providing a site to
cannibalism were the lead driving force for conduct our research. Many thanks to Jim
taking shelter, we would expect to see Dice and Elaine Tulving for hosting us.
more small scorpions under rocks than Finally, we wish to thank Krikor Andonian,
large ones. However, we observed the same Tim Miller, and Reina Heinz for their
number of large and small scorpions (11:11) support and the California Ecology and
in the open in competition trials, so we Conservation program for the opportunity
conclude that temperature remains an to challenge ourselves to design and carry
important influence on scorpions’ choice to out our own research.
take shelter.
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