Behavioral Crypsis in Armadillium vulgare
Behavioral Crypsis in Armadillium vulgare
TAYLOR BROWN
Department of Biological Sciences, Illinois State University, Normal, Il 61761-4160
Word Count: 1,961
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The aim of this research was to determine whether or not pill bugs, Armadillium vulgare,
actively choose cryptic backgrounds as an anti-predator behavior. Two experiments were performed in
this study. The first tested cryptic coloration which is the ability to blend in with the color of the
background. The second experiment tested cryptic texture which is the ability to have camouflage that
resembles the terrain of the habitat. It was found that A. vulgare do not prefer any specific color or
texture in a background. The results showed trends in the data for both experiments. However, neither
experiment had significant differences between the two variables being tested. A. vulgare do not have a
method of crypsis currently.
Keywords: cryptic coloration, cryptic texture, crypsis, Armadillium vulgare, point sampling
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Armadillium vulgare, better known as the pill bug or the roly-poly, has an armored body made
up of plated pieces which covers its back. A. vulgare has a fairly cylindrical body and has the ability to
roll itself into a ball in order to provide protection against predators (Davis et al., 1977). Pill bugs require
moisture so when the surface is too dry to support them, they have another ability to burrow into the
soil to find a more suitable microclimate condition (Davis et al., 1977). Compared to similar species
however, A. vulgare has a relatively higher tolerance to desiccation (Waloff, 1941). Pill bugs are found,
typically, in and around dirt which provides better cryptic coloration for them. Crypsis is another name
for camouflage, or the ability to blend in with the environment which functions to decrease the risk of
predation (Merilaita, 1998). By better understanding the adaptive significance of crypsis in regards to
behavior, we will be more able to fully understand why the ability of crypsis has evolved and the fitness
benefits it can provide. The use of crypsis has long been of interest to many biologists, hunters, and
even our military (Behrens 2009).
Our study was done to provide a better understanding of whether or not pill bugs truly prefer a
cryptic background over one that makes them stand out. This was performed in two different types of
study: background color and background texture. Experiment one tested the cryptic coloration of pill
bugs. The hypothesis was that a prey species will prefer to live on a land color which resembles their
body color in order to decrease the risk of being preyed upon by a predator. By blending in with the
background color, a pill bug could free-roam within that color, but it would be restricted to stay within
that area or risk standing out to predators. This could allow a pill bug to be missed by a predator
scanning for food and therefore continue to reproduce and increase its fitness (Reynolds, 2011).
Experiment two tested if pill bugs preferred a smooth textured background or a rougher, less smooth
background. The hypothesis for experiment two was that a prey species will prefer to live on a land
color which has a texture that resembles their body color, shape, and size in order to decrease the risk
of being preyed upon by a predator. By camouflaging itself with background texture, a pill bug may be
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better at hiding itself from a predator regardless of the ground color. In most land areas, the ground
color can change from season to season which would put the cryptic texture pill bugs at an advantage
over cryptic coloration pill bugs. With the ground color changing, an individual pill bug might not be
able to survive for long periods of time if it is not adapted to the texture of its habitat, thus decreasing
its fitness (Merilaita, 1998).
To test the cryptic coloration hypothesis, pill bugs were placed in a container with two different
color options on the ground for it to choose from. Point sampling was used to record what color the pill
bug was located on every two minutes. The cryptic texture hypothesis was tested by placing pill bugs in
a container with two different texture options for it to choose. Point sampling every two minutes was
also used here to record the texture type the pill bug was located on.
The prediction for the cryptic coloration hypothesis is that pill bugs will choose the brown
colored ground over the white colored ground as it provides a better form of crypsis. The cryptic
texture hypothesis had the prediction that pill bugs would prefer the textured ground over the smooth
ground since it provided better crypsis as well.
Methods
This study was performed on October 18, 2012 in a classroom within the Felmley Hall of Science
Annex at Illinois State University in Normal, Illinois. The pill bugs, A. vulgare, were collected from
natural habitats in Bloomington, Illinois in September 2012. They were maintained in captivity for one
month in a large plastic tub with an adequate amount of soil. The soil was comprised of soil substrate
and decomposing bark mulch. Pill bugs were kept in a laboratory room where they received
unrestricted sunlight from an uncovered window. Their diet was made of lettuce leaves and eggplant
slices. To sustain a humid environment, the living area was misted with distilled water as needed.
Experimental containers were plastic and measured 10x10x9 cm.
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For experiment one, brown absorbent paper covered the entire bottom of the container. A
piece of white absorbent paper was placed on top of the brown paper, covering exactly half of the
bottom. Water was added to completely soak the paper, but not have any flooding. One pill bug was
placed in the container and left alone for four minutes. Once four minutes was up, point sampling was
used to record the location (white or brown) that the pill bug was on. Recordings took place every two
minutes for forty minutes. Three trials were run simultaneously and six trials were run altogether.
Experiment two began by laying brown absorbent paper on the bottom of the container so it
completely covered it. Enough water was added to soak the paper, but so no extra water was lying on
top. To exactly one half of the bottom, black aquarium gravel, total amount of about the size of a U.S.
quarter, was evenly spaced. One pill bug was placed inside the container and allowed to become
accustom to its new surroundings for four minutes. After four minutes, point sampling recorded the
location (gravel or no gravel) every two minutes. This lasted for forty minutes. Three trials were run at
the same time and six total trials were run.
A chi-squared goodness of fit test was used to calculate the statistical significances between the
two different treatments in both experiments. To determine significance, α=0.05 was used. Experiment
one tested between white and brown colored backgrounds. This was the cryptic coloration test.
Experiment two tested between gravel and no gravel backgrounds. This tested the cryptic texture test.
Results
In the cryptic coloration experiment, there was no significance found for a preference of the
white or brown background. It was calculated that with 628 degrees of freedom, X2628= 7.340 with
p>0.05. Table 1 shows the statistical data for cryptic coloration. Figure 1 shows the average amounts
and the standard errors for the two different color treatments.
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In the cryptic texture experiment, again no significance was found between the choices of a
textured background and a non-textured background. With 628 degrees of freedom, the calculations
were as follows: X2628=3.657 and p>0.05. Table 1 shows the statistical data for cryptic texture. Figure 2
is a graph of the averages and the standard errors for the two different textured treatments.
Discussion
In experiment one, even though Figure 1 shows that the pill bugs had a preference for the white
background, the statistical test shows that the differences between the two averages are not significant.
The hypothesis for the cryptic coloration experiment was that a prey species will prefer to live on a land
color which resembles their body color in order to decrease the risk of being preyed upon by a predator.
The prediction was that A. vulgare would choose the brown background as it would provide more
cryptic coloration, thus better protecting itself from predators. Since the results had no significance, we
might as well say that pill bugs do not have a preference for any color background. The hypothesis and
prediction were not supported by the data.
In the texture experiment, Figure 2 shows a slight preference for the graveled texture, but again
the statistical test showed no significances in the differences between the averages. The hypothesis was
that a prey species will prefer to live on a land color which has a texture that resembles their body color,
shape, and size in order to decrease the risk of being preyed upon by a predator. The prediction was
that the pill bugs would prefer the graveled background since the aquarium gravel was roughly the same
color as the bugs and even close to the same size. This would most likely provide them with better
protection against predators. The data collected had no significance between the two different
textures, failing to support both the hypothesis and the prediction.
Overall, Armadillium vulgare do not seem to have any preference for cryptic coloration or
cryptic texture. They did not prefer to blend in with the brown background and did not prefer to blend
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in with the gravel. Perhaps they did not prefer any form of crypsis due to being in captivity. More
experimentation needs to be performed in the field with A. vulgare. Of the two different experiments,
the cryptic texture would seem to have more adaptive significance. If the texture of the habitat has
gravel, or any form of texture, that is the same relative size and color of the pill bug, the ground color
should not make a difference as they can blend in with the texture. Very little research has been
performed on crypsis in Armadillium vulgare and more is needed to provide better results. Merilaita
(1998) found that in another species of isopod, Idotea baltica, it favored crypsis via disruptive coloration
over close background matching.
Perhaps the biggest factor influencing the results was that the pill bugs were taken from their
natural habitat and maintained in captivity. Even though they were fed with enough food, being in
captivity can alter an organism’s behavior. By doing this experiment large scale and by the addition of a
predator, maybe the pill bugs would show a significant preference for a cryptic background like color,
texture, or possibly both. This experiment was run with three trials at a time. By only doing one at a
time, a more accurate assessment could be made for exactly where the pill bug is located when the time
comes to record its location.
Future experiments dealing with crypsis in Armadillium vulgare is essentially limitless. A very
little amount of research has been performed to date and more data needs to be gathered to better
understand the anti-predator behavior of crypsis. The results from this study yielded insignificant
results which show no specific preference for crypsis in pill bugs. With more data and different
experiments, maybe a definite answer could be made regarding cryptic behavior in pill bugs.
This study is important because it shows that pill bugs lack the ability to actively choose cryptic
environments to protect themselves from predators. Crypsis is one adaptive strategy many organisms
use as an anti-predatory behavior. It likely evolved from a process known as the coevolutionary arms
race. Since many predators use vision to locate their prey, prey will have a survival advantage if they
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evolve a form of cryptic camouflage and, as a result, are harder to see (Reynolds, 2011). If pill bugs fail
to perform this behavior, maybe they have another yet undocumented anti-predator behavior.
References
Behrens, R.R. (2009). Revisiting Abbott Thayer: non-scientific reflections about camouflage in art, war
and zoology. Philos. Trans. R. Soc. Lond. B Biol. Sci. 354: 497-501.
Davis R.C., Hassall M, & Sutton S.L. (1977). The vertical distribution of isopods and diplopods in a dune
grassland. Pedobiologia 17: 320–329.
Merilaita, S. (1998). Crypsis through disruptive coloration in an isopod. Proceedings: Biological sciences,
265(1401), 1059-1064. Retrieved from http://www.jstor.org/stable/pdfplus/50884
Reynolds, C. (2011). Interactive evolution of camouflage. Artificial life, 17, 123-136.
Waloff, N. (1941). The mechanisms of humidity reactions of terrestrial isopods. Unpublished manuscript,
Retrieved from http://jeb.biologists.org/content/18/2/115.full.pdf
Tables
Table 1: Statistical Analysis of Both Experiments
Experiment
Cryptic Coloration
Cryptic Texture
Trial Type
Brown
White
Gravel
No Gravel
Total
281
349
339
291
Chi-squared Values
p-value
7.34
1
3.657
1
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Figures
Figure 1: Cryptic Coloration Background Preference in Armadillium vulgare
Figure 2: Cryptic Texture Background Preference in Armadillium vulgare