Rolling by Armadillidium vulgare and Predation.
Introduction
Armadillidium vulgare is a terrestrial isopod with
the capability of rolling into a ball. The purpose of
this study was to determine what effect the rolling
behavior has on predation by a scratching animal.
Previous studies have linked the rolling behavior
of Armadillidium vulgare to various stimuli. Saxena (1957)
was able to induce the behavior by using a hard tactile
stimulus. Salamanders have been observed eating the
isopods by Miller(1938). He reported that large individ-
uals were able to escape predation by rolling into a
ball which the salamanders were unable to swallow.
Waloff (1941) suggested that the behavior may be a re¬
sponse to suddenly changing microclimate conditions.
Observations on Armadillidium vulgare
A set of preliminary studies were conducted to pro¬
vide qualitative insights into possible factors rela-
tingto the rolling behavior.
Humidity
To determine if low humidity conditions stimulate
rolling, isopods were put in a Stender dish set on
Dri-Rite desiccator in a 250 ml covered beaker. None
of the isopods rolled in this 0% relative humidity
condition, supporting the findings of Waloff (1941).
edation
Temperature
To determine if constant, extreme temperatures
stimulate rolling, isopods were placed in three covered
finger bowls with moist paper towels on the bottom.
One was kept at 0°C, one at about 25°C, and the last
at 35°0. At the time of death of the isopods in the
low and high temperatures, no rolling had been observed.
No rolling was seen in the animals at room temperature.
Touch
Tactile stimuli were observed to immediately
initiate the rolling behavior, supporting the find-
ings of Saxena(1957). Touching them with any hard
object, shaking them in a container or in the hand,
or dropping them onto a hard surface all produced roll¬
ing.
Optimal Environmental Conditions
In collecting Armadillidium vulgare from under
iceplant above Agassiz Beach at Hopkins, it was
noticed that the isopods were usually rolled after
the iceplant was lifted up. This observation led to a
hypothesis that the rolled position is the normal rest-
ing state of the isopod when environmental conditions
are the most favorable for survival. Warburg (1964)
and Waloff (1941) both found that the activity of
Rolling by Armadillidium vulgare and Predation
Armadillidium is at a minimum at 100% relative humidity.
The habitat under the iceplant was very moist, probably
reaching 100% relative humidity among the dead leaves
where the isopods were found. The rolled position of
the isopod is an inactive state. Therefore, the hypo¬
thesis that rolling is an inactive state in response
to a very humid, favorable environment seemed reasonable.
An experiment was set up to investigate this. 250 ml
beakers were filled with loose, moist dirt, some deep
enough to allow the isopods to bury, and some with only
a thin layer of dirt. The hypothesis was that animals
that were able to bury themselves in the moist dirt.
where desiccation stress is absent, would be in optimal
conditions and would roll up. An attempt was made to
instantly kill the isopods while still in position in
the dirt by quick-freezing the beakers in an alcohol-
dry ice bath. This failed, but by carefully picking
through the dirt with forceps it was possible to uncover
many isopods without disturbing them. No formal data
was taken due to uncertainty about the accuracy of the
digging method. However, those isopods uncovered with¬
out disturbance were not rolled. With this suggestion
that the rolled position is not the normal, resting
position of Armadillidium vulgare, it was possible to
concentrate on a second hypothisis, the one that became
the working hypothesis for this study.
Rolling by Armadillidi
and Predation
Hypothesis
Since the isopods were probably not usually
in the rolled position, the fact that they were
observed rolled under the iceplant suggested that
lifting the iceplant caused the behavior. In order
to lift the iceplant, roots had to be pulled up also,
which disturbed the soil and dead leaves the isopods
live in. The hypothesis that led to this study was
that a scratching predator, like a bird, could cause
the same kind of disturbance of the soil and organic
litter that uprooting the iceplant did. This dis-
turbance could produce a tactile stimulus for roll¬
ing, or the change in microclimate conditions which
has been suggested as a possible stimulus.
Observations of Chickens,
Scratching Predators, Feeding
on Armadillidium vulgare
Dr. Robin Burnett, in personal communication,
reported observing chickens eating isopods, using a
scratching behavior to find them in leaf litter.
Detailed observations were subsequently made of these
chickens feeding on isopods in an area covered about
an inch deep with dry oak leaves. The scratching
behavior of a chicken, in feeding on animals in litter,
Rolling by Armadillidium vulgare and Predation
consists of two scrapes with each leg, throwing debris
diagonally back and away from the chicken. Then the
chicken quickly jumps straight back scraping a large
amount of debris under its feet, held close together
under its body. The chicken then closely examines the
uncovered patch of dirt and picks up food using quick
stabs with its beak. The inspection of a cleared patch
rarely lasts longer than ten seconds. Isopods were
frequently uncovered and eaten. Armadillidium vulgare
were present in the area in very small numbers, the
dominant species being Porcellio scaber. Armadillidium
were placed in the area so that observations could be
made more easily on their response to the chickens
feeding behavior. When Armadillidium were uncovered by
a chicken's scratch, they often appeared on the bare
ground in the rolled position. They also appeared in
an unrolled state, crawling across the dirt, or lying
on their dorsal surfaces with their legs moving rapidly.
These active isopods were usually eaten immediately by
the chickens. Rolled isopods were also eaten, but
usually after the active ones, and less frequently.
In one case, a chicken uncovered an isopod, which rolled
up. After passing it over, the chicken moved forward
and began to scratch in a new area. Just as the isopod
Rolling by Armadillidium vulgare and Predation
was unrolling it was hit by leaves the chicken had
scratched back from the new area. The isopod immedi¬
ately rolled up again. It eventually unrolled and
moved quickly under the layer of oak leaves.
The Effect of Rolling on Predation:
Materials and Methods
Chickens were therefore used as the scratching
predators for the experimental part of this study.
They are docile and easy to observe and eat isopods
readily. An experiment was conducted to determine
quantitatively what effect the rolled position of
Armadillidium vulgare had on predation by the chickens.
A number of isopods were glued, while in the rolled
position, with a drop of Eastman Kodak 911 glue placed
where the head and uropods join. When the animals were
prevented from opening until after the glue had set,
they were effectively forced to remain rolled. Twenty
of the rolled isopods, and twenty normal isopods were
mixed together and scattered on a patch of bare dirt
near a chicken. One hen was used for all the trials.
After the chicken had eaten about half the isopods, the
remaining ones were collected and the animals of each
type not eaten were counted. Another test of the same
kind, using unrolled animals, half with a spot of glue
on their dorsal surfaces, had shown that the presence of
glue on the isopods produced no significant effect on
Rolling by Armadillidium vulgare and Predation
the feeding. A similar experiment was performed using
ten normal isopods and ten dead, unrolled animals per trial
to detenmine theeffect that simple movement had on preda¬
tion by a chicken. Again, the feeding was stopped when
about half the isopods had been eaten.
Results
The results of the first experiment appear in Table 1.
The results of the second experiment are in Table 2. In
both experiments the unrolled, active animals were eaten
first. In many cases, a moving isopod was neatly plucked
from among several rolled or dead ones that were never
eaten.
Discussion
From these experiments it appears that a moving
isopod draws the attention of a feeding chicken more
than a motionless one. When given a choice, a chicken
eats the moving isopods first. Saxena found the mean
length of time of rolling after a tactile stimulus to
be about ten seconds. If Armadillidium uncovered by
a chicken remained rolled for this length of time,
they would stand a good chance of not being eaten,
as the chickens rarely examined an uncovered area
longer than ten seconds. An isopod which does not
roll when uncovered is very likely to be eaten, as
Armadillidium in the unrolled state is almost always
moving, either crawling, or if on it's dorsal surface.
waving its legs. A rolled isopod could also be pro-
Rolling by Armadillidium vulgare and Predation
pelled away from a chicken when the predator scratches,
due to the sphere shape which rolls well on flat surfaces.
Exactly what causes the isopods to roll when un-
covered by the scratching is unknown. Turning over
a rock under which isopods are living can also cause
rolling. The stimulus could be tactile, changing light
intensity or a sudden change in microhabitat conditions
such as humidity, or temperature. The same stimuli
could elicit the response when a chicken scrapes away
a layer of leaves looking for food.
The extension of the results of this study to con-
clusions about the adaptive significance of rolling to
Armadillidium vulgare are not possible. However, the
behavior does appear to be a successful defense against
some types of predation. Evolutionarily, whether the
behavior was a direct adaptive response to predation
or a response to some other pressure, which had the
effect of limiting some types of predation is a matter
of conjecture. Low humidity, probably the most lethal
pressure the isopods face, has been shown not to be a
stimulus for the behavior. (Waloff) Rather, the isopods
become more active in low humidities. (Waloff, Warburg 1964)
The assumption by Heeley (1941) that rolled isopods were
conserving moisture in the respiratory pleopods in con¬
ditions of low humidity is made even more tenuous by
actual water loss data. Edney (1951) found that more
Rolling by
llidium vulgare and Predation
Arm
than 50% of the water lost by Armadillidium vulgare in
low humidity is through the dorsal surface. This sug¬
gests that were the isopod to roll in low humidity, water
conservation would be minimal, considering the extra
dorsal surface that is exposed when the animal rolls.
Paris (1963) reported that drowning is a major cause of
winter deaths, and this environmental pressure could
have been a factor in the evolution of the rolling be¬
havior.
Summary
1. Scratching by chickens in feeding causes Armadillidium
vulgare to roll up.
2. Chickens eat Armadillidium that are active more than
rolled or dead ones.
Acknowledgement
I wish to thank Dr. Malvern Gilmartin for his help
throughout this project and in preparing the manuscript.
Special thanks go to Dr. Robin Burnett, gentleman farmer,
for the use of his chickens, without which this project
would not have been possible.
Rollir
llidium vulgare and Predation
Literature Cited
Edney, E.B. 1951. Evaporation of Water from Wood-
lice and Glomeris. J. Exp. Biol. 28(1):91-115.
Heeley, William. 1941. Observations on the Life-
Histories of Some Terrestrial Isopods. Proc.
Zool. Soc. Lond. Ser. B. 111(½):79-148.
Miller, Milton A. 1938. Comparative Ecological
Studies on the Terrestrial Isopod Crustacea of
the San Francisco Bay Region. Univ. Calif. Pubs.
Zool. 43(7):113-142.
Paris, Oscar H. 1963. The Ecology of Armadillidium
vulgare in California Grassland: Food, Enemies,
and Weather. Ecol. Monogr. 33(1): 1-22.
Saxena, S.C. 1957. An Experimental Study on Thanatosis
in Armadillidium vulgare. J. Zool. Soc. India.
9(2): 192-199.
Waloff, Nadejda. 1941. The Mechanisms of Humidity
Reactions in Terrestrial Isopods. J. Exp. Biol.
18(2):115-135.
Warburg, Michael R. 1964. The Response of Isopods
Towards Temperature, Humidity and Light. Animal
Behavior 12(1): 175-186.
10
Rolling by Armadillidium vulgare and Predation
TABLE
ROLLED
IROLLET
1
16
18
14
16
5
11
N-20
.001
ABLE
TRIA
NEATEN
IEATE

N-10
01
11