Conglobation of Armadillidium vulgare as a Behavioral Adaptation for Thermoregulation
by Adele Bloodworth
Conglobation is an adaptive, behavioral mechanism acquired by the Armadillidium vulgare (pill bug) used to prevent desiccation and avoid predation. It is not known, however, whether this behavior is also a thermoregulation mechanism in pill bugs. Our objective was to determine if a relationship exists between temperature and conglobation of the A. vulgare.We measured the number of conglobations every two minutes in three different temperature conditions: 6 °C, 14-16 °C, and 21 °C. The results of our experiment indicate a relationship between ambient air temperature and conglobation behaviors; which support the hypothesis that conglobation is indeed an adaptive mechanism of thermoregulation behavior. Our data provide an important insight to understanding the behavioral and physiological adaptations of isopods to ambient physicochemical conditions.
KEY WORDS: Conglobation, Armadillidium Vulgare, Temperature Regulation, Adaptive Mechanism, Isopod
The Armadillidium vulgare (pill bug) is a widely studied crustacean. These terrestrial arthropods are relatively poorly adapted to land as compared to other species. They face many challenges on terrestrial land including temperature regulation, predation avoidance, desiccation, nutrient acquisition, and locomotion. They are confined to certain habitats with moderate temperatures and damp soil. These species often reside under rotting wood and leaves where ambient moisture levels are high. They are generally active at night when the humidity increases and temperatures fall1.
One defense mechanism the pill bugs have acquired is conglobation which is the act of rolling into a ball2. Conglobation is not only a mechanism to protect the pill bugs from predation, but it also prevents desiccation. Evaporation from the pill bugs’ respiratory organs is a critical physiological factor in pill bug survival. Conglobation is one of the mechanisms used by these terrestrial isopods to avoid desiccation1.
Pill bugs have been proposed as viable, promising models for research due to the reliability of inducing repeated behavioral patterns3. Many studies relate conglobation and desiccation of the pill bug; however, there is little research investigating conglobulation as thermoregulation mechanism.
Typically, pill bugs can tolerate a temperature range of -2 °C and 36 °C4. Thermal preferendum of the species is 22 °C to 23 °C. It is known that these isopods cannot survive in extreme temperatures and prefer cooler, more moderate temperatures. In higher temperatures, individuals of this species huddle together in order to prevent water loss4. However, it is not known whether these isopods roll up as an adaptive mechanism to conserve body temperature in colder temperatures. We hypothesize that the pill bugs will roll up as an adaptive mechanism in order to conserve body temperature in cold environments.
In order to test the hypothesis that pill bugs use conglobation as an adaptive mechanism to conserve body temperature, we conducted an experiment in which we measured the number of conglobations of pill bugs in different temperature conditions. We did this by exposing pill bugs to different temperature environments.
A room temperature treatment severed as a control group in all trials. The temperatures were chosen as treatments based on pill bug temperature tolerance ranges (-2 °C and 36 °C)4. We used an ice bath to cool one petri dish to about 6 °C. We used a cold-water bath with less ice to cool a different petri dish to 14-16 °C. We then had the control group at room temperature, 21 °C.
We had three groups of seven pill bugs each and rotated each group to ensure each sample was exposed to each set of environmental conditions and studied in each temperature range. After the first trial was finished, the petri dishes with the pill bugs were removed from their altered environments and the pill bugs were allowed to rest at room temperature for 5 minutes. This allowed them to return to their normal behavior and temperature.
We measured the number of conglobations, by monitoring the pill bugs throughout the 14-minute experiment and recorded the number of pill bugs that were rolled up every two minutes. The experiment was repeated three times.
There were confounding variables we had to mitigate in our experiment. One major variable was the humidity of the petri dishes. We worked to ensure that the humidity stayed in a range of 50-60%. The room temperature was about 54% humidity while the cold was less- staying around 52%. We used humidity pens to measure this data. In a trial run using no pill bugs we cooled the petri dishes and measured the humidity of each. We attempted to combat the difference of humidity by using Saran wrap over the petri dishes; however, when we tested the humidity of each temperature environment, it did not make a significant difference. Another confounding variable was light intensity. We mitigated this by ensuring that all of the environments were under the same amount of light.
The results of our experiment support the hypothesis that conglobation is an adaptive thermoregulation mechanism. Conglobation significantly increased as the temperature of their environment decreased. When we placed the pill bugs in the coldest temperature, they immediately started to move significantly slower. They rapidly exhibited conglobation after being placed in the cold temperature and often did not unroll until exposed to room temperature. The pill bugs at room temperature were very active and did not exhibit conglobation behaviors. The pill bugs in the moderately cold temperature moved slightly slower and rarely exhibited conglobation behaviors. There was very minimal variability in our data across trials indicated by the small standard deviation of each trail (Figure 1). The calculated 95% confidence intervals of each temperature are: 6°C – 14-16°C: (2.19, 2.95), 6°C – 21°C: (2.66, 3.49), 14-16°C – 21°C: (0.0917, 0.9178). Each 95% confidence interval does not contain 0, indicating our p value for each group is less than .05. Therefore, we can conclude that there is a significant difference in the average number of conglobations per minute between the three groups.
Figure 1. The number of conglobations of pill bugs was recorded every two minutes for varying temperatures. There were 7 pill bugs per trial, repeated three times, for a total of 21 pill bug per treatment condition (N=21) from which the average conglobation rates were calculated. This was repeated 3 times with each group of pill bugs experiencing all three temperatures (N=3). This graph shows the average number of conglobations of the pill bugs per minute in each of the temperature environments. The three temperature ranges were 6 °C, 14-16 °C, and 21 °C. There were more pill bugs conglobated in the coldest environment as compared to the other temperatures. SD of 6°C group= .325, SD of 14-16°C group= .417, SD of 21°C group= .125. The calculated 95% confidence intervals of each temperature are: 6°C – 14-16°C: (2.19, 2.95) *, 6°C – 21°C: (2.66, 3.49) *, 14-16°C – 21°C: (0.0917, 0.9178) *. The asterisk indicates a significant p value less than .05.
Our hypothesis that conglobation is related to pill bug thermoregulation was supported by our results. Pill bugs have been found to show behavioral plasticity that is responsive to changes in the environment3. In addition to the known conglobation response behaviors of pill bugs to desiccation and predation avoidance, our data indicated that this behavior is also used in thermoregulation at lower temperatures. It is evident in our experiment that the pill bugs responded to the colder environment by rolling up and moving very slowly, if at all (Figure 1). This was not observed in response to mildly cold or room temperature environments (Figure 1). Conglobation appears to be a behavioral mechanism pill bugs use at temperatures at the low end of their temperature range to conserve internal temperature.
There could have been other interpretations of our hypothesis. Water conservation is a major issue for pill bugs. Conglobation is known to drastically reduce the desiccation of pill bugs1. It is possible that the pill bugs were attempting to conserve water when rolling up in our experiment. However, the humidity of each petri dish was monitored throughout the experiment. There was not a significant difference of humidity across the three environments, so it is not probable that desiccation was increased in one environment leading to skewed results. Also, the less familiar the environment is for the pill bug, the harder it is to predict their behavior3. Laboratory conditions are unfamiliar to these pill bugs; therefore, their behavior may deviate from normal. This could be a potential source of error for our experiment. However, this would affect the pill bugs in each of the three environments, so if it was a defining factor, we would not have seen such clear results.
We found that the pill bugs conglobate in response to the colder temperature to maintain their body temperature. This study is important as it is essential to understand the adaptations of animals and their mechanisms for maintaining homeostasis. The physiological and behavioral adaptations are important for scientists to understand so they can better understand how species evolve and are related to one another. This evolutionary knowledge helps scientists to better understand the relationships among species and their environments. The adaptations of species can better help scientists understand the environments they inhibit and why certain species chose to live in certain places. Global temperature is variable, and it is important to understand how species may react to a change of temperature within their environment. Knowing how pill bugs react to a change in temperature can help scientists predict the effects of global temperature change will have on pill bugs and other, related, species.
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- Raham, G. Pill Bug Biology: A Spider’s Spinach, but a Biologist’s Delight. The American Biology Teacher 48, 9 (1986).
- Horváth, G., Garamszegi, L. Z., Bereczki, J., Urszán, T. J., Balázs, G., & Herczeg, G. Roll with the fear: environment and state dependence of pill bug (Armadillidium vulgare) personalities. The Science of Nature 106, 7 (2019).
- Refinetti, R. Behavioral temperature regulation in the pill bug, Armadillidium vulgare (Isopoda). Crustaceana 29-43 (1984).
Acknowledgements: I would like to sincerely thank my Biology 1108 GLA, Aaron Lenihan, for encouraging me to submit my research paper to The Classic Journal. I would also like to thank the editors of The Classic for guiding me through the revision and publication process.