Thermoregulation activities in bumblebee (Bombus latreille) colonies subjected to different temperature environments
Scientific Writing, Maastricht Science Program, Maastricht, The Netherlands
27, May 2018
Bumblebees (Bombus latreille) are endothermic insects, meaning that they rely upon their ability to internally generate heat as a survival mechanism (Heinrich, 1974). Bumblebees live in colonies, in which they maintain not only the individual bee’s optimum internal temperature through thermoregulation (Britannica, T. E., 2013), but also regulate the ambient temperature needed to safely incubate the larvae of the colony in the brood nest (Weidenmuller, n.d.). The thermoregulation behaviors exhibited by subjecting eight Eurasian and American bumblebee colonies, were observed through research conducted to analyze the outcomes of subjecting the bumble colonies to climate-controlled observation chambers of 20°C and 32°C, with a constant maintained humidity level of 60%. Bumblebees use thermoregulation to generate heat when needed; by repeatedly contracting and relaxing their flight muscles, bumblebees are also conversely able to cool down the colony, by fanning their wings and circulating the surrounding air to lower the temperature in the brood nest, protecting the larvae (Hoyt, 1970), (Weidenmuller, n.d.). Eight bumblebee colonies were analyzed throughout a twenty-day observation period. Measurements of temperature and Carbon Dioxide (CO2) levels were retrieved four times per day, along with daily measurement averages, and recorded in a data set stored on a computer. The four bumblebee colonies introduced to the 20°C climate chamber (with a constant humidity level of 60%) utilized their thermoregulation ability to increase the temperature of the climate chamber through collectively contracting and relaxing their flight muscles, to emit metabolic heat, increasing the initial temperature of the 20°C climate chamber by an average of 9°C each day, effectively protecting the development of the larvae in the brood nest. Interestingly, the four bumblebee colonies in the 32°C climate chamber, performed the same thermoregulation activity, raising the temperature by an average of 2°C each day; providing possible indication that the bumble colonies considered the temperature conditions of the 32°C climate chamber to be too low on average. In the event of detection of dangerously low temperatures, the bumblebees will use thermoregulation, contracting and relaxing their flight muscles and emitting metabolic heat, as well as releasing CO2 as a byproduct of cellular respiration (Hoyt, 1970), (Weidenmuller, n.d.). Recorded CO2 concentration measurements were higher on average in the 20°C climate chamber in which the bumblebee colonies to had to use thermoregulation to increase the temperature to a greater extent than the colonies in the 32°C climate chamber. Correlating the increased thermoregulation activity in lower temperature (20°C) climate chamber, in which the bumblebees raised the temperature significantly, emitting higher levels of CO2 in comparison to the higher temperature 32°C climate chamber, in which less CO2 was emitted by the bumblebees through cellular respiration during thermoregulation.
Maintaining stable temperatures within the brood nest of a bumblebee (Bombus latreille) colony is vital to ensure normal developmental conditions and survival of the larvae of a bumblebee colony. For most species of bees, the average temperature within the brood nest must generally be maintained between 32°C and 35°C (American Bee Journal, 2016). If a significant decrease or increase in nest temperature occurs; below approximately 28°C or above 37°C it may endanger the brood. When bumblebee colonies detect threatening temperature levels, they collectively work to return the nest to optimum temperatures. If nest temperatures are above approximately 37°C, the bees fan their wings to circulate the air and cool the colony down. When temperatures below approximately 28°C are detected by the bees, they are able to generate heat by contracting and relaxing their flight muscles (Arnia, n.d.), (Weidenmuller, n.d.). The maintenance of an optimum temperature by an organism is referred to as thermoregulation (Britannica, T. E., 2013).
Bumblebee colonies (Bombus latreille) from locations in climate regions of America and Eurasia were analyzed in a recent experiment our team conducted to observe the thermoregulation behavior of bumblebee colonies associated with the bee’s defensive abilities of maintaining stable temperatures within the brood nests of their colony to ensure the survival of their larvae. Eight bumblebee colonies were examined after being subjected to specific temperatures. Four colonies each containing bee larvae were exposed to 20°C and the other four colonies also containing bee larvae were exposed to 32°C; both in temperature and light controlled observation boxes. The conducted research analyzed the behavioral responses that the colonies exhibited after exposure to changes in temperature. Measurements of temperature and CO2 levels were recorded four times daily with at least thirty-minute intervals, for a period of twenty days and the averages of each day were recorded in a computer for further analysis. Bumblebees pollinate many of the flowers that provide crops and food for much of the planet. The pollination activities of bumblebees are therefore essential to maintaining the stability within the life cycle on Earth and ultimately the survival of humanity, alarmingly bumblebee populations are declining and the rise of global temperatures relating to climate change may pose an existential threat to the survival of bumblebees if temperatures become too extreme for the development of bumblebee larvae, which may also force bumblebees to adapt to extreme thermoregulation (Mika, 2017). The thermoregulation behavior of bumblebees is necessary to ensure the survival of their brood. It is vital to continue research on thermoregulation in bees and how the bees may be affected by consequences of human activity, such as climate change in order to further understanding and mitigate declining bumblebee populations.
Materials and Methods
The thermoregulation behavior of eight bumblebee (Bombus latreille) colonies which are native to regions in America and Eurasia were analyzed by researchers conducting observations over a twenty-day evaluation period. Ten worker bees and ten pupae were inspected, and individual bees from each colony were numbered and color coordinated, respective to the climate chamber that they were introduced to. Each colony contained a brood nest for the purpose of analyzing the colonies behavioral defenses under different varying temperature environments. the colonies were examined through small wooden observation boxes (20x20x10cm) with Plexiglas covers. Four climate chambers were temperature regulated to 20°C and the other four climate chambers were set to a temperature of 32°C. The experiments were conducted under precise conditions of red light exposure and steadily maintained humidity of 60% in order to avoid uncontrolled external variables. Temperature and CO2 level measurements of each observation box were taken four times per day with intervals of at least 30 minutes in order to increase the accuracy of the experimental data, which was subsequently stored and recorded by a computer. Temperature measurements were taken using a thermocouple reader (Stanford Research System model SR 630). This particular experiment required the use of 4 copper-constantan thermocouples (ø wire 0,2 mm). CO2 measurement reading were taken using a Godart Statham CO2 concentration meter (type 170703772559, nr. 2630).
Results and discussion
Thorough evaluation of the data acquired throughout the duration of this experiment appears to have revealed compelling insights in relation to the mechanisms involved in maintaining the survival of bumblebee offspring through thermoregulation. Of the eight bumblebee colonies examined, the four colonies which were exposed to 20°C (lower than optimal temperatures for the bees) exhibited physiological responses attributed to addressing the conditions threatening the larvae within the brood nest of the colony. Measurements of temperature were taken four times daily for twenty days, the averages of which can be viewed in the first scatter plot below (Figure 1).
Figure 1: Average Daily Temperatures (20°C) Figure 2: Average Daily Temperatures (32°C)
Note: Retrieved from Data Sets: Bumblebee Solutions.xlsx, Provided by Maastricht Science Program
The resulting temperature averages depict the thermoregulatory performance of the bumblebee’s in which the contracting and relaxing of the bee’s flight muscles and the resulting vibration and metabolic heat emittance, increased the temperature of the environment when responding to the threat facing the larvae within the brood nest. The temperature was increased on average by nearly 9°C daily. Although previous research has indicated that bumblebees have the thermoregulation ability to also decrease dangerously high temperatures threatening the survival of brood nest. The thermoregulation activity relating to the decrease in temperature involves the colony collectively “fanning” their wings to circulate the air, cooling down the surrounding environment (American Bee Journal, 2016), (Arnia, n.d.). However, the average daily temperature recorded from the four colonies which were exposed to 32°C, made it quickly evident throughout the experiment that the bees considered the 32°C to be slightly too cool on average. Figure 2 provides these average temperatures, indicating the increase in daily temperature. A mean temperature of 33°C shown in Figure 2 and 28,4°C in Figure 1 demonstrates that the thermoregulatory activity was shown to be higher in the bumblebee colonies exposed to a lower temperature (20°C). Recorded measurements of CO2 concentration (%) suggests a reduction in the CO2 percentage present in the environment of the colony in the climate chamber which was exposed to the higher temperature (32°C). The reverse result was additionally evident, as an average of 0,1975125% CO2 concentration was recorded in the 20°C climate chamber and an average of 0,0749125% CO2 concentration recorded in the 32°C climate chamber. Interpretation of the resulting data of the research conducted over the twenty-day observation period correlates to previous findings of optimal temperature range for maintaining the survival of the brood of bumblebee colonies (Arnia, n.d.), and was found to be between 27°C and 34°C on average (with a constant humidity level of 60%). Bumblebee’s being partially endothermic, detect lower than optimal temperatures, using thermoregulation to return to stable internal temperatures (Heinrich, 1974). If faced with colder temperatures the bumblebees will continuously contract and relax their flight muscles, emitting metabolic heat, as well as releasing CO2 in the process as a byproduct of cellular respiration (Weidenmuller, n.d.). Recorded percentages of CO2 concentration correlated to the lower temperature (20°C) climate chamber; as CO2 concentration was higher on average in the climate chamber which required the bumblebee colonies using thermoregulation to increase the temperature (Hoyt, 1970), (Weidenmuller, n.d.).
The resulting data from this experiment provides strong indications of the link towards higher thermoregulation activities, in which the bumblebee emitted heat to raise the temperature, and higher concentrations of CO2 as a byproduct of cellular respiration. Concentrations of CO2 recorded in the 20°C climate chamber, were higher on average than that of the 32°C climate chamber, providing an empirical correlation. The average temperature measurement data of each climate chamber demonstrates the thermoregulation activity of the eight bumblebee colonies over the duration of the twenty-day observation period, indicating that the bumblebee colonies maintained the temperature of their respective climate chambers (20°C and 32°C) between 27°C and 34°C on average (with a constant humidity level of 60%). Although the results of this research are highly informative, additional experimentation is necessary in order to further analyze how bumblebee colony behavior and thermoregulation activity are affected in response to different ranges in temperature than were examined in this. Additionally, further studies in the field should be considered, as surveying bumblebee colonies in natural conditions may prove to be valuable to analyze alongside presently gathered data. Furthering research into this topic will be increasingly important to tackling the challenges of mitigating the treats facing the survival of bumblebees as global temperatures rise and in developing greater understanding of the roles bumblebees play in many aspects of life cycles on Earth.
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Data Sets retrieved on April 17, 2018, Bumblebee Solutions.xlsx, Provided by Maastricht Science Program