Stress affects every body differently: How bumblebees individually tackle cold stress
- Katie-Lee Browning
- Mar 22, 2023
- 3 min read
By: Katie-Lee Browning
For many insects, prolonged cold exposure leads to a state of paralysis known as “chill coma” which occurs when an ion imbalance disrupts the central nervous system. The time it takes for insects to recover from the chill coma upon re-exposure to warm temperatures is known as the chill coma recovery time (CCRT) and reflects the amount of time it takes for the insect to reestablish ion balance in the body. Interestingly, the temperature at which insects enter a chill coma and the CCRT can vary considerably within and between species. In a recent study, Oyen et al. (2021) used bumblebees (Bombus vosnesenskii) in an effort to understand which environmental and physiological factors explain variation in CCRT. The researchers chose bumblebees as their focus species because they are widely distributed across North America and one of the species that has demonstrated local temperature adaption between populations. The team of researchers hypothesized that variation in climate experienced by these populations would be matched by variation in CCRT. In other words, Oyen and her team expected that shorter CCRTs would be observed in bumblebees located at colder climates. Because male and female bumblebees exhibit vastly different physiologies, the researchers also hypothesized that sex contributes to variation CCRT as well, predicting that males would be more cold tolerant (shorter CCRT) due to their nature of foraging early in the morning while females remain in the nest.
The researchers collected bumblebee queens in the spring from several distinctly different areas of various elevations and temperatures and allowed these queens to form colonies under standard laboratory conditions. Once the colonies were formed, the team removed male and female bumblebees from colonies, weighted them, then placed them in plastic vials submerged in a chiller set to -4° C for 2 hours. After 2 hours, the bees were removed from the water bath and placed into tissue culture microplates at room temperature. The time it took for the bees to become fully responsive in their motor function was recorded as their CCRT.
Across all bees used in the experiment, CCRT ranged from 5.7 to 19 minutes, showing clear variation in CCRT within the species. Because mass may significantly influence thermal tolerance traits, researchers assessed whether CCRT varied with mass and whether mass varied between sexes or among sites. Overall, the results showed that males and females did not differ significantly in mass, but mass, sex, and their interaction influenced CCRT of bees from some sites. This means that males and females did not have distinctly different weights when compared between each other, but the combination of mass and sex in some environmental sites influenced the CCRT. Interestingly, CCRT significantly increased with mass for all bees. For every 100 mg increase in mass, CCRT was approximately 6.7 min longer. CCRT also varied significantly between the sexes, driven by a tendency for males to recover from cold more slowly than females across all sites, which directly contradicts the researchers’ prediction. Overall, female CCRT increased approximately 2.2 min for every 100 mg increase in body mass, and male CCRT only increased approximately 1 min for every 100 mg increase in mass. Neither of the climate variables (i.e., minimum annual temperature and minimum July temperature) explained significant variation in CCRT for females or males.
The hypothesis for this experiment was not supported through the results; variation in CCRT of bumble bees was not explained by the climate of queen collection sites, possibly because the bumblebees used in this study were reared under laboratory conditions rather than in the wild. The researchers speculated that behaviour may explain why CCRT might be unrelated to local climate is that bumblebees. If bumblebees regulate their nest microclimates so they are less influenced by ambient temperatures or shift activity times to avoid exposure to temperature extremes, they would be exposed to a different set of environmental conditions than would be measured with environmental monitoring. Future work investigating microclimates and behaviour in bumblebees will strengthen our understanding of the link between thermal tolerance and bumble bee distributions. This knowledge may also help us understand the implications of mismatches between changing temperatures and thermal tolerance limits that have been proposed to be the causation of the recent range shifts and population declines in bumble bees.
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