Ventilation response thresholds do not change with age or self-reinforcement in workers of the bumble bee Bombus impatiens

The response threshold model is a potential mechanism for task allocation in social insects, and it assumes that workers vary in the levels of task stimuli to which they respond. Furthermore, response thresholds of individual workers may change over time through self-reinforcement (experience), such that workers become more sensitive to task stimuli. However, in addition to self-reinforcement, aging is another process that occurs through time. Distinguishing whether response thresholds change within workers due to self-reinforcement or aging may give insight into the flexibility of this task allocation mechanism. Using a ventilation paradigm, we manipulated workers of Bombus impatiens to have either repeated or lack of exposures to increases in nest air temperature, thereby allowing us to manipulate experience and thus self-reinforcement. Nest air temperature was the task stimulus, and ventilation (fanning) was the behavioral response. We found that ventilation response thresholds do not decrease either with age or experience in workers of B. impatiens, contrary to what has been reported for B. terrestris workers (Weidenmüller, 2004). Instead, we found high levels of intra-individual variation in response thresholds. Our results also show that workers with lower average response thresholds respond to heating events with higher probability than those with higher ventilation thresholds. These results provide insight into the role of the response threshold framework for task allocation; we also discuss how response probabilities may play a role in task allocation among workers.