Ants in isolation: obstacles to testing worker responses to task stimuli outside of the colony context

Abstract

In social insects, division of labor is commonly thought to be driven by differences among workers in their sensitivity, or response thresholds, to task-related stimuli. Despite the wide use of this mechanism throughout social insect research, actual empirical evidence for these thresholds is comparatively scarce. Here, we attempt to fill this empirical gap by testing individual task stimulus response thresholds, their consistency over time, and their relation to behavior in Temnothorax rugatulus ants. We also explored morphological differences in the antenna as one potential neural mechanism generating differences in sensitivity, and thus response threshold variation, across workers. Ants were exposed to different amounts of hungry brood, fungal spores, or sugar— stimuli that appear to drive brood care, grooming, and foraging behavior, respectively. Our measures of response thresholds were not repeatable across two trials for any of the three tested stimuli. In addition, responses to different stimulus intensities (possible response thresholds) were not associated with worker task allocation in the colony with the exception of brood care, in which case the results directly contradicted what the response threshold hypothesis predicts. Workers from different task groups also did not differ in their latency to respond to these stimuli or in the duration of their response. Sensilla density varied across workers but did not predict our measures of response thresholds to any of the tested stimuli. Though this is not what the response threshold hypothesis would have predicted, it is possible that testing ants in isolation may not accurately reflect their behavior in the colony, or that sensitivity to a task stimulus, alone, is not sufficient for driving division of labor. We suggest approaches to testing response thresholds that incorporate the roles of social context and competing task stimuli.