Abstract
Simple SummaryBehavioural specialisation of individuals may improve the performance of groups, but could also limit the ability to switch tasks (behavioural ‘plasticity’) in response to changing group needs. In bumble bee colonies, body size, which is fixed once the bees reach adulthood, influences the tasks that bees perform, meaning that large and small bees often act as specialists. We found that when we experimentally reduced the body-size variation of colonies, some performed less well than normal. Nonetheless, in other colonies, individuals increased task specialisation or effort, which apparently compensated for the absence of large and small workers. These results suggest that both behavioural specialisation and plasticity can be important in collective group performance.Specialisation and plasticity are important for many forms of collective behaviour, but the interplay between these factors is little understood. In insect societies, workers are often developmentally primed to specialise in different tasks, sometimes with morphological or physiological adaptations, facilitating a division of labour. Workers may also plastically switch between tasks or vary their effort. The degree to which developmentally primed specialisation limits plasticity is not clear and has not been systematically tested in ecologically relevant contexts. We addressed this question in 20 free-foraging bumble bee (Bombus terrestris) colonies by continually manipulating colonies to contain either a typically diverse, or a reduced (“homogeneous”), worker body size distribution while keeping the same mean body size, over two trials. Pooling both trials, diverse colonies produced a larger comb mass, an index of colony performance. The link between body size and task was further corroborated by the finding that foragers were larger than nurses even in homogeneous colonies with a very narrow body size range. However, the overall effect of size diversity stemmed mostly from one trial. In the other trial, homogeneous and diverse colonies showed comparable performance. By comparing behavioural profiles based on several thousand observations of individuals, we found evidence that workers in homogeneous colonies in this trial rescued colony performance by plastically increasing behavioural specialisation and/or individual effort, compared to same-sized individuals in diverse colonies. Our results are consistent with a benefit to colonies of large and small specialists under certain conditions, but also suggest that plasticity or effort can compensate for reduced (size-related) specialisation. Thus, we suggest that an intricate interplay between specialisation and plasticity is functionally adaptive in bumble bee colonies.
Highlights
A key organisational principle of insect societies is division of labour, whereby individuals specialise in various tasks such as caring for the brood (“nursing”), guarding the nest, or foraging for resources, meaning that they disproportionately perform these activities compared to the group average [1,2]
We found no evidence for influence of worker body size diversity on the sizes of newly emerged bees reared by the colony (Hypothesis 2a)
We suggest that the decrease in inactivity and the increase in individual effort at least partially compensated for the lack of large and small specialists in homogeneous colonies, explaining the comparable performance of homogeneous and diverse colonies in Trial 1 (Hypothesis 2c)
Summary
A key organisational principle of insect societies is division of labour, whereby individuals specialise in various tasks such as caring for the brood (“nursing”), guarding the nest, or foraging for resources, meaning that they disproportionately perform these activities compared to the group average [1,2]. Extreme examples include size differences of more than fiftyfold in the morphological castes of ant workers [14] and developmental morphological changes in many termites [15] These individual differences are assumed to be functionally linked to specialisation, allowing workers to be more efficient at performing certain tasks, but may come at the expense of decreasing individual behavioural plasticity, e.g., [16,17,18,19]. If these developmentally primed specialists have reduced flexibility, this may result in the need for colonies to balance the proportions of specialists, so that all tasks are performed in accordance with colony requirements. This is why developmentally primed specialists are normally associated with large colonies, where there are enough individuals to provide a buffer against unexpected changes in the colony composition of specialists [3,14,20]
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