Complex systems (e.g. eusocial insect colonies) exhibit emergent behaviours as a result of the interactions of their components. These components often vary in several traits. Such variation may improve system performance by increasing its efficiency or its robustness to environmental change. These two outcomes, efficiency and robustness, are often thought to be in opposition. Therefore, variation may be beneficial only under certain environmental conditions. Here we aim to understand why variation evolved in a particular system, bumble bee (Bombus impatiens) colonies. Workers in these colonies vary in body size, which affects the tasks they perform as well as their starvation resistance, suggesting potential impacts on efficiency and robustness, respectively. We examine how this variation affects colony performance under different environmental conditions and how colonies respond physiologically to these conditions. We maintained colonies of equal biomass but with either variable or less variable worker body sizes using targeted worker removal. We found that colonies with variable body sizes did not produce more brood (i.e. did not show evidence of increased performance) under predictable or unpredictable food environments. However, workers that developed under the unpredictable environment were smaller relative to their weight at eclosion. This effect was due in part to an increase in stored lipids, particularly in smaller workers. These physiological changes may explain why mortality rates did not differ between the predictable and unpredictable environment. Therefore, our finding that size variation did not affect colony performance suggests that size variation may be a neutral trait, present because selection is not acting against it. Our results also suggest that workers respond physiologically to differences in environmental conditions, which is important to consider when testing system robustness.

中文翻译：

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大小变化不能作为大黄蜂的保险；相反，工人在不可预测的环境中增加了重量

复杂系统（例如真社会性昆虫群落）由于其组件的相互作用而表现出紧急行为。这些成分通常在几个特征上有所不同。这种变化可以通过提高系统的效率或对环境变化的鲁棒性来提高系统性能。效率和稳健性这两个结果通常被认为是对立的。因此，变化可能仅在某些环境条件下有益。在这里，我们的目标是了解为什么在特定系统大黄蜂（Bombus impatiens）群体中进化出变异。这些群体中的工人的体型各不相同，这会影响他们执行的任务以及他们的饥饿抵抗力，这分别表明对效率和健壮性的潜在影响。我们研究了这种变化如何影响不同环境条件下的菌落性能以及菌落对这些条件的生理反应。我们使用有针对性的工人移除来维持相同生物量的菌落，但具有可变或更少可变的工人身体尺寸。我们发现，在可预测或不可预测的食物环境下，体型可变的菌落不会产生更多的育雏（即没有表现出性能提高的证据）。然而，在不可预测的环境下发育的工人相对于羽化时的体重来说更小。这种影响部分是由于储存的脂质增加，特别是在较小的工人中。这些生理变化可以解释为什么可预测和不可预测的环境之间的死亡率没有差异。所以，我们发现大小变异不影响菌落性能表明大小变异可能是一种中性特征，存在是因为选择不反对它。我们的结果还表明，工人对环境条件的差异有生理反应，这在测试系统稳健性时很重要。