In social insects, within-colony worker diversity has long been thought to improve colony fitness. Recent studies start to question this assertion in species with moderate diversity. However, demonstrating an absence of effect is difficult as this absence could be due to the measurement of only few life history traits or to artificial conditions. In order to circumvent these limitations, we experimentally decreased worker size diversity within colonies of the ant Temnothorax nylanderi, with or without affecting mean worker size and we reintroduced them in the field for 4 months (spring and early summer). We then thoroughly measured their fitness based on survival, growth, and reproductive success. Our results show that our manipulation did not affect colony fitness. In addition, colonies did not restore diversity to its initial level, further suggesting that worker size diversity is not a key parameter to them. We found the classically observed positive relationship between colony size, colony growth, and reproductive success. Overall, our results confirm that worker size diversity within colony is not necessarily adaptive in species where it is moderate. We discuss the alternative mechanisms that could explain the evolutionary persistence of moderate worker size diversity. Organisms that live in groups can greatly benefit from the emergence of novel group-level traits. For instance, social insects show significant variability in worker size within colonies. This size diversity increases the division of labor among workers and improves colony fitness. However, in species where size diversity is moderate, this relationship may not always be verified. Here, we manipulated both worker size diversity and mean worker size within colonies of the ant Temnothorax nylanderi, we reintroduced them in the field, and we extensively measured colony fitness after 4 months. We found no impact on colony survival, growth, and reproduction. We discuss how social life and its associated cooperation and conflicts could cause variation in worker size, without any positive effect on colony fitness.

中文翻译：

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在蚂蚁 Temnothorax nylanderi 的自然条件下，对工人大小多样性的操纵不会影响群体适应性

在社会性昆虫中，长期以来一直认为群体内工人多样性可以提高群体适应性。最近的研究开始对具有中等多样性的物种的这一说法提出质疑。然而，证明没有影响是困难的，因为这种缺乏可能是由于仅测量了少数生活史特征或人为条件。为了规避这些限制，我们通过实验降低了蚂蚁 Temnothorax nylanderi 群体内的工人大小多样性，无论是否影响平均工人大小，我们将它们重新引入该领域 4 个月（春季和初夏）。然后，我们根据生存、生长和繁殖成功率彻底衡量了它们的适应性。我们的结果表明，我们的操作不会影响菌落适应性。此外，殖民地没有将多样性恢复到初始水平，进一步表明工人规模的多样性不是他们的关键参数。我们发现集落大小、集落生长和繁殖成功之间存在经典观察到的正相关关系。总体而言，我们的结果证实，群体内工人大小的多样性在中等的物种中不一定具有适应性。我们讨论了可以解释中等工人规模多样性进化持久性的替代机制。群居的生物可以从新的群级特征的出现中受益匪浅。例如，群居昆虫在群体内的工人规模上表现出显着的变异性。这种大小的多样性增加了工人之间的分工并提高了群体适应性。然而，在大小多样性适中的物种中，这种关系可能并不总是得到证实。这里，我们操纵了蚂蚁 Temnothorax nylanderi 群体内的工人大小多样性和平均工人大小，我们将它们重新引入野外，并在 4 个月后广泛测量了群体适应性。我们发现对菌落存活、生长和繁殖没有影响。我们讨论了社会生活及其相关的合作和冲突如何导致工人规模的变化，而对群体适应性没有任何积极影响。