What good are soldiers during peacetime? Large worker ants enhance transport capacity during foraging

Polymorphic worker castes have evolved repeatedly in ants, yet the selective advantages of larger workers are not always clear. Because larger workers require greater energetic investment to produce, they must provide disproportionate benefits to colony performance. While larger workers are typically assumed to function in colony defense as soldiers, defensive encounters are sporadic, raising a central question: what maintains these costly individuals during periods of relative peace? Here, we tested whether larger workers enhance foraging performance through increased running speed or improved load transport in the red imported fire ant, Solenopsis invicta. We quantified locomotor performance of workers spanning a 23-fold range in body mass under unloaded and loaded conditions. Larger workers ran faster in absolute terms when unladen, but velocity scaled sublinearly with body size and declined when standardized to body length. When carrying brood, running speed was independent of worker size, and free-running assays showed that larger workers moved faster only during outbound foraging, suggesting that larger workers do not substantially increase transport rates when carrying loads. In contrast, momentum increased steeply and disproportionately with body size across all conditions, reflecting a substantial increase in transport capacity rather than speed alone. Morphological analyses revealed positive allometry of the head, thorax, and abdomen, alongside relatively slender legs, suggesting a reorganization of body form to enhance force production rather than locomotor speed. These results suggest that larger workers function as high-capacity transport specialists, providing one potential explanation for the evolution and maintenance of worker polymorphism in ants beyond colony defense.蚂蚁的多态工蚁品级（polymorphic worker castes）已多次独立演化，但体型更大的工蚁所具备的选择优势始终未完全明晰。由于培育体型更大的工蚁需要投入更多能量，这类工蚁必须为蚁群整体性能带来超出比例的收益，方能维持其存在。尽管通常认为体型更大的工蚁会作为兵蚁参与蚁群防御，但防御性互动实则较为零星，由此引出一个核心问题：在相对和平的时期，是什么因素维持了这类成本高昂的工蚁种群？本研究以红火蚁（Solenopsis invicta）为研究对象，探究体型更大的工蚁是否可通过提升奔跑速度或优化负载运输能力，来改善觅食表现。我们针对体重跨度达23倍的工蚁，分别在无负载与负载两种条件下，量化了其运动性能。无负载时，体型更大的工蚁绝对奔跑速度更快，但奔跑速度与体型呈亚线性比例关系，且经体长标准化后的速度会随体型增大而下降。当携带幼体（brood）时，工蚁的奔跑速度与体型无关；自由奔跑实验结果表明，体型更大的工蚁仅在出巢觅食阶段移动速度更快，这意味着它们在负载状态下并未显著提升运输效率。与之相反，在所有实验条件下，动量（momentum）均随体型增大而急剧且不成比例地提升，这反映出运输能力而非单纯速度得到了显著增强。形态学分析结果显示，工蚁的头部、胸部与腹部均呈正异速生长（positive allometry），而腿部相对纤细，这表明其身体结构发生了重构，以提升力量产出而非运动速度。上述研究结果表明，体型更大的工蚁实为高负载运输特化个体，这为蚁群工蚁多态型的演化与维持提供了除防御功能之外的另一潜在解释。