Ants adjust their tool use strategy in response to foraging risk

1. Ants are among a few invertebrates that can use certain tools. For example, some ants can use debris and soil grains to transport liquid food. Although there has been evidence showing that ants can select the most efficient tools in transporting liquid food, little is known about their flexibility in using a specific tool under environmental pressure. 2. Black imported fire ants, Solenopsis richteri Forel, can use sands as a tool for foraging. We hypothesized that S. richteri can adjust their strategy of tool use in response to various foraging risks. The hypothesis was tested by studying their sand use behavior in response to sugar water with different drowning risks. 3. The results support our hypothesis. S. richteri indeed adjusted their strategy of tool use in response to increased drowning risks. Due to their hydrophobic cuticle, S. richteri can float on water surface to forage directly with the minimal risk of drowning. However, when the drowning risk was elevated by reducing the surface tension of sugar water in a food container by the addition of surfactant, ants used sands to build a structure that could effectively function as a syphon instead of directly foraging inside the container. This sand structure could draw sugar water out of the container then to be collected by the ants, which not only reduced the drowning risk of ants, but also provided a larger operational space for their collection of sugar water. 4. This study demonstrates that ants can not only recognize the increase in foraging risks, but also make corresponding adjustments to their strategy by using a specific tool. Our results also suggest that social insects can be highly flexible in their tool use in response to ecological challenges.1. 蚂蚁是少数能够使用特定工具的无脊椎动物（invertebrates）之一。例如，部分蚂蚁可利用碎屑与土壤颗粒转运液态食物。尽管已有研究表明蚂蚁能够选择转运液态食物时效率最高的工具，但目前学界对其在环境压力下使用特定工具的灵活性仍知之甚少。 2. 黑外来火蚁（Solenopsis richteri Forel）可利用砂石作为觅食工具。我们提出假设：黑外来火蚁能够根据不同的觅食风险调整其工具使用策略。本研究通过观察其在不同溺水风险的蔗糖水溶液环境下的砂石使用行为，对该假设进行了验证。 3. 实验结果支持我们的假设。黑外来火蚁确实会随着溺水风险升高调整其工具使用策略。由于体表具有疏水角质层（hydrophobic cuticle），黑外来火蚁可直接漂浮于水面觅食，溺水风险极低。然而，当通过向食物容器内添加表面活性剂（surfactant）降低蔗糖水溶液的表面张力（surface tension），从而升高溺水风险时，蚂蚁会利用砂石搭建一种可等效为虹吸管（syphon）的结构，而非直接进入容器内觅食。该砂石结构可将蔗糖水溶液从容器中吸出供蚂蚁采集，此举不仅降低了蚂蚁的溺水风险，还为其采集蔗糖水溶液提供了更大的操作空间。 4. 本研究证明，蚂蚁不仅能够识别觅食风险的升高，还能通过使用特定工具对自身策略做出相应调整。我们的研究结果同时表明，社会性昆虫（social insects）在应对生态挑战时，其工具使用行为可具备高度的灵活性。