Urban / rural acorn ant metabolic rate and running speed

Metabolic rates of ectotherms are expected to increase with global trends of climatic warming. But the potential for rapid, compensatory evolution of lower metabolic rate in response to rising temperatures is only starting to be explored. Here, we explored rapid evolution of metabolic rate and locomotor performance in acorn-dwelling ants (Temnothorax curvispinosus) in response to urban heat island effects. We reared ant colonies within a laboratory common garden (25 °C) to generate a lab-born cohort of workers and tested their acute plastic responses to temperature. Contrary to expectations, urban ants exhibited a higher metabolic rate compared with rural ants when tested at 25 °C, suggesting a potentially maladaptive evolutionary response to urbanization. Urban and rural ants had similar metabolic rates when tested at 38 °C, as a consequence of a diminished plastic response of the urban ants. Locomotor performance also evolved such that the running speed of urban ants was faster than rural ants under warmer test temperatures (32 and 42 °C) but slower under a cooler test temperature (22 °C). The resulting specialist-generalist tradeoff and higher thermal optimum for locomotor performance might compensate for evolved increases in metabolic rate by allowing workers to more quickly scout and retrieve resources.变温动物（ectotherms）的代谢率（metabolic rate）预计会随全球气候变暖趋势而升高。但针对温度上升，生物体通过降低代谢率实现快速补偿性进化的潜力，目前才刚刚开始得到探索。本研究以栖居于橡果的蚂蚁（Temnothorax curvispinosus）为对象，探究了其代谢率与运动表现对城市热岛效应（urban heat island effects）的快速进化响应。我们在实验室同质园（common garden）环境下饲养蚁群，以获得实验室出生的工蚁队列，并测试它们对温度的急性塑性响应。与预期相悖的是，在25 ℃条件下测试时，城市种群蚂蚁的代谢率高于乡村种群蚂蚁，这表明城市化可能引发了适应不良的进化响应。在38 ℃条件下测试时，城市与乡村种群蚂蚁的代谢率无显著差异，这源于城市种群蚂蚁的塑性响应减弱。运动表现同样发生了进化：在较高测试温度（32 ℃与42 ℃）下，城市种群蚂蚁的奔跑速度快于乡村种群，但在较低测试温度（22 ℃）下则更慢。由此产生的特化-泛化权衡（specialist-generalist tradeoff）以及更高的运动表现热最适值（thermal optimum），或可通过让工蚁更快地侦察与回收资源，代偿进化带来的代谢率升高。