Data from: High flight costs, but low dive costs, in auks support the biomechanical hypothesis for flightlessness in penguins

Flight is a key adaptive trait. Despite its advantages, flight has been lost in several groups of birds, notably among seabirds, where flightlessness has evolved independently in at least five lineages. One hypothesis for the loss of flight among seabirds is that animals moving between different media face tradeoffs between maximizing function in one medium relative to the other. In particular, biomechanical models of energy costs during flying and diving suggest that a wing designed for optimal diving performance should lead to enormous energy costs when flying in air. Costs of flying and diving have been measured in free-living animals that use their wings to fly or to propel their dives, but not both. Animals that both fly and dive might approach the functional boundary between flight and nonflight. We show that flight costs for thick-billed murres (Uria lomvia), which are wing-propelled divers, and pelagic cormorants (Phalacrocorax pelagicus) (foot-propelled divers), are the highest recorded for vertebrates. Dive costs are high for cormorants and low for murres, but the latter are still higher than for flightless wing-propelled diving birds (penguins). For murres, flight costs were higher than predicted from biomechanical modeling, and the oxygen consumption rate during dives decreased with depth at a faster rate than estimated biomechanical costs. These results strongly support the hypothesis that function constrains form in diving birds, and that optimizing wing shape and form for wing-propelled diving leads to such high flight costs that flying ceases to be an option in larger wing-propelled diving seabirds, including penguins.飞行是一项关键的适应性性状（adaptive trait）。尽管飞行具备诸多优势，但已有多个鸟类类群彻底丧失了飞行能力，其中尤以海鸟类群最为典型，至少有5个独立演化支系演化出了无飞行能力的特征。针对海鸟丧失飞行能力的假说之一认为：在两种不同运动介质间切换的动物，需要在两种介质下的运动功能最大化之间做出权衡（tradeoff）。具体而言，飞行与潜水过程中的能量消耗生物力学模型（biomechanical model）显示：若翼部结构最优适配潜水性能，则其在空气中飞行时的能量消耗将极其高昂。目前已有研究对使用翼部飞行或驱动潜水的自由生活野生动物的飞行与潜水能耗分别进行了测定，但尚未有针对同时兼具两种运动方式的物种的相关测量。同时具备飞行与潜水能力的动物，其运动功能可能处于飞行与非飞行状态的功能边界附近。本研究显示：作为翼驱动潜水者的厚嘴海鸦（Uria lomvia）与足驱动潜水者的远洋鸬鹚（Phalacrocorax pelagicus），其飞行能耗均为脊椎动物中已记录的最高值。鸬鹚的潜水能耗较高，而海鸦的潜水能耗较低，但后者的能耗仍高于无飞行能力的翼驱动潜水鸟类——企鹅（penguins）。就海鸦而言，其飞行能耗高于生物力学模型的预测值，且潜水过程中的耗氧速率随水深增加的下降速率，快于生物力学模型估算的能耗变化速率。上述研究结果有力支持了以下假说：潜水鸟类的身体形态受其运动功能的制约；而为适配翼驱动潜水而优化的翼部形态与结构，会带来极高的飞行能耗，以至于包括企鹅在内的大型翼驱动潜水海鸟，彻底丧失了飞行能力。