Chai P
J Exp Biol. 1998 Apr;201 (Pt 7):963-8. doi: 10.1242/jeb.201.7.963.
As the smallest homeotherms, hummingbirds suffer from low thermal inertia and high heat loss. Flapping flight is energetically expensive, and convective cooling due to wing and air movements could further exacerbate energy drain. Energy conservation during flight is thus profoundly important for hummingbirds. The present study demonstrates that heat produced by flight activity can contribute to thermoregulatory requirements in hovering hummingbirds. The rate of oxygen consumption, as an indicator of metabolic cost, was measured during hover-feeding and compared with that during perch-feeding. In hover-feeding, oxygen consumption increased only moderately between 35 and 5 degreesC in contrast to the sharp increase during perch-feeding over the same temperature range. This result suggests that heat produced by contraction of the flight muscles substituted for regulatory thermogenesis to accommodate for heat loss during exposure to low temperature. With declining air temperatures, the mechanical power requirements of hovering decreased slightly, but metabolic costs increased moderately. As a result, the mechanical efficiency of the muscle in converting metabolic power to mechanical power was reduced. Changes in wingbeat kinematics also accompanied the reduction in muscle efficiency. Wingbeat frequency increased but stroke amplitude decreased when hovering in the cold, suggesting thermoregulatory roles for the flight muscles. Hovering hummingbirds modulated their wingbeat frequency within a narrow range, reflecting the physical constraints of tuning to a natural resonant frequency with an elastic restoring force. We hypothesize that, by forcing the resonant system of the wings and thorax to oscillate at different frequencies, muscle contraction in the cold generates more heat at the expense of mechanical efficiency. This mechanism of modulating the efficiency of muscle contraction and heat production allows flying hummingbirds to achieve energy conservation at low air temperatures.
作为最小的恒温动物，蜂鸟面临着热惯性低和热损失高的问题。振翅飞行能量消耗大，而翅膀和空气运动引起的对流散热会进一步加剧能量消耗。因此，飞行过程中的能量保存对蜂鸟来说至关重要。本研究表明，飞行活动产生的热量有助于悬停的蜂鸟满足体温调节需求。在悬停取食过程中测量了作为代谢成本指标的氧气消耗率，并与栖木取食过程中的氧气消耗率进行了比较。在悬停取食时，在35摄氏度至5摄氏度之间氧气消耗仅适度增加，而在相同温度范围内栖木取食时氧气消耗则急剧增加。这一结果表明，飞行肌肉收缩产生的热量替代了调节性产热，以适应低温暴露期间的热量损失。随着气温下降，悬停的机械功率需求略有下降，但代谢成本适度增加。结果，肌肉将代谢功率转化为机械功率的机械效率降低。翅膀运动学的变化也伴随着肌肉效率的降低。在寒冷中悬停时，振翅频率增加但冲程幅度减小，这表明飞行肌肉具有体温调节作用。悬停的蜂鸟在很窄的范围内调节振翅频率，这反映了通过弹性恢复力调谐到自然共振频率的物理限制。我们假设，通过迫使翅膀和胸部的共振系统以不同频率振荡，寒冷环境中的肌肉收缩以机械效率为代价产生更多热量。这种调节肌肉收缩效率和产热的机制使飞行中的蜂鸟能够在低温下实现能量保存。
