Passive Stay Mechanism Allows Flamingos to Sleep on One Leg

INTRODUCTION How do birds, like flamingos (Phoenicopteridae), sleep while standing on one leg? It is suggested that this behavior reduces heat energy lost through the legs [1]. Yet, the muscular energy required to actively support the body in this pose would likely offset any thermal benefits. In particular, flamingo femurs are oriented horizontally (Figure 1), which places the knee and hip joints in an unfavorable position for generating muscular torques to oppose gravity. We found a passive, gravity-driven mechanism in the proximal pelvic limb of the flamingo that allows sleeping and even dead birds to support their body weight on a single leg without any muscular activation at all.