What is Tendon locking?
'''Tendon locking''' is used by many animal [[species]] which sleep in trees or caves. It lets them sleep without falling off. The system has evolved separately in a number of vertebrates, so it is an example of [[parallel evolution]]. ==Bird feet== The leg arrangement of [[passerine]] birds has a special adaption for perching: a ''tendon-locking mechanism''. A [[tendon]] in the rear of the leg is automatically pulled and tightened when the leg bends. This makes the foot curl stiffly round when the bird lands on a branch. The tendon is locked in place by another tendon. So passerines sleep while perching without falling off.<ref name=Stef>Stefoff, Rebecca 2008. ''The bird class''. Marshall Cavendish Benchmark.</ref><ref>Brooke, Michael and Tim Birkhead 1991. ''The Cambridge Encyclopedia of Ornitholigy''. Cambridge University Press.</ref> == Small mammals == Small animals which sleep in trees or caves usually hang on with their feet: they have a tendon locking device which keeps them hanging on even during sleep.<ref name=Stef/><ref>Brooke, Michael & Birkhead, Tim 1991. ''The Cambridge Encyclopedia of Ornithology''. Cambridge University Press. {{ISBN|0521362059}}</ref><ref>Simmons N.B. and Quinn T.H. 1994. Evolution of the digital tendon locking mechanism in bats and dermaptorans: a phylogenetic perspective. ''Journal of Mammalian Evolution''. POI: 10.1007/BF 01464276 [http://www.mendeley.com/research/evolution-digital-tendon-locking-mechanism-bats-dermopterans-phylogenetic-perspective/]</ref> It works by one tendon running down the leg shifting position slightly so it grips the tendon which closes the toes. Bats independently evolved a similar arrangement.<ref>Quinn T.H. & Baumel J.J. 1990. Chiropteran locking mechanisms: the passive digital lock. ''J. Morphology''. '''109''', 281{{ndash}}208. </ref> Once the tendons are locked, the muscles in their legs and feet can relax.<ref>Haffner M. 1966. A tendon locking mechanism in two climbing rodents. ''Journal of Morphology'' '''229''', 197{{ndash}}227.</ref> Even