biology zoology blog benno meyer rochow claws

Locked in, Locked up, Locked on…

Something that’s difficult to open

You can lock arms with someone, you can lock on to something or be locked in or even be locked up or locked out. But this essay is about animals that possess locking mechanisms. In the tropical waters of Ascension Island in the South Atlantic Ocean, for example, I once caught a pair of matchbox-sized trigger fish and observed their behaviour in my aquarium on board of the research vessel “Walter Herwig”. These denizens of the tropical seas as well as their cousins, the file fishes, can wedge themselves into rock cracks and coral crevices in such a way that it is virtually impossible to dislodge them by pulling at their tails.

You might think that these fish must be mighty strong, but in reality they hardly use any energy at all in this process of “locking in”. Their first dorsal fin ray is a spine that possesses a groove on the backside; but when fully erect a smaller second spine behind the first one is pushed home into the groove of the spine in front, so that the first bigger spine cannot be depressed by external forces, except when the second smaller one is retracted first. A very fine and effective device that is.

Another locking mechanism which requires little energy and does not depend on continuous nervous activity (and thus is not due to tetanic muscular contraction) is the one involved in closing of the two shells of bivalves. Mussels, for instance, can remain tightly shut, locked up, for hours even days. If this was due to sustained muscular work, fatigue would surely set in after a while. The mussel’s adductor muscle, however, shows no fatigue and it is believed that special proteins known as para-myosin filaments, only occurring in molluscs and some other invertebrates, undergo a structural re-organization during contraction for shell closure, termed “crystallization”. Release of this “locking fabric” requires chemicals such as enteramin or serotonin from special nerve fibre endings.

Birds, resting and sleeping on a branch or a wire also use an energy-saving grip technique: in their legs there are in addition to muscles tendinous arrangements, which ensure that the claws clasp a branch or a wire when the body weight pushes on the legs. No energy is required for that, but to release the grip some energy is needed. Certain rope-knots that I had to learn during a week of Pre-Antarctic training at Lake Tekapo, are used by climbers and work in an analogous way: they tighten up when pull is in one direction and loosen when the force is reversed.

But how about animals whose normal stance is always “reversed”, upside-down that is, like bats and sloths? For bats with their reversed and backward facing knees the physiological mechanism has been studied. When they relax having found a substrate to hang from, their body weight causes their toes to become locked in a clasping or grasping position due to the fact that the tendons are directly connected to the toe joints and are being pulled down by the bat’s weight. To unlock the clasping grip and open the clinched toes, energy and muscular actions are required. But what about the sloth? It does not hang by its feet alone, but dangles from a bough or branch of a tree using the mighty claws of its four legs together. How it can sleep in this position and not fall off are questions which to the best of my knowledge have not been satisfactorily investigated yet and wait to be unlocked by an inquisitive zoologist.

© Dr V.B. Meyer-Rochow and, 2019.
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