miércoles, 7 de octubre de 2015

How do bats fly? (¿Cómo vuelan los murciélagos?)

Generalities about requirements to fly
Any animal that wants to fly, including a human that takes a plane, must care about three matters:

- The force of gravity: It is neccessary to generate 'lift' an upwards force   equivalent to, or greater than the downwards force of gravity.

- The force neccessary to 'push' itself forward in the air, and be able  to manoeuvre, i.e. turn, twist, sommersault etc, and sometimes to go backwards. 

- The resistance that air opposes to its advance, air is a viscose medium, this 'drag' resistance will be stronger the faster the animal goes.  Also, the air has currents in it and moving anything through creates vortices and other irregularities of flow.

Efficiency of bats' flight
Anyone  who has watched a bat flying will know bats are good at  this.  They can in  fact fly very well, so it is obvious their wings  overcome the problems  and do all three of these jobs well. But how well?. Although the idea that bats are less skilled flyers than birds, scientists are showing that we have to review this assumption.

  • Bats use less energy to fly: Studies that  compared oxygen consumption among birds, insects and bats of  similar sizes (a hummingbird, a small bat and a large moth, for example)  use   less energy to fly.
    • Bats are extremely  manouverable animals, often capable of rapid changes  in direction (prey  capture, manouvering, capture evasion, etc). This ability has raised attention not only in biologists but in aerospace investigators. "Bats seem to be mostly specialized for agile and maneuverable flight in complex environments," according to Geoffrey Spedding, a   University of Southern California. "In   broad generalities, bats are characterized by a darting, sharply   turning and maneuvering flight. This can be seen as they wheel about   catching insects, or flit from flower to flower," Spedding added.

    So, how do bats do it so efficently?

    To oppose the force of gravity the generalized trick among all flying animals is to use their body, and specially the wings to generate a negative pressure above them that 'sucks' them up. The 'Lift' force is generated by a combination of the shape of the  wing and the  passage of air across it.
    Basically this is because the  wings of bats  are not flat,  but are shaped like an aerofoil - meaning  they are an  irregular concave shape.   Because of the curvature of the  wing the air  that moves over the top of the wing has further to travel  to get  across the wing, thus it speeds up.  This causes the air pressure  above  the wing to drop because the same amount of air is exerting its   pressure over a greater area. Therefore, any given point experiences   less pressure. This effectively sucks the wing up.
    Meanwhile  the air going below the wing experiences the opposite  effect. It slows  down, generates more pressure and effectively pushes  the wing up.  Hence a bat with air moving over its wings is pulled up  from above and  pushed up from below at the same. The more curved the  aerofoil, and the  greater the speed of the airflow, the greater the  lift, providing the  degree of curvature does not impede the flow of air.
    From  this you can realise that larger wings will generate more lift  than  smaller wings. The adverse side of this is that the larger your  wings  are the greater the energy requirements of flapping, and the  greater  stresses there are on the physical structure of the wings.  Larger wings  are also harder to turn, which means reduced  maneouvrability.  Therefore there are limits to how large an animals  wings can actually  be.
    Secondly we can see that flying  faster generates more lift per unit  of time than flying slowly.   However it also generates more drag, and,  as drag is proportional to  the third power of the speed, with each unit  increase in speed the  costs of overcoming the created drag not only  increase, but increase  faster than the energetic benefits generated by  the increased lift,  hence a point is soon reached where flying faster  costs more than it is  worth.

But, what are the secrets of bats' wings?
the wings of bats have an amazing versatility of movements and shape control that seems to be based in:
- Their membranes and multiple joints.
- Their long loose muscles embedded in the skin.
- Their stretchy tendons.

The wing structure of bats and birds differs. Birds have feathers projecting back from   lightweight, fused arm and hand bones. Bats have flexible, relatively   short wings with membranes stretched between elongated fingers.
Spedding   said while birds can open their feathers like a Venetian blind, bats   have developed a twisting wing path that increases the lift during the   upstroke.
The joints and membranes of their wings give bats great control of the shape of them,as slow motion videos of bats flying show.
Unlike insects and birds, which  have relatively rigid wings that can move in only a few directions, a bat’s wing contains more  than  two  dozen joints that are overlaid by a thin elastic membrane  that can  stretch to  catch air and generate lift in many different ways. This gives bats an   extraordinary amount of control over the  three-dimensional shape their  wings  take during flight.

What is more, their  wings have long muscles embedded in the skin, running front to  back,  and not attached to any bones. Scientists had suspected that  these  muscles probably helped shape the wings in flight, but evidence  was  lacking.
Now,  in experiments at  Brown University with Jamaican fruit bats, investigators have found signs that  the muscles do  indeed contract on the downstroke when bats are flying.

One more surprise are that  bats' stretchy bicep and tricep  tendons are crucial for storing and  releasing the energy the creatures  require for takeoff.
Taking off of those critters has long perplexed biologists. This activity requires a great expense of energy, and bats seem to be in great anatomical disadvantage for getting it.

A group of scientists at Brown University investigated the matter, using XROMM (X-ray Reconstruction of Moving Morphology) technology that integrates   three-dimensional renderings of animals' bone structures into X-ray   video. (XROMM data allow researchers to conduct detailed analyses of   animals' muscle mechanics and anatomy as the creatures moves.) The team   looked in particular at Seba's short-tailed bats --  fruit bats -- X-raying the creatures as they lifted themselves off  the  ground. Analyzing the videos that resulted, the researchers made a   discovery: bats seem to take off into the air by stretching out the   tendons that anchor their bicep and tricep muscles to their bones. They   then compress the tendons to release energy and power their flight   upward.
It seems, in other words, that  bats' stretchy bicep and tricep  tendons are crucial for storing and  releasing the energy the creatures  require for takeoff. As research lead Nicolai Konow explained it:   "By combining information about skeletal movement with information   about muscle mechanics, we found that the biceps and triceps tendons of   small fruitbats are stretched and store energy as the bat launches  from  the ground and flies vertically."
The  bats' stretchy-muscle  analysis seemed to be confirmed by the team's  use of another technology:  fluoromicrometry, in which small, chemically  labeled markers are  implanted directly into muscle -- which in turn  allows researchers to  measure changes in muscle length during  contractions with high  precision.
And that's a big finding: most scientists had previously believed, Smithsonian Magazine points out,   that small mammals' tendons are too stiff, and too thick, to be   stretched at all. The X-rays revealed otherwise, however, and the Brown   team presented their findings last week at a meeting of the Society for Experimental Biology.   And they've presented their videos to the rest of us, thankfully, so   that we may be appropriately astounded and creeped out by the unique   biology of bats.
How Bats Take Flight, Revealed by X-Ray.

Why Bats Are More Efficient Flyers Than Birds
High Metabolism Fueled Evolution of Bat Flight
How Bats Take Flight, Revealed by X-Ray.

2 comentarios:

  1. I wonder if pterosaurs used that tendon trick?

    1. I really don't know, the surprise with bats was that it was thought that this kind of stretchy tendons could only be found in larger mammals.
      But, total guess here, I wouldn't be surprised to read some material about stretchy tendonds and loose muscles related to pterosaurs flight. Now, can you imagine one of those larger pterosaurs taking off...