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
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 : "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, , 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 . 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.
., using (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 -- 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.