ENHS Nature Trails December 2000 Article 2

Natural History and You - The President's Forum
by Nathan Tublitz

Flight Adaptations in Birds, Bats, and Insects

Of all the various forms of locomotion, perhaps the most fascinating is flight. Flight has arisen but four times in the history of the animal kingdom: in the insects, the now-extinct pterodactyls, birds and bats. The main reason for the scarcity of flying taxa is that flapping flight is the most energetically costly form of locomotion. Muscle powered wings must be evolved and metabolism must be increased 10-fold to cope with the energetic demands of flying. Once achieved, however, flight accrues wonderful benefits to the chosen few. Flying species have access to a less crowded and almost limitless ecological niche, are able to escape from grounded predators, and are able to overcome geographic obstacles to spread to diverse environments. This freedom has enabled flying organisms to evolve rapidly and broadly; there are over 9000 species of birds, and it is estimated that about 25% of insect species--over 10 million different species!--have a winged phase. Among the 4200 mammalian species that exist today, 1000 (~25%) are bats. Because flight has evolved independently in the birds, bats and insects, each has evolved unique adaptations to cope with the demands of flight.

Insects were the first animals to occupy terrestrial environments about 400 million years ago as a result of overcrowded seas. Insect species increased so quickly on land that they soon radiated upwards into the skies to fill previously unoccupied niches. It has been hypothesized that insect flight arose from the gill-like appendages of aquatic insects; however, flight appears to have arisen independently at least five times in insects. To successfully achieve flight, insects evolved several adaptations: ultra-lightweight wings powered by highly potent, specialized flight muscles, and unique wing specializations. Insect wings, like sails on a sailboat, are razor thin and strong, with batten-like veins providing the necessary tensile strength and support. They are controlled by massive flight muscles located in the thorax or midsection of the insect. Insect flight muscles are the fastest contracting muscles in the animal kingdom, capable of contracting up to 1000 times each second. Muscles require energy to contract and superfast-contracting insect flight muscles need buckets of energy. The muscles themselves store and release much of this energy when needed. The rest is provided by the highly nutrient insect blood which bathes insect flight muscles in a luxurious broth. The heaviest flying insects can only fly with 4 wings and must flap those wings in a figure eight pattern to generate enough thrust for a successful lift off. Some large flying insects such as grasshoppers even have a weighted, tumor-like growth at the tip of their wing to enhance downward thrust. The largest known insects, the extinct giant dragonflies with their three foot wing spans must have been a sight to behold 250 million years ago!

Long after flight evolved in insects, it also developed in birds. Birds evolved from reptiles some 150 million years ago, and even the oldest known fossil bird, Archeopteryx lithographica, has all the external morphological specializations for flight, including prominent quilled feathers. Like insects, birds support flight through a variety of adaptations, including very well developed flight muscles, rapid heart rate and wing beat frequency, a large output of oxygenated blood by the heart, a short circulatory time from heart to wing muscles and back again, and a remarkably efficient oxygen exchange system in the lungs. One only has to see the acrobatic precision-hovering of a hummingbird or imagine the 35000 km annual migration of the arctic tern to fully appreciate the need for these adaptations.

Although mammals evolved from reptiles before birds, they remained flightless for about 100 million years, only then did mammals with bat-like features appeared in the fossil record. Like birds, bats are excellent flyers with some covering distances of over 500 km in one night. Their wings are webbed extensions of their front digits, and like insect wings, are very thin membranous structures. Wings in bats are flapped by extremely strong forearm muscles, the strongest set of muscles in the bat. Bats generate loft because of their unusually large wing to body size ratio and maintain flight due to exquisitely tuned blood biochemistry that allows for high aerobic capacity for long periods of time. These specializations combine to produce the highly complicated flight behaviors seen in many bats, including the ability to fly with one wing while capturing prey in the other.

The attainment of flight by insects, birds and bats is a classic example of the ability of evolution to provide elegant solutions to the age-old problem of finding new food resources and unfilled niches. But unraveling the physiological and morphological adaptations underlying flight is not easy. As Orville Wright said, "Learning the secret of flight from a bird is a good deal like learning the secret of magic from a magician. After you once know the trick, you see things that you did not notice when you did not know exactly what to look for".

Photo thanks to R.W. Scott, "Birds in Flight," @

http://www.gregscott.com/rwscott/rwscott.htm

This website has exceptionally high quality photos of birds in flight as well as links to other sites.

Nathan Tublitz
Professor of Biology
Institute of Neuroscience
University of Oregon
Eugene OR 97403
Phone: 1-541-346-4510 FAX: 1-541-346-4548

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