Origin of avian flight
Around 350 BCE,
In March 2018, scientists reported that
Flight characteristics
For
The mechanics of an avian's wings involve a complex interworking of forces, particularly at the shoulder where most of the wings' motions take place. These functions depend on a precise balance of forces from the muscles, ligaments, and articular cartilages as well as inertial, gravitational, and aerodynamic loads on the wing.[4]
Birds have two main muscles in their wing that are responsible for flight: the pectoralis and the supracoracoideus. The pectoralis is the largest muscle in the wing and is the primary depressor and pronator of the wing. The supracoracoideus is the second largest and is the primary elevator and supinator. In addition, there are distal wing muscles that assist the bird in flight.[5]
Prior to their existence on birds, feathers were present on the bodies of many dinosaur species. Through natural selection, feathers became more common among the animals as their wings developed over the course of tens of millions of years.[6] The smooth surface of feathers on a bird's body helps to reduce friction while in flight. The tail, also consisting of feathers, helps the bird to maneuver and glide.[7]
Hypotheses
Pouncing Proavis model
A theory of a pouncing proavis was first proposed by Garner, Taylor, and Thomas in 1999:[8]
We propose that birds evolved from predators that specialized in ambush from elevated sites, using their raptorial hindlimbs in a leaping attack. Drag-based, and later lift-based, mechanisms evolved under selection for improved control of body position and locomotion during the aerial part of the attack. Selection for enhanced lift-based control led to improved lift coefficients, incidentally turning a pounce into a swoop as lift production increased. Selection for greater swooping range would finally lead to the origin of true flight.
The authors believed that this theory had four main virtues:
- It predicts the observed sequence of character acquisition in avian evolution.
- It predicts an Archaeopteryx-like animal, with a skeleton more or less identical to terrestrial theropods, with few adaptations to flapping, but very advanced aerodynamic asymmetrical feathers.
- It explains that primitive pouncers (perhaps like Microraptor) could coexist with more advanced fliers (like Confuciusornis or Sapeornis) since they did not compete for flying niches.
- It explains that the evolution of elongated rachis-bearing feathers began with simple forms that produced a benefit by increasing drag. Later, more refined feather shapes could begin to also provide lift.[8]
Cursorial model
A cursorial, or "running" model was originally proposed by Samuel Wendell Williston in 1879. This theory states that "flight evolved in running bipeds through a series of short jumps". As the length of the jumps extended, the wings were used not only for thrust but also for stability, and eventually eliminated the gliding intermediate. This theory was modified in the 1970s by John Ostrom to describe the use of wings as an insect-foraging mechanism which then evolved into a wing stroke.[9] Research was conducted by comparing the amount of energy expended by each hunting method with the amount of food gathered. The potential hunting volume doubles by running and jumping. To gather the same volume of food, Archaeopteryx would expend less energy by running and jumping than by running alone. Therefore, the cost/benefit ratio would be more favorable for this model. Due to Archaeopteryx's long and erect leg, supporters of this model say the species was a terrestrial bird. This characteristic allows for more strength and stability in the hindlimbs. Thrust produced by the wings coupled with propulsion in the legs generates the minimum velocity required to achieve flight. This wing motion is thought to have evolved from asymmetrical propulsion flapping motion.[10] Thus, through these mechanisms, Archaeopteryx was able to achieve flight from the ground up.
Although the evidence in favor of this model is scientifically plausible, the evidence against it is substantial. For instance, a cursorial flight model would be energetically less favorable when compared to the alternative hypotheses. In order to achieve liftoff, Archaeopteryx would have to run faster than modern birds by a factor of three, due to its weight. Furthermore, the mass of Archaeopteryx versus the distance needed for minimum velocity to obtain liftoff speed is proportional, therefore, as mass increases, the energy required for takeoff increases. Other research has shown that the physics involved in cursorial flight would not make this a likely answer to the origin of avian flight. Once flight speed is obtained and Archaeopteryx is in the air, drag would cause the velocity to instantaneously decrease; balance could not be maintained due to this immediate reduction in velocity. Hence, Archaeopteryx would have a very short and ineffective flight. In contrast to Ostrom's theory regarding flight as a hunting mechanism, physics again does not support this model. In order to effectively trap insects with the wings, Archaeopteryx would require a mechanism such as holes in the wings to reduce air resistance. Without this mechanism, the cost/benefit ratio would not be feasible.
The decrease in efficiency when looking at the cursorial model is caused by the flapping stroke needed to achieve flight. This stroke motion needs both wings to move in a symmetrical motion, or together. This is opposed to an asymmetrical motion like that in humans' arms while running. The symmetrical motion would be costly in the cursorial model because it would be difficult while running on the ground, compared to the arboreal model where it is natural for an animal to move both arms together when falling. There is also a large fitness reduction between the two extremes of asymmetrical and symmetrical flapping motion so the theropods would have evolved to one of the extremes.[11] However, new research on the mechanics of bipedal running has suggested that oscillations produced by the running motion could induce symmetrical flapping of the wings at the natural frequency of the oscillation.[12]
Wing-assisted incline running
The
Birds use wing-assisted inclined running from the day they hatch to increase locomotion. This can also be said for birds or feathered theropods whose wing muscles cannot generate enough force to fly, and shows how this behavior could have evolved to help these theropods then eventually led to flight.[18] The progression from wing-assisted incline running to flight can be seen in the growth of birds, from when they are hatchlings to fully grown. They begin with wing-assisted incline running and slowly alter their wing strokes for flight as they grow and are able to make enough force. These transitional stages that lead to flight are both physical and behavioral. The transitions over a hatchling's life can be correlated with the evolution of flight on a macro scale. If protobirds are compared to hatchlings their physical traits such as wing size and behavior may have been similar. Flapping flight is limited by the size and muscle force of a wing. Even while using the correct model of arboreal or cursorial, protobirds' wings were not able to sustain flight, but they did most likely gain the behaviors needed for the arboreal or cursorial model like today's birds do when hatched. There are similar steps between the two.[19] Wing-assisted incline running can also produce a useful lift in babies but is very small compared to that of juveniles and adult birds. This lift was found responsible for body acceleration when going up an incline and leads to flight as the bird grows.[20]
Arboreal model
This model was originally proposed
The evolutionary path between arboreality and flight has been proposed through a number of hypotheses. Dudley and Yanoviak proposed that animals that live in trees generally end up high enough that a fall, purposeful or otherwise, would generate enough speed for aerodynamic forces to have an effect on the body. Many animals, even those which do not fly, demonstrate the ability to right themselves and face the ground ventrally, then exhibiting behaviors that act against aerodynamic forces to slow their rate of descent in a process known as parachuting.[22] Arboreal animals that were forced by predators or simply fell from trees that exhibited these kinds of behaviors would have been in a better position to eventually evolve capabilities that were more akin to flight as we know them today.
Researchers in support of this model have suggested that Archaeopteryx possessed skeletal features similar to those of modern birds. The first such feature to be noted was the supposed similarity between the foot of Archaeopteryx and that of modern perching birds. The
Synthesis
Some researchers have suggested that treating arboreal and cursorial hypotheses as mutually exclusive explanations of the origin of bird flight is incorrect.[25] Researchers in support of synthesizing cite studies that show incipient wings have adaptive advantages for a variety of functions, including arboreal parachuting, WAIR, and horizontal flap-leaping.[26] Other research also shows that ancestral avialans were not necessarily exclusively arboreal or cursorial, but rather lived on a spectrum of habitats. The capability for powered flight evolved due to a multitude of selective advantages of incipient wings in navigating a more complex environment than previously thought.[25]
See also
- Origin of birds
- Bird flight
- Flying and gliding animals
- Insect flight
- Tetrapteryx, a four-winged stage proposed by William Beebe; hindlimb feathers on Microraptor and Anchiornis have been interpreted as evidence of four-winged gliding.
Footnotes
- PMID 29535376.
- ^ Guarino, Ben (13 March 2018). "This feathery dinosaur probably flew, but not like any bird you know". The Washington Post. Retrieved 13 March 2018.
- S2CID 37866029.
- S2CID 4379208.
- PMID 17766290.
- PMID 22304966.
- ^ "Bird Anatomy & Bird Parts". All-Birds. Retrieved 9 April 2016.
- ^ PMC 1690052.
- JSTOR 27849060. Retrieved 14 November 2020.
- S2CID 29012467.
- S2CID 29012467.
- PMID 31048911.
- (Web). Scientists believe they could be a step closer to solving the mystery of how the first birds took to the air. BBC News. Retrieved 25 January 2008.
- S2CID 6323207.
- ^ Senter, P. (2006). "Scapular orientation in theropods and basal birds, and the origin of flapping flight" (Automatic PDF download). Acta Palaeontologica Polonica. 51 (2): 305–313.
- PMID 17488937.
- S2CID 7496862.)
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: CS1 maint: multiple names: authors list (link - S2CID 40712093.
- S2CID 15166485.
- PMID 17488937.
- ).
- ^ PMID 21558180. Archived from the original(PDF) on 2014-04-25. Retrieved 2014-04-24.
- ]
- S2CID 36259402.
- ^ S2CID 535424.
- . Retrieved 14 November 2020.
References
- Baier, D.B.; Gatesy, S.M.; Jenkins Jr, F.A. (2007). "A critical ligamentous mechanism in the evolution of avian flight". Nature. 445 (7125): 307–310. S2CID 4379208.
- Chatterjee, S. 1997. The Rise of Birds. The Johns Hopkins University Press. Baltimore. p. 150-151, 153, 158.
- Chatterjee, S.; Templin, R. J. (2002). "The flight of Archaeopteryx". Naturwissenschaften. 90 (1): 27–32. S2CID 25382695.
- Elzanowoski, A. 2000. "The Flying Dinosaurs." Ed. Paul, G. The Scientific American Book of Dinosaurs. p. 178.
- Feduccia, A. 1999. The Origin and Evolution of Birds. Yale University Press. London. p. 95, 97, 101, 103–104, 136.
- Garner, J.; Taylor, G.; Thomas, A. (1999). "On the origins of birds: the sequence of character acquisition in the evolution of avian flight". Proceedings of the Royal Society of London. Series B: Biological Sciences. 266 (1425): 1259–1266. PMC 1690052.
- Gill, F. 2007. Ornithology. W.H. Freeman and Company. New York. p. 25, 29, 40–41.
- Lewin, R (1983). "How did vertebrates take to the air?". Science. 221 (4605): 38–39. PMID 17738003.
- Morell, V (1993). "Archaeopteryx: early bird catches a can of worms". Science. 259 (5096): 764–765. PMID 17809336.
- Ostrom, J (1974). "Archaeopteryx and the origin of flight". The Quarterly Review of Biology. 49: 27–47. S2CID 85396846.
- Paul, G. 2002. Dinosaurs of the Air. The Johns Hopkins University Press. London. p. 134-135.
- Videler, J. 2005. Avian Flight. Oxford University Press. Oxford. P. 2, 91–98.
- Zhou, Z (2004). "The origin and early evolution of birds: discoveries, disputes, and Perspectives from fossil evidence". Naturwissenschaften. 91 (10): 455–471. S2CID 3329625.
External links
- Flight of the Archaeopteryx (journal article)
- Arboreal argument
- How Birds Got their wings Phys.org February 24 2023
- Origin of the propatagium in non-avian dinosaurs Zoological Letters February 23 2023