227:(gnateaters). The trachea are covered in partly ossified rings known as tracheal rings. Tracheal rings tend to be complete, while the bronchial rings are C-shaped and the unossified part has smooth muscles running along them. The trachea are usual circular or oval in cross section in most birds but are flattened in ibises. The trachea is simple and tubular in ducks. The last few tracheal rings and the first few bronchial rings may fuse to form what is called the tympanic box. At the base of the trachea and at the joint of the bronchi a median dorsoventral structure, the pessulus, may be developed to varying extents. The pessulus is bony in passerines and provides attachment to membranes, anteriorly to the semilunar membranes. The membrane that forms part of the first three bronchial rings is responsible for vibrating and producing the sound in most passerines. These membranes may also be attached to the pessulus. In some species like the hill-myna,
348:, a measure of friction between a fluid and a vessel wall. In continuous breathers, such as birds and mammals, the trachea is exposed to fluctuations of wall shear stress during inspiration and expiration. In simulations with a simplified airway conducted by Kingsley et al. (2018), fluctuations in flow patterns led to localized wall shear stress, with the highest stress during exhalation at the tracheobronchial juncture. Localized stress may have provided selective pressure for an airway support located at the tracheobronchial juncture to maintain airway patency. Understanding whether these forces would have favored the evolution of soft tissue or cartilage requires further experimentation.
395:
exchange. Efficiency permits more “dead space” within the avian trachea, allowing the trachea to lengthen without a subsequent decrease in tracheal diameter. With a longer trachea, the avian vocal system shifted to a range in which an overlap between fundamental frequency and first tracheal resonance was possible. Without the critical tracheal length, mammals were unable to achieve an ideal length-frequency tracheal combination. At this point in avian evolution, it may have become advantageous to move the vocal structure upstream to the syringeal position, near the tracheobronchial juncture.
406:(shags). Longer necks likely predisposed Aves for syrinx evolution. Because of the correlation between neck length and tracheal length, birds are considered to have an “acoustically long trachea.” Technically, this refers to a tube where the lowest resonant frequency of a vibrating object (i.e. the syrinx) is four times longer than the length of the tube. A shorter tube would be less efficient; a longer tube would cause wave-form skewing. In most mammalian species and their therapsid ancestors, tracheal length was not sufficient to facilitate a boost in vocal efficiency.
441:
311:
arose after selection for acoustic function. Conversely, the larynx could have retained some vocal capabilities, though at a diminished capacity. The syrinx then evolved to supplement sound production, which would have been followed by the loss of the larynx as a sound source. The former scenario would have led to a “silent” period in the evolution of avian ancestors. The current fossil record does not provide definitive evidence for whether the function of the larynx was lost before the syrinx was gained.
352:
size, relative neck length, and larynx position relative to the hyoid apparatus (i.e. the bones that suspend the tongue and larynx) are all known to have changed across
Dinosauria evolution. Coupled with respiratory shifts, these characteristics may have favored syrinx evolution in birds. Distinct airway geometries in Mammalia and Archosauria may have also impacted syrinx evolution: the bronchi in crocodiles and humans, for example, diverge at different angles.
43:
98:
240:
252:
267:
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inertance at the input of the trachea. In songbirds, this is achieved by matching fundamental frequency with the first vocal tract resonance. Using physical and computational models, Riede et al. discovered that because of the dynamics between inertance and tracheal length, a structure in the syringeal position can be significantly more efficient than a structure in the laryngeal position.
233:, there is wide gap between the second and third bronchial semirings where large muscles are attached, allowing the inner diameter to be varied widely. Other muscles are also involved in syringeal control, these can be intrinsic or extrinsic depending on whether they are within the syrinx or attached externally. The extrinsic muscles include the sternotrachealis from the sternum.
410:
birds with leklike mating systems have evolved to use louder sounds and a wider range of frequencies during displays; wood warblers with higher trill performance have higher fitness. While the specific acoustic advantage of the ancestral syrinx remains speculative, it is evident from modern avian diversification that sexual selection often drives vocal evolution.
361:
respiration. Because of this, vibratory tissue precursors must have, at most, briefly predated the attachment of the first muscles to the trachea to clear the airway for respiratory function. Therefore, the two pairs of extrinsic muscles present in the ancestral syrinx were likely selected to ensure that the airway did not collapse during non-vocal respiration.
476:, whereas females have a smaller sized bulla. There are multiple key differences that distinguishes a male's syrinx from a female's. Males have a large bulla located on the left side of the trachea, and the tracheosyringeal rings that line the trachea are thicker in male mallards than in females. Within the trachea there is a structure called the
485:
decreases the force absorbed from the air moving through the syrinx, making a louder, higher pitched sound. On the other hand, males have a lot of fat and connective tissue in their bulla, which absorbs much more power from the moving air. This coupled with their thicker membranes leads to less vibrations and a duller, lower pitched sound.
374:
setting, vocal pressures must have been central to the morphological shift. Though these experiments do not account for the role of the syrinx in more metabolically challenging behaviors, such as flight, Reide et al. put forth a compelling theory about the selection for the syrinx in response to increased vocal efficiency.
386:
origin of Aves and during the late
Jurassic period, theropod-lineage dinosaurs underwent stature miniaturization and rapid diversification. It is possible that during these changes, certain co mbinations of body-size dependent vocal tract length and sound frequencies favored the evolution of the novel syrinx.
351:
Continuous breathing alone, however, would not have provided enough pressure for the development of the novel syrinx. Mammals also respire through continuous breathing, yet they did not evolve the novel structure. Additional structural components must therefore be considered in syrinx evolution. Body
327:
The archosaurian shift from larynx to syrinx must have conferred a selective advantage for crown birds, but the causes for this shift remain unknown. To complicate matters, the syrinx falls into an unusual category of functional evolution: arising from ancestors with a larynx-based sound source, the
186:
The position of the syrinx, structure and musculature varies widely across bird groups. In some groups the syrinx covers the lower end of the trachea and the upper parts of the bronchi in which case the syrinx is said to be tracheobronchial, the most frequent form and the one found in all songbirds.
409:
With bolstered vocal efficiency due to longer necks, the syrinx may have been retained in Aves by sexual selective forces. Acoustic communication is essential for courtship, territorial defense, and long-range communication, all of which greatly impact an organism's fitness. For example, polygynous
373:
While a need for structural support may have given rise to an organ at the tracheobronchial juncture, selection for vocal performance likely played a role in syrinx evolution. Riede et al. (2019) argue that because birds with deactivated syringeal muscles can breathe without difficulty within a lab
364:
Further fossil data and taxonomical comparisons will be necessary to determine whether structural modifications of the syrinx unrelated to sound, such as respiratory support during continuous breathing or in flight, were exapted in the development of a vocal organ. Additionally, further research on
360:
Additionally, syrinx musculature was necessarily selected for maintaining respiratory function. Because sound is produced through the interaction of airflow and the self-oscillation of membranes within the trachea, a mechanism is necessary to abduct structures from the airway to allow for non-vocal
328:
syrinx contains significant functional overlap with the structure it replaced. In fact, there is no evidence that an original, simplified syrinx could produce calls with a larger frequency range or longer or louder calls than an alligator-like larynx, which would have potentially increased fitness.
314:
The fossil record does, however, provide clues for the evolutionary timeline of some syringeal elements. For example, increased mineralization at the tracheobronchial juncture is likely a late-arising feature in avian evolution. Despite new discoveries of preserved avian tracheobronchial rings from
484:
The nature of the sounds produced by males and females are different due to these differences in the syrinx. Females have a louder call because the space inside their bulla is not lined with a lot of fat or connective tissue, and the thinner tympaniform membrane takes less effort to vibrate. This
385:
Efficiency, however, is influenced significantly by non-linear interactions of trachea length, phonation threshold pressure, and frequency. Riede et al. therefore conclude that the evolution of a simple syrinx may be tied to specific combinations of vocal fold morphology and body size. Before the
377:
This theory involves vocal tract length and the dynamics of airflow. While both the larynx and the syrinx produce sound through the interaction of airflow and self-oscillating valves, the syrinx is located deeper in the respiratory tract than the larynx. This is a critical distinction between the
394:
Diversification in theropod stature may explain why birds alone capitalized on the efficiency of the novel structure. Importantly, birds generally have longer necks than mammals. This distinction is due to the unidirectional flow of the avian respiratory system, which increases efficiency of gas
310:
There is uncertainty about the relationship between the larynx and syrinx during this morphological shift, but there are two predominant evolutionary possibilities: regimes unrelated to sound production could have led to the loss in vocal function of the larynx. A new structure, the syrinx, then
381:
Inertance must be considered alongside frequency—when a tube is lengthened beyond a quarter wavelength, standing waves interfere with sound production. Thus, acoustic theory predicts that to maximize energy transfer, birds must develop an appropriate length-frequency combination that produces
306:
from the same epoch. Before this discovery, syringeal components were thought to enter the fossil record infrequently, making it difficult to determine when the shift in vocal organs occurred. An intact specimen from the late
Cretaceous, however, highlights the fossilization potential of the
480:
that divides the trachea in half where the two bronchus branch out. The pessulus is ossified, and lined with tympaniform membranes that influence the sound production depending on its thickness when the air runs past the pessulus, causing vibrations. The membranes in males are thick and
152:, caused by air flowing through the syrinx. This sets up a self-oscillating system that modulates the airflow creating the sound. The muscles modulate the sound shape by changing the tension of the membranes and the bronchial openings. The syrinx enables some species of birds (such as
340:
Due to airway bifurcation, the tracheobronchial juncture was present at the origin of multiple lungs in tetrapods. In bird-lineage archosaurs with bifurcated airways, the evolution of an increased metabolic rate and continuous breathing exposed airway walls to altered amounts of
299:
Within the avian stem lineage, the transition from a larynx-based sound source to a tracheobronchial syrinx occurred within
Dinosauria, at or before the origin of Aves about 66-68 million years ago. The earliest fossilized record of syringeal remains is from a single specimen of
378:
structures, as the length of the air column above and below a sound source affects the way energy is conveyed from airflow to oscillating tissue. The longer and narrower the tube, the greater the inertance (i.e. the “sluggishness” of air) and the easier it is to produce sound.
429:, have distinctly different syrinxes between males and females. This difference is significant given that sexing birds is difficult at younger stages. Birds that exhibit sexual dimorphism in the syrinx can present itself at around 10 days in Pekin ducks
336:
While the evidence is limited, selection for non-acoustic characteristics, such as structural support and respiratory function, may have contributed to the evolution of a syrinx-like structure at the tracheobronchial juncture.
449:
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the
Cenozoic, these structures have not been recovered from Mesozoic archosaurs. This might be a product of weak mineralization in the bronchi and trachea of Mesozoic archosaurs, a condition which would inhibit
319:. Thus, a shift towards a mineralized structure may have been preceded by many key avian adaptations, including respiratory shifts, increases in metabolic rates, and feather ornamentation.
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Selection for long necks, while highly variable, is often driven by beneficial feeding adaptations. Specifically, long necks facilitate underwater predation, evident in the extant genera
199:. The syrinx may also be restricted to the trachea and this is found in a very small number of bird groups that are sometimes known as tracheophonae, a subset of the
421:
leads to different syrinxes in birds, and the degree of differences varies. Some species do not present differences between sexes while others, like the mallard
1534:
Wilson, Robert E.; Sonsthagen, Sarah A.; Franson, J. Christian (August 2013). "Sex
Determination of Duck Embryos: Observations on Syrinx Development".
1478:
Frank, T.; Probst, A.; König, H. E.; Walter, I. (April 2007). "The Syrinx of the Male
Mallard (Anas platyrhynchos): Special Anatomical Features".
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Frank, T.; Walter, I.; Probst, A.; König, H. E. (December 2006). "Histological
Aspects of the Syrinx of the Male Mallard (Anas platyrhynchos)".
988:
645:
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74:
89:, 5: membrana tympaniformis lateralis, 6: membrana tympaniformis medialis, 7: second group of syringeal rings, 8:
1006:"Airway wall stiffening increases peak wall shear stress: a fluid-structure interaction study in rigid and compliant airways"
183:, lack a syrinx and communicate through throaty hisses. Birds do have a larynx, but unlike in mammals, it does not vocalize.
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172:
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tetrapod tracheas is necessary to understand potential constraints in the evolution of unique airway morphologies.
239:
1432:
Goncalo, C.C. (2014). "Birdsong performance and the evolution of simple (rather than elaborate) sexual signals".
599:
Suthers, R.A. (October 1990). "Contributions to birdsong from the left and right sides of the intact syrinx".
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316:
251:
117:
1621:
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Tenney, S.M.; Bartlett, D (1967). "Comparative quantitative morphology of the mammalian lung: trachea".
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is possible, with muscles on the left and right branch modulating vibrations independently so that some
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1290:"Similarity of Crocodilian and Avian lungs indicates unidirectional flow is ancestral for archosaurs"
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ancestral structure and may indicate that the syrinx is a late-arising feature in avian evolution.
1231:"A Middle Jurassic abelisaurid from Patagonia and the early diversification of theropod dinosaurs"
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646:"Systematic relationships and biogeography of the tracheophone suboscines (Aves: Passeriformes)"
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The syrinx may be restricted to the bronchi as in some non-passerines, notably the
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in mammals, the syrinx is located where the trachea forks into the lungs. Thus,
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Proceedings of the
National Academy of Sciences of the United States of America
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Proceedings of the National Academy of Sciences of the United States of America
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1172:"Songbirds tune their vocal tract to the fundamental frequency of their song"
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Large bulla in a male (left) and smaller bulla structure in a female (right).
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can produce more than one sound at a time. Some species of birds, such as
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nontransparent, but the females have thinner, sheer membranes.
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1366:"Understanding selection for long necks in different taxa"
531:
The Audubon Society Encyclopedia of North American Birds
1235:
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528:
922:"The evolution of the syrinx: an acoustic theory"
390:The evolution of neck length and sexual selection
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973:The Biology of the Avian Respiratory System
734:"Identity and novelty in the avian syrinx"
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472:Male ducks have a large tracheal bulla
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332:Selection for tracheobronchial support
101:Syrinx (serial 5) seen just below the
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138:of mammals. The sound is produced by
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560:Larsen, O. N.; Franz Goller (2002).
78:Schematic drawing of an avian syrinx
1294:Integrative and Comparative Biology
566:The Journal of Experimental Biology
700:10.1111/j.1469-7998.1972.tb01353.x
146:(the walls of the syrinx) and the
130:. Located at the base of a bird's
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1574:Anatomia, Histologia, Embryologia
134:, it produces sounds without the
1586:10.1111/j.1439-0264.2006.00701.x
1548:10.3184/175815513X13739900273488
1492:10.1111/j.1439-0264.2006.00737.x
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369:Selection for vocal efficiency
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665:10.1016/S1055-7903(02)00034-9
1343:10.1016/0034-5687(67)90002-3
939:10.1371/journal.pbio.2006507
433:Anas platyrhynchos domestica
981:10.1007/978-3-319-44153-5_3
452:1: last free cartilaginous
203:passeriformes that include
93:, 9: bronchial cartilage
81:1: last free cartilaginous
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1137:Journal of Neurophysiology
1122:Form and Function in Birds
1059:Royal Society Open Science
164:) to mimic human speech.
29:
1022:10.1007/s10439-010-9956-y
468:, 9: bronchial cartilage.
356:Selection for musculature
284:Suboscines and a shoebill
50:This article needs to be
1364:Wilkinson, D.M. (2012).
1149:10.1152/jn.1996.76.1.287
295:An evolutionary timeline
126:) is the vocal organ of
27:The vocal organ of birds
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290:Evolution of the syrinx
245:The syrinx of hornbills
32:Syrinx (disambiguation)
1536:Avian Biology Research
1331:Respiration Physiology
1248:10.1098/rspb.2012.0660
847:Birdsong and Evolution
469:
445:
323:Evolutionary causation
317:preservation potential
144:membrana tympaniformis
142:of some or all of the
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1288:Farmer, C.G. (2015).
845:ten Cate, C. (2004).
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578:10.1242/jeb.205.1.25
219:(typical antbirds),
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1080:10.1098/rsos.150636
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615:1990Natur.347..473S
474:(bulla syringealis)
215:(ground antbirds),
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920:Riede, T. (2019).
872:(6215): e1253293.
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181:New World vultures
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609:(6292): 473–477.
419:Sexual dimorphism
414:Sexual dimorphism
230:Gracula religiosa
223:(tapaculos), and
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217:Thamnophilidae
173:lateralization
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1627:Bird sounds
849:. Elsevier.
272:The ostrich
205:Furnariidae
167:Unlike the
136:vocal folds
1616:Categories
140:vibrations
112:(from the
1462:205997942
505:Whistling
495:Bird call
404:Cormorant
201:suboscine
197:nightjars
177:songbirds
123:pan pipes
1602:22651510
1594:17156094
1556:84204362
1508:43746431
1500:17371385
1454:22030736
1392:22171805
1316:26141868
1275:10473833
1267:22628475
1216:16567614
1107:15408254
1099:27069652
1040:20162357
958:30730882
926:PLOS ONE
894:24228777
886:25504729
824:27732575
778:30249637
673:12099801
586:11818409
527:(1980).
489:See also
478:pessulus
466:bronchus
462:pessulus
458:tympanum
149:pessulus
87:pessulus
1400:6176477
1351:6058337
1258:3385738
1207:1459391
1184:Bibcode
1157:8836225
1090:4821263
1067:Bibcode
1031:3034653
949:6366696
866:Science
832:4389926
804:Bibcode
769:6187200
746:Bibcode
631:4351834
611:Bibcode
193:cuckoos
154:parrots
132:trachea
52:updated
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602:Nature
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400:Cygnus
169:larynx
160:, and
120:" for
118:σύριγξ
116:word "
110:syrinx
1598:S2CID
1552:S2CID
1504:S2CID
1458:S2CID
1396:S2CID
1271:S2CID
1103:S2CID
890:S2CID
828:S2CID
649:(PDF)
627:S2CID
511:Notes
456:, 2:
343:wall
162:mynas
158:crows
128:birds
114:Greek
1590:PMID
1496:PMID
1450:PMID
1388:PMID
1347:PMID
1312:PMID
1263:PMID
1212:PMID
1153:PMID
1095:PMID
1036:PMID
985:ISBN
954:PMID
882:PMID
820:PMID
774:PMID
669:PMID
582:PMID
541:ISBN
257:The
195:and
189:owls
108:The
103:crop
1582:doi
1544:doi
1488:doi
1442:doi
1438:178
1419:287
1378:doi
1339:doi
1302:doi
1253:PMC
1243:doi
1239:279
1202:PMC
1192:doi
1180:103
1145:doi
1085:PMC
1075:doi
1026:PMC
1018:doi
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944:PMC
934:doi
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812:doi
800:538
764:PMC
754:doi
742:115
696:doi
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661:doi
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574:doi
570:205
537:995
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