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Scaling of axial muscle architecture in juvenile Alligator mississippiensis reveals an enhanced performance capacity of accessory breathing mechanisms
Journal of Anatomy, Volume: 239, Issue: 6
Swansea University Author: Kayleigh Rose
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DOI (Published version): 10.1111/joa.13523
Quantitative functional anatomy of amniote thoracic and abdominal regions is cru-cial to understanding constraints on and adaptations for facilitating simultaneous breathing and locomotion. Crocodilians have diverse locomotor modes and variable breathing mechanics facilitated by basal and derived (a...
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Quantitative functional anatomy of amniote thoracic and abdominal regions is cru-cial to understanding constraints on and adaptations for facilitating simultaneous breathing and locomotion. Crocodilians have diverse locomotor modes and variable breathing mechanics facilitated by basal and derived (accessory) muscles. However, the inherent flexibility of these systems is not well studied, and the functional spe-cialisation of the crocodilian trunk is yet to be investigated. Increases in body size and trunk stiffness would be expected to cause a disproportionate increase in mus-cle force demands and therefore constrain the basal costal aspiration mechanism, necessitating changes in respiratory mechanics. Here, we describe the anatomy of the trunk muscles, their properties that determine muscle performance (mass, length and physiological cross- sectional area [PCSA]) and investigate their scaling in juvenile Alligator mississippiensis spanning an order of magnitude in body mass (359 g– 5.5 kg). Comparatively, the expiratory muscles (transversus abdominis, rectus abdominis, ili-ocostalis), which compress the trunk, have greater relative PCSA being specialised for greater force- generating capacity, while the inspiratory muscles (diaphragmaticus, truncocaudalis ischiotruncus, ischiopubis), which create negative internal pressure, have greater relative fascicle lengths, being adapted for greater working range and contrac-tion velocity. Fascicle lengths of the accessory diaphragmaticus scaled with positive allometry in the alligators examined, enhancing contractile capacity, in line with this muscle's ability to modulate both tidal volume and breathing frequency in response to energetic demand during terrestrial locomotion. The iliocostalis, an accessory expira-tory muscle, also demonstrated positive allometry in fascicle lengths and mass. All accessory muscles of the infrapubic abdominal wall demonstrated positive allometry in PCSA, which would enhance their force- generating capacity. Conversely, the basal tetrapod expiratory pump (transversus abdominis) scaled isometrically, which may in-dicate a decreased reliance on this muscle with ontogeny. Collectively, these find-ings would support existing anecdotal evidence that crocodilians shift their breathing mechanics as they increase in size. Furthermore, the functional specialisation of the diaphragmaticus and compliance of the body wall in the lumbar region against which it works may contribute to low- cost breathing in crocodilians
allometry, archosaur, axial anatomy, breathing, crocodilian, flexibility, locomotion, muscle architecture, ventilatory mechanics
Faculty of Science and Engineering
This research was supported by fund-ing from the National Science Foundation (NSF- 17565187) to DAC and the Biotechnology and Biological Sciences Research Councils (BBSRC– BB/I021116/1) to JRC.