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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 specialisation 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 muscle 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\u2010sectional area [PCSA]) and investigate their scaling in juvenile <jats:italic>Alligator mississippiensis<\/jats:italic> spanning an order of magnitude in body mass (359\u00a0g\u20135.5\u00a0kg). Comparatively, the expiratory muscles (<jats:italic>transversus abdominis<\/jats:italic>, <jats:italic>rectus abdominis<\/jats:italic>, <jats:italic>iliocostalis<\/jats:italic>), which compress the trunk, have greater relative PCSA being specialised for greater force\u2010generating capacity, while the inspiratory muscles (<jats:italic>diaphragmaticus<\/jats:italic>, <jats:italic>truncocaudalis ischiotruncus<\/jats:italic>, <jats:italic>ischiopubis<\/jats:italic>), which create negative internal pressure, have greater relative fascicle lengths, being adapted for greater working range and contraction velocity. Fascicle lengths of the accessory <jats:italic>diaphragmaticus<\/jats:italic> 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 <jats:italic>iliocostalis<\/jats:italic>, an accessory expiratory 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\u2010generating capacity. Conversely, the basal tetrapod expiratory pump (<jats:italic>transversus abdominis<\/jats:italic>) scaled isometrically, which may indicate a decreased reliance on this muscle with ontogeny. Collectively, these findings would support existing anecdotal evidence that crocodilians shift their breathing mechanics as they increase in size. Furthermore, the functional specialisation of the <jats:italic>diaphragmaticus<\/jats:italic> and compliance of the body wall in the lumbar region against which it works may contribute to low\u2010cost breathing in crocodilians.<\/jats:p>","DOI":"10.1111\/joa.13523","type":"journal-article","created":{"date-parts":[[2021,7,24]],"date-time":"2021-07-24T01:45:43Z","timestamp":1627091143000},"page":"1273-1286","update-policy":"https:\/\/doi.org\/10.1002\/crossmark_policy","source":"Crossref","is-referenced-by-count":4,"title":["Scaling of axial muscle architecture in juvenile <i>Alligator mississippiensis<\/i> reveals an enhanced performance capacity of accessory breathing mechanisms"],"prefix":"10.1111","volume":"239","author":[{"given":"Kayleigh A. R.","family":"Rose","sequence":"first","affiliation":[{"name":"Department of Biosciences College of Science Swansea University Wales UK"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Peter G.","family":"Tickle","sequence":"additional","affiliation":[{"name":"School of Biomedical Sciences Faculty of Biological Sciences University of Leeds Leeds UK"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Ruth M.","family":"Elsey","sequence":"additional","affiliation":[{"name":"Louisiana Department of Wildlife and Fisheries Rockefeller Wildlife Refuge Grand Chenier LA USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-2913-5406","authenticated-orcid":false,"given":"William I.","family":"Sellers","sequence":"additional","affiliation":[{"name":"Department of Earth and Environmental Sciences Faculty of Science and Engineering University of Manchester Manchester UK"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"suffix":"II","given":"Dane A.","family":"Crossley","sequence":"additional","affiliation":[{"name":"Department of Biological Sciences University of North Texas Denton TX USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-0211-1786","authenticated-orcid":false,"given":"Jonathan R.","family":"Codd","sequence":"additional","affiliation":[{"name":"Faculty of Biology, Medicine and Health University of Manchester Manchester 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