Beavers are well-known for their ability to fell large trees through gnawing. Yet, despite this impressive behavior, little information exists on their masticatory musculature or the biomechanics of their jaw movements. It was hypothesized that beavers would have a highly efficient arrangement of the masticatory apparatus, and that gnawing efficiency would be maintained at large gape. The head of an American beaver, Castor canadensis, was dissected to reveal the masticatory musculature. Muscle origins and insertions were noted, the muscles were weighed and fiber lengths measured. Physiological cross-sectional areas were determined, and along with the muscle vectors, were used to calculate the length of the muscle moment arms, the maximum incisor bite force, and the proportion of the bite force projected along the long axis of the lower incisor, at occlusion and 30° gape. Compared to other sciuromorph rodents, the American beaver was found to have large superficial masseter and temporalis muscles, but a relatively smaller anterior deep masseter. The incisor bite force calculated for the beaver (550–740 N) was much higher than would be predicted from body mass or incisor dimensions. This is not a result of the mechanical advantage of the muscles, which is lower than most other sciuromorphs, but is likely related to the very high percentage (>96 %) of bite force directed along the lower incisor long axis. The morphology of the skull, mandible and jaw-closing muscles enable the beaver to produce a very effective and efficient bite, which has permitted beavers to become highly successful ecosystem engineers.
Beaver Dissection Masticatory muscles Bite force Rodent
This is a preview of subscription content, log in to check access.
The authors thank Dr Andrew Kitchener of National Museums Scotland for providing the beaver specimen, and Mrs Sue Taft from the Department of Engineering, University of Hull for use of her dermestid beetle colony. Thanks are also due to Gwen Haley and the staff of the X-ray department at The York Hospital for CT scanning the skull and mandible. We are grateful to two anonymous reviewers for their helpful comments.
Blanga-Kanfi S, Miranda H, Penn O, Pupko T, DeBry RW, Huchon D (2009) Rodent phylogeny revised: analysis of six nuclear genes from all major rodent clades. BMC Evol Biol 9: 71CrossRefPubMedPubMedCentralGoogle Scholar
Brandt JF (1855) Beiträge zur nähern Kenntniss der Säugethiere Russlands. Mém Acad Imp Sci St Pétersbourg, Sér 6 9: 1–375Google Scholar
Caspari E (2003) Animal Life in Nature, Myth and Dreams. Chiron Publications, Hendersonville, NCGoogle Scholar
Cox PG, Jeffery N (2011) Reviewing the morphology of the jaw-closing musculature in squirrels, rats and guinea pigs with contrast-enhanced microCT. Anat Rec 294: 915–928CrossRefGoogle Scholar
Cox PG, Kirkham J, Herrel A (2013) Masticatory biomechanics of the Laotian rock rat, Laonastes aenigmamus, and the function of the zygomaticomandibularis muscle. PeerJ 1: e160CrossRefPubMedPubMedCentralGoogle Scholar
Druzinsky RE (2010a) Functional anatomy of incisal biting in Aplodontia rufa and sciuromorph rodents – Part 1: Masticatory muscles, skull shape and digging. Cells Tissues Organs 191: 510–522CrossRefPubMedPubMedCentralGoogle Scholar
Druzinsky RE (2010b) Functional anatomy of incisal biting in Aplodontia rufa and sciuromorph rodents – Part 2: Sciuromorphy is efficacious for production of force at the incisors. Cells Tissues Organs 192: 50–63CrossRefPubMedPubMedCentralGoogle Scholar
Druzinsky RE (2015) The oral apparatus of rodents: variations on the theme of a gnawing machine. In: Cox PG, Hautier L (eds) Evolution of the Rodents: Advances in Phylogeny, Functional Morphology and Development. Cambridge University Press, Cambridge, pp 323–349CrossRefGoogle Scholar
Freeman PW, Lemen CA (2008) A simple morphological predictor of bite force in rodents. J Zool 275: 418–422CrossRefGoogle Scholar
Herrel A, Spithoven L, Van Damme R, De Vree F (1999) Sexual dimorphism of head size in Gallotia galloti: testing the niche divergence hypothesis by functional analyses. Funct Ecol 13: 289–297CrossRefGoogle Scholar
van Spronsen PH, Weijs WA, Valk J, Prahl-Andersen B, van Ginkel FC (1989) Comparison of jaw-muscle bite-force cross-sections obtained by means of magnetic resonance imaging and high-resolution CT scanning. J Dent Res 68: 1765–1770CrossRefPubMedGoogle Scholar
Waterhouse GR (1839) Observations on the Rodentia with a view to point out groups as indicated by the structure of the crania in this order of mammals. Ann Mag Nat Hist 3: 90–96, 184–188, 274–279, 593–600Google Scholar
Woods CA (1972) Comparative myology of jaw, hyoid, and pectoral appendicular regions of New and Old World hystricomorph rodents. Bull Am Mus Nat Hist 147: 115–198Google Scholar
Woods CA, Hermanson JW (1985) Myology of hystricognath rodents: an analysis of form, function and phylogeny. In: Luckett WP, Hartenberger J-L (eds) Evolutionary Relationships among Rodents: A Multidisciplinary Analysis. Plenum Press, New York, pp 685–712Google Scholar
Woods CA, Howland EB (1979) Adaptive radiation of capromyid rodents: anatomy of the masticatory apparatus. J Mammal 60: 95–116CrossRefGoogle Scholar
Wright JP, Jones CG, Flecker AS (2002) An ecosystem engineer, the beaver, increases species richness at the landscape scale. Oecologia 132: 96–101CrossRefGoogle Scholar