Functional Anatomy of Incisal Biting in <i>Aplodontia rufa</i> and Sciuromorph Rodents – Part 1: Masticatory Muscles, Skull Shape and Digging

Abstract

Traditionally, rodents have been grouped into suborders distinguished largely on the basis of characteristics of the jaw adductor muscles and other features of the masticatory apparatus. The three classic suborders are: Sciuromorpha (squirrels), Myomorpha (rats and mice), and Hystricomorpha (porcupines and the South American caviomorph rodents). Protrogomorph rodents are thought to represent the primitive condition of rodent masticatory muscles. Aplodontia rufa, the mountain beaver, is the only living protrogomorphous rodent. The present work is a detailed comparison of the masticatory apparatus in A. rufa and Marmota monax, the woodchuck. But the mandibular region of A. rufa appears remarkable, unlike anything found in other rodents. Is A. rufa a reasonable representative of the primitive, protrogomorphous condition? A.rufa is a member of the aplodontoid-sciuroid clade with a wide and flat skull. The large temporalis and mandibular apophyses of A. rufa are features related to its relatively wide skull. Such features are found in less dramatic forms in other sciuromorphous species and the basic arrangement of the masticatory muscles of A. rufa is similar to the arrangement seen in sciuromorphs.

Digital dissection of the masticatory muscles of the naked mole-rat,Heterocephalus glaber(Mammalia, Rodentia)

“…As a proportion of total jaw adductor muscle mass (32%), it is very large compared to other species in the Hystricognathi e.g., Cavia (11%: Cox & Jeffery, 2011), Hydrochoerus (5%: Müller, 1933) and Hystrix (17%: Turnbull, 1970). However, it should be noted that temporalis muscles of similar relative size have been recorded in Aplodontia rufa (34%) and many sciuromorphs (25%–30%) by Druzinsky (2010), and may in fact be the primitive condition for rodents. The highly reduced size of the eye in the naked mole-rat may have partly facilitated the increase in size of the temporalis, enabling its anterior expansion into the relatively unoccupied orbit (Lavocat, 1973).…”

“…The highly reduced size of the eye in the naked mole-rat may have partly facilitated the increase in size of the temporalis, enabling its anterior expansion into the relatively unoccupied orbit (Lavocat, 1973). It has also been suggested that a large temporalis may result from the widening and flattening of the skull seen in fossorial species (Druzinsky, 2010). Such a large temporalis is likely to be acting as a powerful elevator of the jaw (Hiiemae, 1971), producing substantial forces at the teeth.…”

“…In addition, to understand how naked mole-rat masticatory muscles are similar to or differ from rodents more generally, the results here were compared to published descriptions of other rodents including sciuromorphs, myomorphs and hystricomorphs, as well as the only living protrogomorph, Aplodontia rufa . The literature consulted was as follows: Müller, 1933 ( Hydrochoerus ); Greene, 1935 ( Rattus ); Schumacher & Rehmer, 1962 ( Cavia , Rattus ); Wood, 1965 ( Marmota , Myocastor , Ondatra ); Turnbull, 1970 ( Sciurus , Rattus , Hystrix ); Woods, 1972 ( Proechimys , Echimys , Isothrix , Mesomys , Myocastor , Octodon , Ctenomys , Erethizon , Cavia , Chinchilla , Dasyprocta , Thryonomys , Petromus ); Weijs, 1973 ( Rattus ); Woods & Howland, 1979 ( Capromys , Geocapromys , Plagiodontia , Myocastor ); Woods & Hermanson, 1985 ( Capromys , Geocapromys , Plagiodontia , Myocastor , Echimys , Octodon , Erethizon , Coendou , Dasyprocta , Atherurus , Thryonomys , Petromus ); Offermans & De Vree, 1989 ( Pedetes ); Ball & Roth, 1995 ( Sciurus , Microsciurus , Sciurillus , Tamiasciurus , Tamias , Glaucomys ); Thorington & Darrow, 1996 ( Aplodontia , Paraxerus , Funisciurus , Myosciurus , Heliosciurus , Protoxerus , Funambulus , Calliosciurus , Tamiops , Xerus , Atlantoxerus , Ratufa ); Olivares, Verzi & Vassallo, 2004 ( Aconaemys , Octomys , Tympanoctomys , Spalacopus , Octodon , Octodontomys ); Satoh & Iwaku, 2004 ( Cricetulus , Mesocricetus , Phodopus , Tscherkia ); Satoh & Iwaku, 2006 ( Onychomys ); Satoh & Iwaku, 2009 ( Neotoma , Peromyscus ); Druzinsky, 2010 ( Aplondontia , Cynomys , Tamias , Marmota , Ratufa , Sciurus , Thomomys ); Hautier & Saksiri, 2009 ( Laonastes ); Hautier, 2010 ( Ctenodactylus ); Cox & Jeffery, 2011 ( Cavia , Rattus , Sciurus ); Baverstock, Jeffery & Cobb, 2013 ( Mus ); Becerra et al, 2014 ( Chinchilla , Ctenomys , Octodon ).…”

Functional Anatomy of Incisal Biting in <i>Aplodontia rufa</i> and Sciuromorph Rodents – Part 2: Sciuromorphy Is Efficacious for Production of Force at the Incisors

“…Five woodchucks (M. monax) and eight mountain beavers (A. rufa) obtained for the study [Druzinsky, 1989[Druzinsky, , 1995[Druzinsky, , 2010 were dissected. Single specimens from 5 sciuromorphous species, from the collections of the Field Museum of Natural History, Chicago, were also dissected [Druzinsky, 2010, table 1].…”

“…Single specimens from 5 sciuromorphous species, from the collections of the Field Museum of Natural History, Chicago, were also dissected [Druzinsky, 2010, table 1]. Skulls and mandibles of each species dissected were studied from the collections of the Department of Oral Anatomy, the FMNH, and the Division of Mammals, American Museum of Natural History, New York [Druzinsky, 2010, …”