Bite‐force performance of the last rhynchocephalian (Lepidosauria:Sphenodon)

Abstract

We present the first empirical measurements of bite-force performance from adult Sphenodon (Rhynchocephalia), the only extant non-squamate lepidosaur. Using raw bite-force data, we calculated maximum bite forces at the anterior and posterior extremes of the lower tooth row: 81.8 N and 163.5 N (female) and 119.1 N and 238.3 N (male). Combining our results with published data from juvenile animals, we calculated scaling coefficients of bite force on linear morphometrics of body and head size as c. 2.7 (anterior) and c. 3.5+ (posterior). These exceed isometric scaling predictions (2.0), yet are similar to those for other non-avian reptiles. This supports previous views that Sphenodon cannot bite as hard as agamidlizards. We discuss the role of bite force in the behavioural ecology of Sphenodon, propose that the lower temporal bar, unique among extant lepidosaurs, does not necessarily constrain bite force, and evaluate possible effects of other morphological characteristics on bite-force performance.Keywords Rhynchocephalia; skull morphology; ontogeny; feeding; tuatara INTRODUCTIONOne of New Zealand's most iconic animals is the tuatara (Sphenodon) (Parkinson 2002). It is the largest endemic non-avian reptilian and has significant cultural importance for Maori (Sharell 1966; Mot 1997;Ramstad et al. 2007). Currently restricted to approximately 35 small islands and the subject of concerted conservation efforts Mlot 1997;Gaze 2001;Parkinson 2002;Mitchell et al. 2008), Sphenodon is of great scientific significance because it is the only extant member of the Rhynchocephalia, a group that was diverse and globally distributed during the Mesozoic (Apesteguia & Novas 2003;Evans 2003;Jones 2008;Jones et al. 2009). Being the closest living relative (sister group) to Squamata (lizards, snakes, amphisbaenians;Rest et al. 2003;Evans 2003), Sphenodon has been the subject of much research and has played an essential role as an outgroup taxon in comparative studies of squamate biology (e.g., Schwenk 1986Schwenk , 2000McBrayer & Reilly 2002;Vitt et al. 2003;Herrel et al. 2007;Evans 2008; Jones et al. in press).There has been a long-standing interest in numerous aspects of Sphenodon biology, including its dietary ecology and social behaviour, as well as the unique morphology of this taxon's feeding apparatus. The diet of Sphenodon is diverse and comprises ants, moths, caterpillars,

Comparison between in vivo and theoretical bite performance: Using multi-body modelling to predict muscle and bite forces in a reptile skull

“…Peak in vivo bite force measurements were recorded from an adult male Sphenodon (skull length ¼62 mm) housed at Dallas Zoo (Jones and Lappin, 2009). The animal was encouraged to bite a custom designed piezoelectric isometric force transducer (Type 9203, Kistler Inc., Switzerland), upon which bite point position and force magnitude were recorded.…”

“…The animal was encouraged to bite a custom designed piezoelectric isometric force transducer (Type 9203, Kistler Inc., Switzerland), upon which bite point position and force magnitude were recorded. Strips of leather were glued to the bite force plates to provide purchase and to avoid damage to the animal's teeth during the experiments (see Jones and Lappin, 2009). All bite trials were filmed in order to record the exact position of each bite along the tooth row, allowing bite forces obtained from different trials to be standardised and compared (Lappin and Husak, 2005;Jones and Lappin, 2009).…”

“…Here we assess whether this model can provide realistic predictions of bite force based on muscle force magnitudes calculated in a standard way from physiological cross-sectional area (PSCA) multiplied by muscle stress. Predicted bite force is compared to in vivo bite force data (Jones and Lappin, 2009). …”

Reliable quantification of bite-force performance requires use of appropriate biting substrate and standardization of bite out-lever

“…ImageJ) (Jones and Lappin, 2009). The fulcrum of the lever, the jaw joint, can be identified in some taxa with external landmarks.…”

“…In several studies multiple species have been compared, including some analyses of over 15 species Aguirre et al, 2002;van der Meij and Bout, 2004;Santana and Dumont, 2009;Vanhooydonck et al, 2010;Santana et al, 2010;Freeman and Lemen, 2010). The majority of research has been conducted on lepidosaurs (lizards and tuatara) (Herrel et al, 1999;Lappin and Husak, 2005;Anderson et al, 2008;Jones and Lappin, 2009;D'Amore et al, 2011;Cameron et al, 2013), in part because of their diversity of feeding and reproductive biology, as well as their experimental tractability.…”