How hard can a tuatara bite?

By Hilary Miller 06/01/2010

ResearchBlogging.orgAs a geneticist, I’m only rarely let out of the lab to chase after my study animal, the tuatara.  I count these occasions as a gift, where I get to feel like a “real” biologist and learn to talk knowledgably about the ecology and habits of tuatara (which, lets face it, are generally of more interest to the lay person than their genes).  I also count myself lucky that I’ve never been bitten by a tuatara – although I have helped extract other people’s fingers from the mouths of tuatara and can confirm that it is an eye-watering experience.

We now know exactly how hard a tuatara can bite, thanks to a recent study published in the Journal of the Royal Society of New ZealandMarc Jones (University College London) and Kristopher Lappin (California State Polytechnic University) have measured bite force in adult tuatara and found that a male tuatara could produce a bite force of up to 238 Newtons.  Jones and Lappin measured bite force using a custom-designed isometric force transducer.  They report that the tuatara needed little encouragement to bite onto the leather-covered bite plates, and that “once biting commenced the tuatara would maintain its grip with considerable reluctance to release”.  Something that will come as no surprise to those who have been on the receiving end of a tuatara bite!

When comparing their results with previously published data from juvenile tuatara, Jones and Lappin discovered that adult tuatara bite proportionately harder than juveniles, even when the difference in skull size between adults and juveniles is accounted for.   This phenomenon has been observed in many other reptiles, but the reason for it is unclear. 

Knowing how hard a tuatara can bite is of more than just academic interest to researchers interested in knowing how much it will hurt when they get bitten.  Bite force is linked to many aspects of behaviour, influencing, for instance, the range of potential food items that an animal can consume, and the outcome of competitive interactions.  In some lizard species, bite force is a better predictor of territory size and reproductive success than body size.   Although yet to be confirmed in field studies, this may also be the case for tuatara, as males in particular aggressively compete for territories and access to females.

It appears that tuatara cannot bite as hard as agamid lizards of the same size, and Jones and Lappin speculate that agamid lizards could have out-competed rhynchocephalians in the late Mesozoic, contributing to their demise everywhere except New Zealand.  However, they point out that “it is unknown how the bite force of the modern day tuatara compares to that of Mesozoic rhynchocephalians, which were very diverse in terms of their tooth shapes and skull structure. Therefore it is unlikely that all but one rhynchocephalian lineage went extinct because of prey competition with a single group of lizards”.  

Marc E. H. Jones, & A. Kristopher Lappin (2009). Bite-force performance of the last rhynchocephalian Journal of the Royal Society of New Zealand, 39 (3), 71-83