Lizard lifestyle has no impact on their biting force

When lizards fight or feed, the amount of biting force they use is closely related to their size.

Despite the wide range of skull shapes, tooth types, lifestyles, and habitats of lizards and related reptiles, research published by scientists at the Museum has found that size is the best predictor of how forcefully the animals can bite. 

This link could have implications for what we know about the evolution of lizards, suggesting selection on their body size is linked closely to selection on bite force.

It might also help us to predict the abilities of their ancient relatives, and perhaps even other reptiles such as dinosaurs. 

Dr Natalie Cooper, senior researcher at the Museum and co-author of the study, says, 'There's huge variety in how forcefully different lizards can bite. It might seem obvious that bigger ones bite more forcefully, but lots of previous work has theorised that other traits, like diet and lifestyle, should be equally important. However, we don't find any evidence for this.'

The scientists have called for more research to expand upon their findings, published in Proceedings of the Royal Society B, to assess if what they found holds true for other lizard species.  

How does biting affect evolution?

Aside from eating, lizards use their mouth for a variety of purposes, including fighting, catching prey and defending mates. Having greater bite force might allow a lizard to tackle prey and eat food it otherwise could not, making biting force an important part of natural selection. 

Co-author Dr Marc Jones explains, 'In the past, people have used sprint speed and running endurance as a measure of an animal's performance, but bite force is now becoming important too.

'It is a physiological behaviour that can be quantified and related to various aspects of ecology that are related to survival. It's important to the animal as an individual, but also the ecology of its species.'

While studies have been carried out on the bite of individual species, or small groups, a wider look at how bite force varies across a major group hadn’t been carried out. This research looked to change that. 

The researchers decided to focus their analysis on a group of reptiles known as the Lepidosauria, which includes lizards, limbless reptiles known as amphisbaenians and lizard-like animals called tuatara. The group also includes snakes, but these were excluded from the study as few studies have sampled the bite force of these animals.

Lead author Justin Isip, a PhD student at the Museum, says, 'The lepidosaurs are interesting as they show greater variation in diet than most people assume and even include some unusual specialised species that that feed on a diet of snails.

'There has also been a lot of previous research into bite force in this group for us to build on.' 

Ahead of their research, the scientists predicted that bite force would be greater in larger species, herbivores, non-burrowers and animals with teeth attached firmly to the jaw bone's crest. Following its completion, they found that only size was borne out in the data. 

Does lifestyle affect bite force?

Using over 20 years of data, the researchers looked at how bite force was affected by different traits such as diet, head dimensions and species.

This was used to create a series of models, which could then be used to test out the predictions they had made.

Overall, the main differences in bite force were due to size. Larger animals have larger jaw muscles, increasing the force they can exert with their mouth relative to their body size.

The relationship even held out for the herbivores in the sample, which was unexpected as previous studies suggested they had more forceful bites than both carnivores and omnivores.

'I think the answer for the herbivory question was interesting because it's discussed a lot in scientific literature,' Natalie says, 'but it has never been tested using so many species so it was quite exciting to get a negative result. It showed that herbivores bite more forcefully because they're big, not because they eat plants.'

However, some types of lizard didn't neatly follow the size bite-force pattern. Skinks, a widespread group of lizards, fit the pattern based on head dimensions but not for body size.

Burrowers, meanwhile, bite more forcefully than expected for their head dimensions. It had been suggested that because these animals need to push themselves through the soil, bite force would be reduced as the lizards adapt to have smaller heads. 

Instead, these results suggest that they can continue to exert a lot of force even with small heads. The researchers suggested that bite force in burrowers might be particularly important to break apart prey before swallowing, as their smaller heads have limited gape and cannot swallow large food whole.

Though the findings have important implications for our knowledge of lizards, more needs to be done to increase available data on this topic. This is the largest study of its kind to date, but bite force data was only available for around 2% of lizards.

This is because some species are too small or not aggressive enough to be easily sampled, while many others have never been researched.

The scientists hope that further work, and efforts to make the right data more freely available, will be able to present a clearer picture of how biting adaptations affect lepidosaurs.

'There are many other groups, such as small herbivores, that haven't been studied or are very much understudied,' Justin says. 'It would be good to get bite force data from these in the future to develop our understanding further.'