Tooth morphology and biomechanical implications
Overall, our result highlight two morphotypes: long, thin, highly curved
teeth with a thinner layer of hard tissue (long pulp cavity running
along the entire length of the tooth) versus short, stout, straighter
with thicker layer of hard tissue (short pulp cavity). Long, slender
teeth are associated with feeding factors that require a good grip on
the prey, whether the prey is slippery, or soft, or if there is no solid
substrate to support and facilitate feeding. Thin teeth undergo high
stresses during penetration into the prey due to bending and are more
susceptible to failure (Bar-On, 2019). Thin teeth are also highly
affected by even the slightest axial force that cause high stress
(Bar-On, 2019; Rajabizadeh et al., 2020). Thus, a certain amount of
bending might be beneficial to avoid breakage, especially in a feeding
context where the prey cannot be correctly restrained. Short and stout
teeth are associated with hard or long prey which both impose either
high and/or repeated loading on the teeth but the stout shape allows to
decrease the peak stress that originates from the compression force and
concentrates it in the tip (Bar-On, 2019; Rajabizadeh et al., 2020).
Rajabizadeh and colleagues (2020) compared the mechanical properties
associated with tooth shape in two sister species; one that eats hard
prey and has short, stout teeth versus a generalist with more slender
teeth. They used finite element analyses to compare von Mises stress and
deformation during loading from various angles on the two teeth. They
demonstrated that, as suggested by Bar-On’s results (2019), snake teeth
barely undergo any stress when applying a tangential force to the tip.
Deviation of the applied force from the tangent of the tip imposes a
higher and more widely distributed stress in the slender tooth than in
the stout one. These results suggest that some of the morphological
variation we highlight here may be related to mechanical adaptations of
the teeth to dietary constraints. Yet, the two compared shapes are far
from representing the large variability in tooth morphology in snakes
and many functional aspects of snake teeth remain unexplored such as the
effect of curvature or the effect of variation in the inner shape of the
teeth on its biomechanical properties.
Conclusions based on the teeth of other vertebrates are hardly
applicable to snakes. Snake teeth fulfill different functions than those
of other vertebrates; they play a major role in prey capture and
intra-oral transport, but they are rarely used to reduce the size of
prey items. Although snakes have acquired constraining diets such as
durophagy, the function of their teeth is not to crush (except forFordonia ) but to transport the whole prey into the digestive
tract. Therefore, the trade-off highlighted for the teeth of durophagous
vertebrates, between convex teeth that reduce the force needed to break
a hard item but increases the strain in the tooth versus concave tooth
that reduces the strain but require higher forces to break the prey,
does not apply to durophagous snakes (Crofts, 2015). Our study shows
that prey mechanical properties are not the only drivers of tooth
morphology, but feeding behavior, and more globally feeding ecology,
impose a variety of constraints that impact their size and shape. Future
investigations of the biomechanics of snake teeth may help establish the
link between their morphological and behavioral variability and would
enrich our understanding of tooth evolution and function in vertebrates.
Experimental designs (Kundanati et al., 2020), simulations (Bar-On,
2019; Rajabizadeh et al., 2020) and analytical tools (Huie et al., 2022)
have recently been developed and can be used to better understand the
dental biomechanics of snakes using the shapes highlighted in the
present study, in a functionally relevant context.