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Limb kinematics mask performance differences in varanids from different habitats
Given the ever increasing thread on arboreal adaptations in theropods, this
recent paper from the Clemente lab seems pertinent. It may provide some insight
into what we can look for in the fossils of animals that may have been tree
dwellers but show only incipient adaptations for tree climbing (if that).
Kinematics are pretty much off the table (short of modeling kinematic ranges),
but changes in limb biomechanics may be reflected on the bones via changes in
muscle attachment angles. Alternatively, this could show that the whole thing
is a crapshoot and that putting small theropods in trees is, at best, ambiguous.
Clemente, C.J., Withers, P.C., Thompson, G., Lloyd, D. 2013. Lizard tricks:
Overcoming conflicting requirements of speed vs climbing ability by altering
biomechanics of the lizard strideLizard tricks: Overcoming
conflicting requirements of speed vs climbing ability by altering biomechanics
of the lizard stride.
Adaptations promoting greater performance in one habitat are thought to reduce
performance in others. However, there are many examples of where, despite
habitat differences, such predicted differences in performance do not occur.
One such example is the relationship between locomotory performance to habitat
for varanid lizards. To explain the lack of difference in locomotor performance
we examined detailed observation of the kinematics of each lizard's stride.
Differences in kinematics were greatest between climbing and non-climbing
species. For terrestrial lizards, the kinematics indicated that increased femur
adduction, femur rotation and ankle angle all contributed positively to changes
in stride length, but they were constrained for climbing species, probably due
to biomechanical restrictions on the centre of mass height (to increase
stability on vertical surfaces). Despite climbing species having restricted
stride length, no differences have been previously reported in sprint speed
between climbing and non-climb
explained by climbing varanids using an alternative speed modulation strategy
of varying stride frequency to avoid the potential trade-off of speed vs
stability on vertical surfaces. Thus, by measuring the relevant biomechanics
for lizard strides, we have shown how kinematic differences among species can
mask performance differences typically associated with habitat variation.