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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.

http://jeb.biologists.org/content/early/2013/07/16/jeb.089060

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. 


Abstract

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. 

Jason