Re: Lizard study asks: Why go bipedal?

[Dann Pigdon <dannj@alphalink.com.au>]

Jura writes: 

> Recently announced paper from Clemente et al, decides to tackle the "why do so many lizards run bipedally" issue. 
>
> Clemente, C.J., Withers, P.C., Thompson, G., Lloyd, D. 2008. Why Go Bipedal? Locomotion and Morphology in Australian Agamid Lizards.J. Exp. Bio. 211: 2058-2065 
>
> Available here: 
>
> http://jeb.biologists.org/cgi/content/abstract/211/13/2058 
>
> ... All is not perfect though as there are some gripes to be had with the 
> paper. For one, the authours mention other times in which vertebrates have 
> evolved bipedalism. They mention dinosaurs, birds, kangaroos, and primates. 
> They argue that freeing up of the forelimbs might allow for the their use in 
> alternate ways...
> Furthermore, what the heck do kangaroos get out of having their forelimbs freed 
> up? I've seen wallabies use their forearms to help hold food, but kangaroos in 
> general just seem to use them for standing.

Macropods forelimbs are used for: 

* Low-speed locomotion, via a pentapedal gait (the tail is used as a fifth 
limb)
* Combat, whereby males wrestle with each other to compete for females. I've 
heard that kicking out at each other is a last resort, and often initiated 
by the losing male.
* Temperature regulation, by licking the forelimbs and relying on 
evaporation to cool the blood flowing through them. 

I doubt that having the forelimbs free is necessary for the last two. It 
would seem that high-speed bipedalism in macropods has little to do with 
freeing the forelimbs for other purposes, and more to do with a more 
efficient locomotion method in a continent where poor soils and low-quality 
plants tend to select for energy-efficiency. Hopping gaits have evolved 
independantly in rodents as well, usually in environments were energy 
efficiency is also highly selected for (mostly deserts). 

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Dann Pigdon
GIS / Archaeologist              http://geo_cities.com/dannsdinosaurs
Melbourne, Australia             http://heretichides.soffiles.com
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