Lizard study asks: Why go bipedal?

[Jura <pristichampsus@yahoo.com>]

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

Abstract:

_____________________________________________________________________

Bipedal locomotion by lizards has previously been considered to provide a 
locomotory advantage. We examined this premise for a group of quadrupedal 
Australian agamid lizards, which vary in the extent to which they will become 
bipedal. The percentage of strides that each species ran bipedally, recorded 
using high speed video cameras, was positively related to body size and the 
proximity of the body centre of mass to the hip, and negatively related to 
running endurance. Speed was not higher for bipedal
strides, compared with quadrupedal strides, in any of the four species, but 
acceleration during bipedal strides was significantly higher in three of four 
species. Furthermore, a distinct threshold between quadrupedal and bipedal 
strides, was more evident for acceleration than speed, with a threshold in 
acceleration above which strides became bipedal. We calculated these thresholds 
using probit analysis, and compared these to the predicted threshold based on 
the model of Aerts et al. Although there was a
general agreement in order, the acceleration thresholds for lizards were often 
lower than that predicted by the model. We suggest that bipedalism, in 
Australian agamid lizards, may have evolved as a simple consequence of 
acceleration, and does not confer any locomotory advantage for increasing speed 
or endurance. However, both behavioural and threshold data suggest that some 
lizards actively attempt to run bipedally, implying some unknown advantage to 
bipedal locomotion.

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The conclusions reached by the authours are certainly fascinating. They tested 
many of the questions I, myself, have had about lizard bipedality. 
Surprisingly, endurance and speed both were found to be inconsequential. Those 
were the two forces that I figured would have driven the push towards 
bipedalism. Apparently this is not the case. The only correlate found was that 
bipedalism tends to go with a faster acceleration. Other than that, the 
authours are viewing bipedalism as more of a side effect of speedy locomotion, 
rather than anything else. To quote one of the authours: "The lizards were 
pulling a wheelie."

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. They cite primate tool use and bird wings as an example of this. However, 
birds inherited their bipedal stature from dinosaurs. What were theropods using 
their forelimbs for? If one believes the majority of paleo-art out there, the 
answer is apparently nothing (seriously guys, have them grab something with 
those claws). 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. 

The point is that bipedal lizards aren't the only vertebrates with forearms 
that don't appear adapted for anything other than locomotion.  

Another complaint, and by far the biggest, is the persistent use of the broken 
molecular phylogenetic tree for squamates that posits iguanians to be nested 
well within scleroglossa; ignoring the multitude of reversals that would be 
required to pull this off. 

Because of this, the authours erroneously concluded that bipedalism evolved 
only once in the lacertilian tree and was lost a multitude of times, with a 
putative re-acquirement in varanids. 

Another minor complaint comes from the very slight use of _Chlamydosaurus 
kingii_; the only lizard known to be a "true" biped (see: Shine & Lambeck 
1989). Given that the authours were trying to spot differences between bipeds 
and quadrupeds, I can understand the use of lizards like _Ctenophorus_, with 
their greater spectrum of gaits. However, in doing so they should have 
qualified their conclusions better in regards to how lizards obtain a bipedal 
stance. In _Chlamydosaurus_, bipedal trotting is attained from a standing 
start. Quadrupedal stance is only seen when stopping to eat. Furthermore, 
foraging runs and escape runs use two different gaits, with the latter gait 
more akin to that of other facultatively bipedal lizards. Judging from the 
stats given in the paper, it seems apparent that the _C.kingii_ used in this 
study were mostly running away. 


Final complaint is with the supplementary movies found here: 

http://jeb.biologists.org/cgi/content/full/211/13/2058/DC1

The movies go by way too fast. Thus making them hard to watch.

Aside from those complaints the overall paper is pretty good. The fact that 
_Lophognathus gilberti_ was found to run bipedally 95% of the time is very 
interesting, as it suggests another possible true biped among the lizards.

The authours noted that despite any advantage in speed, or endurance, some 
lizards intentionally push their center of mass towards their hips early on in 
the running phase in order to more quickly obtain a bipedal gait. The reasons 
behind this are unclear, but do suggest that bipedalism confers some advantage.

One advantage alluded to, but never really elaborated on, was the faster 
acceleration noted in bipeds. Though maximum speed was no different than in a 
quadruped, this speed was obtained faster. Ecologically I could see this being 
very advantageous. When one is trying to avoid a predator, maximum top speed is 
probably less important than reaching that top speed as fast as possible. 
Especially if the goal is to reach a nearby hole, or tree.

Bringing this around to archosaurs, how does one figure this affects views on 
archosaur bipedality? If there is no speed benefit (barring the acceleration 
component), and no increase in endurance by removing the "Carrier's 
constraint," then what other reasons would cause a quadruped to go the bipedal 
route?

Jason


"I am impressed by the fact that we know less about many modern [reptile] types 
than we do of many fossil groups." - Alfred S. Romer