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Lizard study asks: Why go bipedal?
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:
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
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:
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
"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