So, why go bipedal?

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As human beings this might come off as somewhat of a “duh” question.

“To free our hands up, of course.”

Ah, but like most things in life, the common sense answer is not the right one.

Consider all the bipedal animals alive today. We have humans, obviously; kangaroos, birds, a few lemurs, and a whole swath of lizards. Of these, the “free up the forelimbs” argument really only holds true for humans, lemurs and birds. What of kangaroos and all those lizards? Exactly what does one benefit from when going bipedal?

Do you go faster, run longer, or gain a better vantage point?

 

C.kingii during a typical foraging run.

All these questions were asked, and somewhat answered in a recent paper by Clemente et al.

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

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.

The conclusions reached, were interesting and quite unexpected.

I’d say the most surprising part would have to be the discovery that both endurance and speed 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.

In fact, the only correlate that the authors found was that a switch to bipedalism resulted in an increase in acceleration. Short of that, the authors viewed bipedalism as more of a side effect of speedy locomotion, rather than anything else.

As one authour put it: “The lizards were pulling a wheelie.”

There are some gripes (niggles if you will) with the paper. For one, the authors assert that a switch to bipedalism allowed birds to incorporate their forelimbs into wing design. While being bipedal certainly allowed for this, it could not have been the cause. Birds descended from dinosaurs, and very, very very few dinosaurs had wings. Theropods were sporting freed forelimbs for some 80 million years, or so (probably longer given the proposed ancestors of dinosaurs). Wings were not the cause, simply a benefit. Something else had to have spurred the evolution of bipedality.

Side note: What the heck were theropods doing with those forelimbs anyway? Most paleo artists tend to draw theropods with their arms tucked to the side, yet work by Carpenter (2002) has shown that there was some considerable range of motion in theropod forelimbs. They weren’t brachiating from tree to tree, or anything, but they could certainly do a heckuvalot more than just tuck their arms to the side. Even T.rex with its embarrassingly short forearms, had a surprisingly large range of motion to them. So what’s up paleo art guys? Let’s see some theropods putting their arms to use.

The largest SNAFU in the paper comes from the cladogram that the authors chose. They chose to go with the broken molecular tree used by Townsend et al, which asserts that Iguanians are actually Scleroglossan lizards. This might sit all fine and well when looking at molecules, but it utterly falls apart upon a morphological assessment. In order for Iguanians to fit in the Scleroglossan family tree, they had to have undergone a tonne of morphological reversals, including the re-softening of the tongue and the re-evolution of both postorbital bars (surprisingly, the latter is actually not out of the realm of impossibility as tuataras have apparently done just that).

Due to this tree choice, the authors 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 authors 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.

The final complaint comes from the supplementary movies given.

Frankly the movies are just too short. I have to go super slow-mo just to see anything. So that’s a bummer.

Overall the paper is rather good. The authors discovered that Lophognathus gilberti runs bipedally 85% of the time. This suggests that C.kingii is not the only truly bipedal lizard out there.

The authors also observed 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 not discovered during this experiment.

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.

If one lizard has a top speed of 12km/hr and another has a top speed of 8km/hr, and the goal (burrow) is only 50 meters away, then that extra speed isn’t going to mean much. Especially if the slower lizard is able to hit its top speed faster.

Apparently lizards “pull a wheelie” because in their ecosystem it pays to be a drag racer, rather than a Daytona 500 car.

~Jura

References

Carpenter, K. 2002. Forelimb Biomechanics of Nonavian Theropod Dinosaurs in Predation. Concepts of Functional Engineering and Constructional Morphology. Vol. 82(1): 59-76.

Shine, R., & Lambeck, R. 1989. Ecology of Frillneck Lizards, Chlamydosaurus kingii (Agamidae), in Tropical Australia. Aust. Wildl. res. Vol. 16: 491-500.

Townsend T, Larson A, Louis E, Macey JR. 2004. Molecular Phylogenetics of Squamata: The Position of Snakes, Amphisbaenians, and Dibamids, and the Root of the Squamate Tree. Syst Biol. Vol. 53(5):735-57.

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5 Responses to So, why go bipedal?

  1. Avatar Sockmonkey
    Sockmonkey says:

    Maybe it’s to assist in breathing as well. Reptiles that don’t use gular pumping basically have to hold their breath during a quadrupedal sprint due to the way lateral flexing squeezes their lungs.

    • That is certainly possible. However, the authors noted that endurance was negatively correlated with bipedality. Lizards that ran bipedally tended to do so for a smaller amount of time.

      That said, their analysis wasn’t particularly robust for this factor (r2 = 0.40 for phylogenetically corrected data), so it remains a possibility. I agree with you that bipedal lizards should be able to run and breathe at the same time. What I would like to see is a setup that directly measures breathing ability while moving bipedally.

  2. Avatar Sockmonkey
    Sockmonkey says:

    Yeah, an escape sprint tends to be mostly anerobic anyhow.
    I was thinking of how Chlamydosaurus kingii goes bipedally while foraging.
    I’m wondering how it’s “jogging” endurance would compare to lizards that can maintain a “jog” with gular pumping.
    How many lizards can do a gallop like crocodiles sometimes do?
    That gait should allow sufficient breathing as well.

  3. These are all good questions that currently lacking any good answers. Although C. kingii is known to move bipedally, there is remarkably little information on how it does so and what the physiological ramifications of this locomotion are.

    As for the bounding gallops that we see in crocodylians, I haven’t seen any video or literature references to lizards locomoting this way. The closest thing that comes to mind is a video I saw years ago at the Society of Integrative and Comparative Biology Meeting (SICB). It was on how lizards avoid obstacles while running and it featured a collared lizard (Crotaphytus collaris). The researchers filmed the lizards moving over obstacles. In one video in particular, I remember seeing the lizards hop on one rock and immediately hop to the next rock and then hop off and continue running. The move looked nearly identical to a croc gallop. So I would say that the capability of bounding is there in some lizards, but I don’t know if any species has ever taken advantage of it.

  4. Avatar Sockmonkey
    Sockmonkey says:

    It would seem odd if none of them did. On the one hand, some habits are just hardwired into an animal’s brain, but on the other, they often do a lot of weird unexpected crap too.
    I could picture one of the more leggy lizards doing a kangaroo-like bounding with with the tail counterbalancing and the front legs mostly preventing a faceplant.