Monday, 14 October 2013

Lizard tricks: overcoming conflicting requirements of speed versus climbing ability by altering biomechanics of the lizard stride

So recently I managed to publish a paper in the Journal of Experimental Biology about my dissertation work looking at the locomotion in Australia's biggest and baddest lizards the monitor lizards. I called it 'Lizard Tricks', not only cause I think they are doing something neat, but because I think what we see in these lizards would be a nice little engineering principal we could use in the design of robots or something else equally awesome. Ignoring for now that it took me over 6 years to figure this all out, I'd like to try and explain how I finally got to this point, and where we are with our understanding of locomotion in these beasts.

All good studies should always start as simply as possible, and this is certainly where we began. In our earliest studies we compared the morphology of different species with their retreat site (Thompson et al. 2008). What we saw is that burrowing species tended to be different, from those that retreat into tree hollows, as shown in this figure. Well it mostly works, except for V. mertensi, the water monitor from the North of Australia. Look at him, sitting in there among the other tree species. 

But never mind, for most species at least there is some morphological differences between these groups, which I interpreted as an adaption between tree-climbing species, and those that spend their time on the ground. I should mention here that the morphological differences we did see, were not like those seen in other groups, such as short legs in climbing species, but no matter I thought, we do see some differences. Great, so I saw a different in morphology which was related to habitat. Now what?

Well following the paradigm of Arnold (1983), morphology is only likely to be related to ecology, through its influence on performance variables, since this is what selection can act upon. Not the shape of an animal per se, but how does that shape confer an advantage to make it run faster, leap further and do all the other lizardy things they do. And so this is what we did. 

We examined the evolution of speed among varanid lizards, which was related to size, but once we removed this size effect we found no relationship between speed and climbing ability (Clemente et al. 2009a). We also checked differences in endurance capacity, but found no differences there either (Clemente et al. 2009b). So what was going on? What did these differences in morphology mean, if they didn’t translate into differences in performance. Well it turns out that they did, but we weren’t looking at the right performance traits. But before I go into that I’d like to talk about another project I was working on at the same time. I was interested in how lizards change their gait as they get bigger.

One impressive feature of monitor lizards is their huge variation in size. So if you were interested in the effects of size on gait, you couldn’t hope for a better group. The reason we expected gait to change in these lizards, was a previous study by Andy Biewener (1989) in mammals showed a neat principal, whereby small animals like cats and mice, had a crouched posture, but big animals like horses and elephants, had a much more upright posture. The idea was that bigger animals change their posture to be more upright since this allows them to support more of their weight along the long axis of the bone, where compressive stresses dominate, rather than bending the bone in the middle. 

Think about a drinking straw, which is very easy to bend in half, but much much harder to crush end to end. Anyway, this works really well in mammals, I thought the same principal must be true for monitor lizards right? Wrong. I had forgotten which group of lizards I was working with. Monitors are like that awkwardly developing teenager, who just refuses to assimilate just out of spite, so I shouldn’t have been surprised when there was no relationship between upright postures and body size, as seen in this figure above. It varied from species to species, but not in the way I expected.

After some searching I did find another possible mechanism through which lizards could mediate size related stress in limb bones (by changing femur rotation), but it didn’t explain where the variation in upright posture was coming from. And that is how I ended up at the current study.

You see climbing species are faced with certain problems that us land walking species don’t have to face, the most obvious of which is falling off the dammed thing you are climbing, to escape whatever douchebag animal is trying to eat you. One way climbing species avoid falling is by lowering the body centre of mass which reduces the torque due to gravity.

Becoming less upright is a good way to do this, and this is certainly what these lizards appeared to be doing. They adopt a sprawling posture, which is kind of like being forever in the pushup position, close to the ground. But the variation I had seen in upright posture before, was infact better represented by whether lizards lived in trees or not.

So part of my problem was solved in that I had figured out where the variation in posture was coming from, but this created another problem, speed. You see there are two ways of becoming faster, one is to take longer steps the other is to take more steps in the same period of time. Becoming more upright is popular among terrestrial lizards, not only because it looks cool (check out that cover photo), but also because it increases the effective length of your limbs and allows you to take longer steps, increasing stride length and therefore speed. So we looked at the biomechanics of the stride, and this is exactly what we found among terrestrial monitors. Terrestrial lizards take longer steps as they run faster. However, climbing lizards, for perpetual pushup related reasons we talked about above can’t do this, so we might expect them to be quite a bit slower. But as we saw above this is not the case, there was no difference in speed between climbing and terrestrial monitors. 

So how are climbing monitors keeping up with their terrestrial brethren? Well the answer it seems is that they are changing the other methods of modifying speed, stride frequency. By taking more steps more often, they have overcome the problems of staying stable while climbing while at the same time not sacrificing speed. And so that is the story of Lizard Tricks. There is still lots to figure out, like whether these climbing lizards, with their different gaits are actually putting more stress on limb bones, but for now that’s a problem for future Chris to deal with.       

Friday, 19 July 2013

Jumping beans in Slow motion

Between 2010 and 2012 I worked at the Rowland institute at Harvard university. Most of my time was spent researching the muscular dynamics of frog muscle, by building robotic frogs, but every now and then we liked to goof off scientifically. That chance came the one day when the head of my workgroup, Chris Richards, brought in a mexican jumping bean. Being from Australia, I had not seen one of these before, and that is sad. Cause they are friken awesome! The bean itself is the seed of a shrub, the genus Sebastiania. Inside this bean is growing the larve of a little brown moth (Cydia deshaisiana). The moth makes a hollow inside the bean, as it eats the bean from within. But what makes it amazing is that it is able to thermoregulate by moving the bean from hotter places (such as in the sun) to cooler shadier places. It does this by 'jumping' - hence the name. The actual mechanism of how this tiny moth can get the whole bean to jump is unknown, and would probably require some xray cinematography, which sadly we didn't have at the institute. What we do have though, is the next best thing, very sensitive force plates and high speed cameras.
Having several dead lines to attend to, i decided the best use of my day, would be to ignore them all and play with the little bean. I set to work on it, and this is what i got out.

First i was interested in the amount of ground force the little bean is capable of producing. I used a honeywell force cell to measure it, jumping up from a smooth plastic surface. The force plot is show below.

Here the x-axis shows time in ms and the y-axis shows vertical force (in newtons). Peak force was about 0.14N. The body weight of the bean (both worm and seed) was 0.134g therefore, this bean was jumping with a force over 100 times its body weight!!

The second thing i noticed in my force traces is there was a smaller first peak, followed by the major peak, about 500 ms later. This was present in almost all of the force traces i got.
I decided to use the photron high speed camera (photron FASTCAM SA3 model 60K-M2) to take a look at what was going on and this is what i found. The film is taken at 1500 fps, and is slowed down 60 times.

The first peak on the force trace does appear obvious in the movies as a little shuffle backwards, before the main jump. My guess is that the bean is shifting its position inside the shell in preparation for the jump (possibly storing energy) then shifting weight quickly causing the bean to jump up.

Can the water dragon (Intellagama lesueurii) run on water?

Can the water dragon (Intellagama lesueurii) run on water? 

I have long been impressed with the ability of the South American Basilisk lizard to run on water. There are plenty of videos of it on youtube, for example a short one here. 

Its pretty amazing. Looking at this video alone shows as two important aspects of its locomotion, 1) it is able to lift the whole body out of the water, and 2) it is able to do so for quite long stretches (around 10-15m). This ability has been documented quite well by a series of papers by the group at Harvard University, particularly Tonia HsiehThey have done some great work, including describing how smaller lizards are better able to support body weight than larger lizards , modelling 3D forces and recording 3D kinematics of the lizards stride. Below is a gif showing some of the detailed kinematics of the lizard stride which i stole from George Lauders lab webpage. 

(NOTE: just click on the gifs if they are not running)

One other important point reported in these papers, based on the description given in Hsieh (2003), is it seems the kinematics change when running on water, such that the limb moves behind the hip, rather than being both infront and behind the hip.   

"whereas the hindlimb kinematics of other lizards on land are typically symmetrical (i.e. limb excursions anterior to the hip are of similar magnitude to the limb excursions aft of the hip), basilisks running through water exhibit much greater excursions aft than they do anterior to the hip. "

This seems to be shown quite well in the gif above. So given this information on how Basilisk runs on water, we can then ask the question, Can the water dragon also run on water? 
I was led to believe it may be able to from two dominate and convincing lines of logic. 1) they are water dragons! - it might behoove them to be able to do so and 2) i heard reports of the juvenile lizard being observed doing so from a fellow researcher. 

So i modified the lizard racetrack which i have here at the University of Queensland, by placing a short water filled aquarium across their path which they must cross to get to the other side. Then i sat back and filmed them using the fastec high speed camera system. And this is a typical (read absolutely best) result below

Well the first thing i noticed is that they are no basilisk lizards. The body is not held out of the water and progress is significantly slowed. The first step step seems hardly effective at all, and the second step is much deeper, and seems like a breaking step, with the foot held flat. However, the following step seem to have some similarity to those of the basilisk. From steps 3 onwards, the foot does not appear to be pushed as far forward, and much of the stroke seems to be posterior of the hip, as in basilisk. Secondly the trapped air bubbles on the foot are interesting, and these are also observed for basilisk, where they are thought to be the result of tiny fringes along the toes of the south american lizard. Such fringes however, are not obvious in the water dragons. Below is a snapshot of the bubble being dragged down on the trailing edge of the foot. 

So i am unsure what to make of this all. If does look like they are capable of some run/swim locomotion, but it certainly falls short of the amazing prowess of the basilisk. These juveniles tend to avoid the sinusoidal swimming pattern, with tucked in arms, as seen in adults. Though the top view does appear to show that this is still employed, in conjunction with the feet, suggesting that this is indeed a uniquely adapted form of locomotion. 

Here are some lesser impressive ones. Though notice that the right hind foot is actually brought out of the water - suggesting they could be using surface effects to give more downward force. 

And this one below shows a similar stroke. 

So that is as far as i have got. Let me know whether you think it is sufficiently interesting to warrant detailed kinematic analysis, or whether you think water dragons are just a little retarded when it comes to running on water. 

Finally i leave you with what happens after several trials and the dragons know the water is coming up. It led me to believe, that for water dragons, they sure do not like water!