Well, it’s time for the Grand Reveal of what the picture in the previous post is (reproduced below). The guesses ranged from bird to wallaby/kangaroo to the stuff of nightmares. And indeed Nick Gardner got it right first, it is a wallaby. Specifically, it is a Bennett’s Wallaby (Macropus rufogriseus rufogriseus), which is the Tasmanian island form related to the Red-necked Wallaby, and an animal that has gone feral (along with mara and other cool critters) in Whipsnade Zoo near the RVC. You can tell it is a wallaby and not a bird, because there is an “Achilles tendon” attaching to a calcaneal tuber (“heel bone”) on the back side of the limb (shown with asterisk below) that birds lack, and if you look closely the toes are hairy, lack bird-like claws, and a few other details like the profile of the musculature are very different; more mammalian than avian. The stump of the muscular tail (cut off) is also a clue. Although the avian similarity in the case of wallabies is still striking, which is one reason I chose this image. Well done, Nick!
I found this picture in my archives and remembered when it was taken back in ~2005- some lab members received some frozen wallaby legs and thawed them out to use in experiments. They tried to compress the legs in an Instron machine (mechanical testing system; partly visible at the top of the pic) to see what the passive, springlike properties of the legs are like in a wallaby, vs. the properties they could measure in a living animal. (The shiny white reflective areas in the pic are for tracking joint motions) And I thought it was a freaky cool pic, so I shared it.
I also posted that pic because my team has done some in vivo analysis of the leg properties in such animals (previous news story here; paper in preparation), and because we use this technique of loading cadaveric legs in such machines quite routinely. We did this for elephant feet to study how the “sixth toe” of elephants works, and we’re analyzing data (as I write) for how elephant feet and rhino feet deform or move when loaded similarly. This method has a long history; we didn’t invent it; perhaps most famously used for studying horse limb mechanics [pdf example], which have a lot of passive properties (almost everything below the elbow/knee is non-muscular). Many animals’ limbs are tendinous/elastic toward their distal end (toes), so the limbs tend to become less actively controlled by the nervous system and become more of a mechanical control system (sometimes involving a non-neural “preflex“) in that region; although it’s all a matter of relative degree of passive:active control in different situations, species, and limbs.
The picture below shows an x-ray of an elephant’s whole hind foot, in which you should be able to see the bones (brighter white) of the foot surrounded by a lot of soft tissue, mostly more passive kinds like fat, skin, fascial sheets, ligaments and tendon.
Here (further below) is a preliminary image from our elephant foot studies in progress, intended to reveal the passive properties/motions of the feet so we can figure out how those properties are combined with more active control, and how actively elephants control their feet vs. other, possibly more ‘passive-footed’ animals like horses. This is interesting from anatomical and evolutionary perspectives, and for helping with foot health problems that are serious concerns for such animals– more about that later. The arrow in the picture below shows where a lot of the motion is: at the knuckle (metatarsophalangeal) joints of the toes; the rest of the foot tends to rotate around these mobile joints. We can’t peer inside living elephant feet to see if they actually do this, but we can compare the external motions, pressure patterns, and other data from living and dead elephant feet to see how they match up, which is what we’re doing now — and we’re doing the same thing with rhinos, which have cool 3-toed, more “hoof-like” feet, as opposed to the 5-toed, fatter feet of elephants. To get this image, we’ve had to put the foot inside a custom-made device using a car jack to apply a constant load, and a wooden framework to hold the specimen still, and then run it through a CT machine in unloaded and then loaded states to see how the bones move. Here is what the crazy apparatus looks like, with enthusiastic undergrad for scale:
This, below, is a right hind foot (pes) of an Asian elephant, shown from the inside of the foot (toes are numbered 1-4 from the big toe/hallux toward the outside of the foot; 5th toe is not visible). The yellow image is the relaxed, unloaded foot; the green is after applying a large load equivalent to the animal standing on one foot (or running quickly). Notice how the third metatarsal (the long bone that the arrowhead is touching) for the unloaded state is in front of that for the loaded state, whereas yellow and green images of the bones toward the tip of the toes are overlapping more, indicating they did not move much/at all. That tells us that the motion is occuring at the joint indicated (“knuckle”), which makes sense anatomically, because that joint looks like it has a lot of mobility.