Keeping on your toes (Frustrating Feet III)

 My last post was placed here on 16 December, and it is already almost March. The reason for that delay is that I had to get The Book ready for the publisher. And I did! The files have now all been sent off, neatly categorised by chapter and number. My wife and I proofread the text of over 60,000 words once more. I also went over all full-page illustrations again, as the page format was broader than I had originally used (there will probably be close to 200 illustrations in total). Luckily, digital paintings allows many ways to alter illustrations, and I only had to paint new parts of paintings a few times.

Right! Back to posting. Feet again this time. Let’s focus on toes.  

Why have toes at all? As discussed in the first post on feet, some animals get away without any toes at all (crabs) and others have just one toe per foot (horses). Feet, rather obviously, contact the ground. For small animals ‘contact’ may primarily mean the ability to actively adhere to the substrate, also discussed in the first foot post. For large walking animals, gravity will ensure that the contact surface is pressed against the ground.

That contact surface can be relatively small and hard, such as a hoof, and such small contact surfaces work best when the substrate is hard, so the foot will not sink into it. In contrast, the contact surface will need to be large if there is a risk of sinking into soft soil. There are two ways to enlarge the surface area.

Click to enlarge: source here

‘Elephant feet’
The first variant is a foot with embedded toes, ensheathed to form a sort of club foot ending in one large flat round surface, such as the soles of elephant or sauropod feet. In elephant and sauropod feet the toes are bundled up to form a kind of cone, broad side down. Such a structure is useful to spread weight through the toes across the shared sole.

T.rex foot; click to enlarge; source here

‘Bird feet’
The other variant consists of long independently mobile toes, such as ostriches and theropods have (I know, birds are dinosaurs). I will approach the question of whether independent toes are useful for two circumstances: dealing with uneven terrain and walking.

Which variant is best under which circumstances is not something I have been able to find in the literature, so, as always, students of speculative biomechanics find themselves on an insecure footing (sorry for that one). As an aside, perhaps courses on biomechanics should use speculative biology as a teaching tool: it forces students to ask questions that normal biology, dealing with observable facts, regards as givens.

Conforming to uneven ground; Click to enlarge; copyright Gert van Dijk

Dealing with uneven terrain
If that elephant foot with a large continuous foot surface is stiff and does not deform easily, placing one side of the foot on an elevation such as a stone will create a large bending moment. That can be solved by providing the ankle joint with some leeway to turn in all directions: the foot will rotate a bit relative to the leg until both sides of the foot will contact the ground. Mind you, that ankle motion must be tightly controlled, or else the ankle joint may move too far and is hurt (that's how you sprain an ankle).

In variant two, with independently movable toes, the ankle joint does not have to do anything as the solution then lies in the toes themselves. If such a foot is placed on uneven terrain, each toe can move until it contacts the ground. In reality, you can have a combination of the two designs

The two variants, one with big round feet and one with splayed independent toes, seem to be linked to lifestyle: sauropods, with big plodding feet, were immense, slow, and herbivorous while theropods were smaller (still large!), fast, and carnivorous. The dinosaur example has the advantage that the two groups have a shared ancestry, so the answer cannot lie in a different starting point. There must be a functional reason, and that is why I summed up size, speed and feeding method. The feeding method is probably not crucial, but lifestyle is, in the sense that an agile animal must have legs and feet to allow agility. Agility hare can be defined as meaning not only speed, but also the ability to turn, accelerate, absorb hard landings, get a grip, etc. I think that independent toes provide more agility than toes that are encased in a common sheath. My reason to think so is that animals with large round feet also had vertical leg bones, while those with independent toes tend to have leg bones that are slanted with respect to the vertical. Such ‘crouching’ postures require much energy just to stand still but allow jumping and hard landings.     

The image above overlays an elephant with T. rex at the same scale. Notice how the legs of T. rex are slanted while the elephant has vertical legs.     

Additional toe propulsion; click to enlarge; copyright Gert van Dijk

Walking
Elephants and brontosaurs on one side, and ostriches and T. rex on the other side. Both types of animals walk well, so one is not necessarily superior to the other. But we can speculate about consequences of foot designs.

There is probably one more factor of interest here, one that has to do with the direction of toes relative to the direction of walking. Toes usually point forward, or a bit to the side as well as forward. Is that a given? On Earth, very few animals have toes that point backwards. Some walking birds, such as roadrunners, have backwards toes, but you can argue that these legs are there only because they fit the Bauplan of animals that have to grasp branches or need to hold on to tree trunks, for which backwards-pointing toes are useful.  

A good reason to have forward-pointing toes is that they contribute to walking, by moving backwards relative to the ankle. The main propulsive force of the leg of an ostrich or a T-rex must come from movement in the hip, knee and ankle joints. But moving the toes backwards will still add force. The situation is similar to throwing a tennis ball: you can grip it in your fist but also with the tips of your fingers. To throw it far, the main acceleration will come from simultaneous extension of the shoulder, elbow and wrist joints, but if you hold the ball with your fingers, straightening your fingers will add a bit of acceleration. Not much, but worthwhile.

The image above aims to show that: the blue triangle shows where the heel/ankle touches the ground, and the red triangle where the tip of the toe touches the ground. By rotating the toe backwards the animal effectively gains that additional distance each step.  

The toes of T-rex probably also added a bit of acceleration by moving back in unison with the other leg joints. In contrast, the toes of elephants and brontosaurs probably hardly moved at all compared to their ankle joints. Does that put these clubfooted animals at a speed disadvantage? Yes, it does, but that toe assembly is part of a large set of traits that together illustrate that the animal is designed for weight carrying at the price of agility.

In summary, independently movable toes offer more agility and add propulsive force, which is why they may be tied to a more active lifestyle.

Barlowe's rayback; click to enlarge; copyright W Barlowe

Consequences for speculative biology
You probably know Barlowe's Expedition book. I always admired his paintings skills, but at the same time I also always felt that he often gave too much weight to 'alienness' , disregarding biological likelihood. In the past, I discussed why I though a planet filled with blind animals would be very unlikely (Here; the short answer is that vision seems to evolve very readily). Something I hadn't mentioned yet is that I always felt it odd that his very large agile bipeds had feet like those of elephants. I had an inkling that such feet did not fit such a lifestyle. I still cannot prove that, but at least I can now provide some reasoning rather than just a feeling.

Rusp toes; clich to enlarge; copyright Gert van Dijk

A second consequence is how toes should be placed on alien feet. As you may know, Furahan hexapods have jointed legs with three large segments and a few smaller segments at he end. There are there to provide that extra acceleration in running animals.  You will also find that rusps have two toes, or rather nails, at the side of their feet. These do not help with acceleration; rusps are fairly slow and do not benefit from a little bit of additional acceleration. As they have many feet, the surface of each does not have to be large. The bottom of the feet amounts to a horizontal strip, with nails digging in at either end. Rusps like a solid grip!