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Quadruped Anatomy for Animators

By Morgan Williams
Cinema 4DAfter Effects

The questionable quadruped—a quandary of modern animation—quamped many animators' quests and has been considered quisquos at best...quizzity at worst. Now before you coin us quipsome, let's dive into how to rig quadrupeds for animation

When it comes to designing, rigging, and animating characters, I get more questions about quadrupeds than almost any other subject.  
We all know how much variety and unpredictability exists in the world of motion design.  A good motion designer should be able to handle anything that’s thrown at them. It’s one of the reasons I believe it’s important for motion designers to study character animation. You never know when the need to animate a character might come up.
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But you also never know what kind of a character that might be. Sure, very often it’s going to be a human, but it’s just as likely to be a dog, a donkey, or a dinosaur. And that’s not to mention all the possibilities for animals outside the world of quadrupeds, such as birds, fish, and insects. We won’t get into that can of worms—which you may also have to animate! 
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Angry Birds Movie - Columbia Pictures
Most character animation curriculum, for good reason, begins with the humanoid biped. It’s the most common character you’re likely to encounter, as well as being the one you’re most familiar with—since you, well, are one. But of course there are lots of other animals in the world that you might be called upon to bring to life.
Now I obviously don’t have the time or space to address every creature on earth, but I thought it would be great to talk a little bit about what is probably the second most common type of character you might encounter besides the humanoid biped: the quadruped.

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Biped vs. Quadruped Locomotion

Let’s start by defining our terms a bit so we’re on the same page. When we talk about “quadrupeds” or “bipeds”, we’re not talking about any specific zoological classification like a genus or a species; we’re talking about the animals' means of locomotion.
So let’s break it down:
  • Biped  -  An organism whose terrestrial locomotion is accomplished with 2 rear limbs or legs.
  • Examples include humans obviously, but also kangaroos, some specific dinosaurs, and birds - who are actually feathered, bipedal dinosaurs!
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Ostrich walking - Eadweard Muybridge
  • Quadruped  -  An organism whose terrestrial locomotion is accomplished with 4 limbs or legs.
  • Most (but not all!) quadrupeds are vertebrate animals, including dogs, cats, cattle, reptiles, unicorns, and dragons.
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Horse running - Eadweard Muybridge
It’s also important to make a distinction between Quadrupeds and Tetrapods.
  • Tetrapod  -  An animal with 4 limbs.
  • Examples include amphibians, reptiles, mammals, and birds.
But here’s where it starts to get weird:  Not all quadrupeds are tetrapods, and not all tetrapods are quadrupeds!  For example, a bird is a tetrapod because it has 4 limbs; 2 legs and 2 wings, but it moves on land with it’s 2 back legs, making it a biped.  And a praying mantis is a quadruped, because it walks on its 4 hind legs, but it has 6 limbs total so it is not a tetrapod!

Understanding Quadruped Anatomy

Now that we’re clear about what we’re talking about, let’s focus on how to begin to understand quadruped anatomy so we can design, rig and animate these beasties.
Comparative anatomy goes a long way to making sense of the anatomy of quadrupeds. All vertebrates share essentially the same skeletal structure, but the bones are shaped and proportioned differently depending on their use and the specific needs of the animal. This similarity of structure is called homology.  So the good news is that once you understand one skeleton, you basically understand them all.
If you took my Rigging Academy course, you probably saw these illustrations showing the comparative anatomy between limbs of a human and a cat:
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But comparative anatomy works even as we branch out into much more diverse animals.  Note this comparison between the limbs of several very different creatures:
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Homology of vertebrates - Encyclopedia Britannica
A careful study of comparative anatomy will not only help you understand the anatomy of fairly “realistic” animals but it’s also invaluable when you need to anthropomorphize an animal!
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Brian the dog from Family Guy - Fox Broadcasting Company
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Anthropomorphized cow from the Veggemo spot by Giant Ant
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Elephant Sketches by Heinrich Kley

Focus on the Feet

As you look at comparative anatomy, you’ll note that there’s not much of a difference in the skeletal structure of the torso of vertebrates.  There’s the skull, the spine including the neck and possibly a tail, the ribcage and the pelvis.  And all are basically arranged and used in a similar way
Where things get more specialized is in the limbs, particularly the feet or paws.  The bones are basically the same as we’ve learned from comparative anatomy, but the use of these bones is broken down into 3 slightly different categories.

Plantigrades 

These are animals - either bipedal or quadrupedal - that walk with the bones of the feet, hands or paws (the carpal, metacarpal, tarsal and metatarsal bones), flat on the ground.  Examples include primates (humans), bears, mice, rabbits, kangaroos, etc.  You can see in the bear gif below how the “palms” of the hands/paws and the bottoms of the feet from the toes to the heel land flat on the ground with each step. 
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Bear Walk Cycle by Dane Romley

Digitigrades

These are animals that walk on the bones of the digits or toes (the phalanges), with the wrists and ankles elevated above the ground.  Examples include most non-hooved vertebrates like dogs, cats, most mammals and dinosaurs.  Note how in this dog gif, it’s just the fingers/toes/phalanges that meet the ground.  The first joints just above the toes on the front legs are the wrists, and the long bones extending up from the toes on the back legs is actually the foot (most digitigrades and ungulates have very long feet) and the first joint above the toes is the ankle.
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Dog Walk Cycle by Jess Morris

Unguligrades or Ungulates

These are animals that walk on the tips of the toes using hooves.  Examples include all cattle, pigs, giraffes, deer, and elephants (which are known as “sub-ungulates”).  Ungulates are very similar to digitigrades in that the wrists and ankles are again off the ground, as you can see in this horse gif.  The difference between the ungulates and digitigrades is very subtle in that the ungulates are walking on the tips of the toes (rather than with the toes/fingers/phalanges flat on the ground) on the “hooves” which are highly adapted, specialized and strengthened “toenails” or “fingernails”. Note again the very long foot bones on the back legs.
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Horse Walk Cycle by Richard Williams

Anatomy Vs. Design

Now with all this semi-scientific talk which is an important part of understanding the creatures we’re trying to animate, we also need to remember that we’re often not animating animals that are particularly realistic.  
As we stylize, exaggerate and/or anthropomorphize various creatures, it’s important to adjust our rigging and animation decisions based on the design we’re creating. A real dog is of course a quadrupedal digitigrade, but a cartoon dog like Brian from Family Guy is a bipedal plantigrade, and must be rigged and animated accordingly.
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Stewie and Brian from Family Guy - Fox Broadcasting Company
So we have to analyze the anatomy of our specific character’s design, and not be constrained by the “reality” of that particular animal.

Rigging and Animation Issues

If you are animating a quadrupedal character using a rigged puppet, a clear understanding of it’s anatomy - particularly the skeletal anatomy - is critical.
If you are working in After Effects, the wonderful Duik Bassel script for rigging and animation already has prefabricated arm and leg “structures” for plantigrades (both bipedal and quadrupedal), digitigrades and ungulates which will all auto rig correctly giving you exactly the structure and motion you need.

Plantigrades

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Digitigrade

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Ungulate

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Rainbox, the developer of Duik Bassel promises more “prefabricated” structures in the future which will probably include full quadrupedal skeletons.
If you are working in Cinema 4D, the Character Builder can help you create a pretty effective digitigrade or ungulate quadruped rig automatically, but you might need to do some more manual rigging for plantigrade quadrupeds.
A correctly rigged puppet will go a long way to helping the animator craft anatomically “correct” poses and movements for a quadruped.  But whether you are animating with a rig, hand drawing your animation or using some other method, a clear understanding of the anatomy of the animal in question is a must.  And just as you research the anatomy, you should also make sure you do extensive research into the movement of that creature. 
The Disney studio pioneered the practice of actually bringing live animals to the studio to have the artists study their anatomy and movement, and while you may not be able to get an elephant to come to your studio, you do have the internet where you can find lots of video reference of almost any beastie you can think of.
Animal Gaits, Rigged by Ram Krish, Directed and Animated by Stephen Cunnane
The best way to meet the challenge of designing, rigging and animating quadrupeds is to understand them inside and out. I think this is actually one of the most fun and rewarding aspects of character animation. 
How lucky are we to be able to spend an afternoon studying aardvarks, lizards or zebras as part of our job?
I hope this article has helped you feel more comfortable working with quadrupeds the next time you’re called upon to bring one of these creatures to life.

Continue Your Journey

Want to learn more? Are you ready to dive into the work of rigging and character animation?  Check out Morgan’s two courses, Rigging Academy and Character Animation Bootcamp!