Various vertebrate skeletons.
The countless vertebrate species on Earth are, fundamentally, remarkably similar to one another in body structure. Thus, once you gain a basic understanding of the vertebrate body structure, it is very simple to draw any variety of vertebrate. This tutorial is a look at the vertebrate body structure, and will offer rough guidelines to drawing three different kinds of vertebrates: a humanoid biped, a dog- or cat-like quadruped, and a predatory, raptor-like biped.
1. A Starting Point
All terrestrial vertebrates (animals with a backbone) share several common skeletal characteristics. You’re pretty much guaranteed to find a skull, backbone, and ribcage in any vertebrate. If they have arms/forelegs, they most likely have shoulder blades and a collarbone, and hind legs usually come with hip bones. This gives us our starting point: the easiest place to begin drawing a vertebrate organism:
Note the simplified structures: a line for the backbone, and rough ovals for the head and ribcage. Though I haven't drawn them yet, there are hints of the pelvic bones in the quadruped and raptor; this is because the hips are very important to those animals' posture and balance, while it is safe to assume that the human's pelvis is directly below his ribs. The lines drawn inside the ovals suggest 3-dimensionality; it’s a good idea to get your perspective down from the very beginning, so you can fix any errors as you go.
The first differences between our three body types can be seen here, as well. The humanoid’s backbone connects to its skull at the base, so that its line of sight is perpendicular to its spine. The spines of the quadruped and raptor, however, connect to the rear of the skull, putting the line of sight parallel with the spine. Additionally, the humanoid ribcage is much shallower and wider than the others’ when viewed from the front; a creature whose arms extend forward from its spine (rather than outward like a human’s) has a much narrower, deeper ribcage.
2. First Steps
From the side:
Legs are absolutely key in drawing a vertebrate, as they define the creature’s interaction with the world – where it touches the ground, and how it balances. There are two major “types” of leg structure among vertebrates: digitigrade (the animal walks on its toes, with its “heel” off the ground – this includes animals like cats, horses, and birds) and plantigrade (the animal walks with its entire foot on the ground – examples include humans, other primates, and most rodents). You can see the difference between the two above. Beyond simple looks, they also describe how the animal balances its weight. The raptor and humanoid are both bipedal, but the raptor relies on its tail for balancing on its bent-kneed, digitigrade legs, while the human stands upright with knees locked. Leg structure can tell you a lot about the lifestyle of an animal.
From the front:
Like the ribcage, the hip bones of our three subjects differ based on the direction that their legs extend from the body; the quadruped and raptor’s hipbones are slim, allowing the legs to extend downward. The humanoid’s hips are wider, since the legs extend parallel to the spine. Additional width in all three animals comes from the structure of the thigh bone, which is essentially identical among the three: the leg connects to the hip via a ball-and-socket joint, with the “ball” extending from the leg bone at an angle to meet the socket of the hip.
Our first major difference in structure is the shape and alignment of the shoulder blades. On the cat, they extend up the side of the ribcage; this shape is important, as it shows you where and how the arms/forelegs connect to the torso. The human shoulder blade is much flatter, lying along the back. The raptor’s forelimbs are very similar to the cat’s, due to the vertical nature of the ribcage.
The “hands” of our three subjects will differ slightly; the humanoid and raptor both have similar wrists and grasping fingers. The cat’s paw is similarly constructed, but much blunter, with longer wrist bones and shorter digits. Other animals may have more unusual forelimb constructions.
So now we have the basic skeletal structure down for all three of our subjects, and we’re ready to take them fully into three dimensions.
3. We Want To Pump You Up
Muscles define the shape of the animal over its bones, and can greatly affect the final appearance of the animal in surprising ways. However, there are many points on the body at which the bone structure *does* define body shape. The skull (for the most part) is one. Body outline will generally follow the spine down the back and wrap around the ribcage, as well, and the elbows and knees are generally only just below the surface. However, muscles are essential for filling in the rest. In most vertebrates, abdominal muscles shape the stomach area to extend it smoothly below the ribcage, but the musculature of the limbs may vary. Below, you can see the major, shape-defining muscle structures of each of our subjects.
Humanoid:
The shoulder and pectoral muscles are the most obvious form-describing muscles in the upper body, and the biceps, triceps, and forearm muscles serve to define the arms. The quads define the front of the thigh, while the buttocks, inner thigh muscles, and calves shape the back and lower leg. Tendons stretching around the ball of the heel help to shape the foot.
Quadruped:
Two muscles define and move the shoulder and upper foreleg. The tricep shapes the “elbow” region, and the digital extensors shape the lower foreleg. A pair of muscles similar to the humanoid’s quads shape the upper leg and thigh, while the lower leg is subtly shaped by a muscle equivalent to the human calf. The “foot” is composed mostly of tendons, and follows the shape of the bones.
Raptor:
The forelegs are essentially the same in musculature to those of the quadruped, due to the similarity of function and the underlying structure of the ribcage and arm bones. The hind legs, due to their larger size and the raptor’s bipedal nature, have much more substantial muscles, though the anchor points are again similar. Note how the thigh muscle anchors and extends from the hip bone. The tail is interesting to note, because of its role as a balancer to the rest of the body (since the raptor’s weight is centered on its hips).
We’ve now broken down the body into a series of major muscle groups, and can return to our drawings. As we simplified the head and ribcage originally, we can similarly simplify the muscle groups as appropriately shaped ovals, resulting in the following:
4. More Than One Way To Skin A Cat
Last, we need to put skin over our creatures. For the most part, simply extend a smooth outline around the defined muscle groups; in cases of smooth-skinned creatures like primates and reptiles, this will most often be all you need to do. Animals with looser skins, like a cat or rabbit, may look substantially different, as their muscles move freely under their skin. The biggest concern when cleaning up and applying a skin is figuring out where the body’s outline will create creases or overlaps – for example, a human’s armpit, or the crook of the elbow or knee. The three-dimensionality suggested by the ovals should allow you to make good guesses, but when in doubt, USE ADDITIONAL REFERENCE IMAGES. Google Image Search is not just your friend; it is your friend, comrade, confidant, and fairy godmother all rolled into one lovely little blue-and-white, always-available package.
Remember the musculature you described in step 3, and use it to shape the skin and the final appearance of your subject. Use reference images to get the proper shape and placement of details like claws, ears, and facial features. Don't be afraid to tweak your design if it doesn't look quite right; for instance, I deepened the ribcage of my quadruped to give it a stockier, more threatening appearance. If all went right, you’ve ended up with a naked but well-structured subject; from there, you can add feathers, fur, clothes, armor, war-paint... anything you want.
The ovoids you used to shape the ribcage, head, and major muscles will also help to define areas of light and shadow if you choose to shade your drawing – pick a location for your light source (upper left is a common favorite), and think in your mind about which parts of a given body part will be in the light and in shadow. Again, reference images can help if you are having trouble visualizing it yourself; find a similar pose and light source and see which parts are lit and which are not. Below is a quick shading job of our three subjects, the final product of this brief tutorial:
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As a parting “gift,” here is a list of proportions I have compiled of the human body. These are average values, but following them will help you learn to create realistically proportioned humanoids. Once you have familiarized yourself with what appears “natural,” you will be able to tweak proportions to produce more interesting and unusual creatures.
1) The human body:
a. The human body is 7.33 "heads" tall, from crown to feet.
b. The waist is approximately 3 heads down.
c. The crotch is approximately 4 heads down.
d. Knees are approximately two heads up from the feet.
e. When the arms are relaxed, fingertips are approximately 3 heads up from the feet.
f. The shoulders are approximately 3 head-widths across.
2) The human head:
a. The distance from chin to crown is approximately equal to the distance from front to back of the skull.
b. Eyes are approximately halfway between the chin and the crown.
c. The base of the nose is approximately halfway between the chin and the eyes.
d. The lips are approximately halfway between the chin and the base of the nose.
e. The top and bottom of the ears are approximately level with the eyes and base of the nose, respectively.
f. The corners of the mouth are approximately aligned with the pupils of the eyes.
A last piece of advice: only practice can improve your drawing ability. Sketch all the time. Sketch from photos and from life to teach yourself body shape and position. The more you practice, the better you will get and the more natural it will come to you.