Comparing bones: scapulas (shoulder blades)

This is my first ever post about comparing the same bone from lots of different animals from my collection. It's different to my normal blog posts because I had to do a lot of research, but interesting because I got to look at how each animal has adapted to where they live and what they do.

Everything about scapulas I have found on the internet is very complex and scientific. I try not to write my blog like that, and I only try to use words I understand myself to make my posts easy to understand. I've picked the scapula for my first one of these posts because it is a relatively big bone that varied quite a lot between each animal type. I've tried to look at each animal and how it lives to understand why its scapula ended up that way, so some of what I have written is guesswork, and some is from reading books and the internet. So here it is.

What is a scapula ?

The scapula is a flat, thin, triangular bone usually with a ridge along the length of it. In humans it is on the back where the arms attach to it at the shoulders. In all animals I can think of that that walk on four legs it is on the side of the body, like this:

It is very unusual because it is only attached to other bones at the shoulder, and doesn't properly attach with the spine. It is only attached to the shoulders with muscles but this also means it can move around a lot depending on the position of the front legs (or arms). In skeletons in museums I have seen it sideways, low on the ribs or high up to the level of the top of the spine depending on the pose of the animal.

I picked eleven mammal scapulas from my collection, one of which is on my bat skeleton so isn't in the picture below.

I have put them together in the groups I think they fit best in.

Ruminant (grass eating mammal) scapulas

Ruminants are the group of mammals that eat greenery and have a stomach that is in four parts. They usually eat grass or other plants, chew it a bit, swallow, then later bring it up and chew some more. They do this because they can't properly digest it in one go. The ruminant scapulas I have are red deer, roe deer and sheep.

They are triangular with almost straight edges. The bit at the bottom fits over the top of the humerus (upper arm bone) like if you put your hand palm down over your fist. It fits well but not as well as the hip joint which is a deeper socket. The socket is called the glenoid cavity but I'm just going to call it the socket to keep it simple. These are left scapulas, because the ridge goes nearest the head. The flat part (called the costal side) goes against the ribs, and the ridge side is on the outside (called the dorsal side)

The reason why scapulas are so flat is so that there is lots of room for muscles to attach. The ridge along the middle scapula is called the spine which is a pretty stupid name because it could get confused with the animal's actual spine. 

Deer and sheep have different lives. Deer are amazing at running and jumping, and are super fast and flexible. Sheep are slow runners, rubbish jumpers (apart from the ones you knit) and spend most of their day standing or moving slowly, only moving fast for small distances. The two deer scapulas are longer and thinner in proportion to the sheep which is shorter and faster. So maybe having a long thin scapula  is better for the muscles that make you better at running and jumping ?

This next picture shows the ridges better:

If scapulas are flat to let muscles attach, maybe a bigger ridge (spine) means that animal is stronger at moving the shoulder blade in a certain direction. The red deer has a big ridge, but it was from an adult stag (male) and male red deer are bigger and stronger than females. The roe deer was from a doe (female) and I don't know about the sheep. 

The hook which comes off the shoulder socket to the left here is called the coracoid process and some muscles attach here.

A vet told me that muscle sits on top of the flat bit of the scapula up to the level of the top of the ridge, meaning this red deer (which I call Roger) was very muscular indeed. 

The other thing about ruminants is that they use their legs for running, and so move from front to back, but don't often need to move them strongly from left to right. The muscles which pull the legs from front to back are probably stronger than the muscles which pull the legs forward because when the legs are pulling back they are on the ground and doing all the work.

The shape of the shoulder socket is different on the deer to the sheep. On the deer it is almost round and deeper. On the sheep it is uneven and shallower. That probably makes it a stronger join for deer when running over uneven ground, so it is less likely to get dislocated. Sheep don't do a lot of running so this isn't important for them.

Seal scapulas

The seal is a lot different from the ruminants. It isn't a triangle at all, but almost like a circle with a bite taken out of it. There aren't really any straight lines at all. The sheep and deer scapulas had the ridge almost at the left hand edge, but this one is almost in the middle.

Seal front arms are very different to other mammals. They use them for swimming, but hardly ever to support their weight. So the muscle groups should be very different from sheep and deer.

The ridge is still there but it is tiny. So whatever muscles a red deer had lots of, the seal doesn't need much of. Or maybe the seal has very strong forearms/front legs for swimming, so more of the scapula is for muscles between the scapula and humerus (upper arm bone). It doesn't have a hook by the socket (coracoid process) at all.

The underneath of the seal scapula isn't flat like the deer, but is rippled which is strange. Maybe muscles go on here to pull it back towards the body.

Medium-sized mammal scapulas

I picked scapulas from my fox Pharoah, my cat Kitty and my otter Abraham.

I grouped the fox cat and otter together on size but their front legs do different things. A fox is good at running and digging. A cat is a fast runner but can also climb. Otters are fatter and slower but are very good swimmers. But their scapula shapes are similar. The ridge is in the middle. The front edge, nearest the head, is curved and the back edge, nearest the back legs, is straight.

The ridge is a good size and ends is a slight hook over the shoulder joint. The cat scapula has the extra hook (the coracoid process) coming out of the side of the shoulder socket . The fox has one as well but not quite as big. 

Although the three animals do different things with their shoulder blades, they all end up quite similar though.

Small mammal scapulas

This rabbit scapula is long and narrow with two straight sides like the scapula of the deer, but has a curved top like the fox cat and otter. This is probably because it runs extremely fast, like deer, but needs strong front paws to dig burrows. It has a big hook (coracoid process) by the socket.

The ridge height is about the same proportion as the female roe deer but not as big as the red deer stag.

Hedgehogs and grey squirrels don't seem to have much in common except their size. Hedgehogs are slow moving, and squirrels can be very fast. This hedgehog scapula was from my juvenile hedgehog skeleton called Oscar. The squirrel scapula is from my grey squirrel skeleton Xavier.

Hedgehogs use their front paws for digging dens, and squirrels use them for climbing. These are different type of movement.  Hedgehogs need their paws to go forward and back, and squirrels need to grip food or branches, which moving paws together or apart. These movements need different groups of muscles on the scapula.

Both hedgehog and squirrel scapulas have a special bit of bone which comes off the middle ridge of scapula and hangs over the socket. The cat scapula had this as well. It is called the acromion and it is on the scapula of animals that have collarbones (or clavicles). Humans have clavicles, which are thin bones that go across the top of the chest from the shoulder to the top of the sternum (front of the ribcage). Animals that have clavicles are those that need to grip their arms together as it helps hold the scapula in place. But for animals that need to run fast and move the scapula fast, it isn't a good idea as it would restrict the movement. That's why hedgehogs and squirrels have them but rabbits and deer don't.

This is the same scapulas turned over and it's better to see the acromion. You can also see something strange ! The squirrel has an extra ridge on the inside (the costal side). This must be where it can have extra muscles attached. It also has a massive hook (coracoid process). If all these animals were the same size, I think the squirrel would have the strongest shoulders by far.

Bat scapulas

The last scapula I have is from my pipistrelle bat Ziggy. Bats are mammals too, like all the other animals here, even though they fly. This is tiny, maybe less than 1cm across, and is so thin it is see through. Ziggy was eaten by dermestid beetles who left tiny bits of tissue to hold the bones together. You can see the left humerus (upper arm bone) and clavicle (behind) still attached and the ball of the humerus joining the scapula. This scapula is huge, almost the length of the whole bat rib cage.

This is it from a different angle with it held by tweezers. At the top you can see the acromion and the coracoid process hook at the top, (at least I think it is that way round but it is difficult to tell)

Scapulas are even more important to bats. A deer with a damaged scapula can still run on three legs, but a bat with a damaged scapula wouldn't be able to fly. That must be why bat scapulas are huge in comparison.

This is the first time I have done a scientific study post on comparing bones. I've guessed some of it but hope I have got most of it right. If it's popular I'll do more like this in the future !

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