White-tailed Deer Sternum

Fig 1. Unknown bones I found in the woods, 2023.12.09.

I went for a walk by myself the other day to scope out an area I was going to be going with my Learn The Land program with the Guelph Outdoor School. I wanted to see which areas would be worth investigating and get a sense of how long it would take to get to different landmarks I thought might be worthwhile to go with them. While I was out in a part of the forest I didn’t even consider would be that interesting, I came across a bone, or a series of bones rather, which I wasn’t familiar with. I had to take some photos and knew I would be looking it up when I got home.

My first thought was a spinal column but with the spinous process’ chewed off. But if they were chewed off, and I have seen that before, there would usually be a stub of where they once were. And if it were some sort of spine, where would the vertebral foramen (the holes in the vertebrae where the spinal cord runs through and is protected)? I didn’t see any trace of that either. It had to be something else. A buck has about 327 bones in their body, so I don’t feel bad for not yet knowing them all. It feels good actually, to come across bones I have not seen before so I can come home and investigate.. but sometimes I can’t sort it out right away. I ended up putting the photos away in my database labelled as “unknown” and nearly forgot about them. It wasn’t until I was more recently looking up metacarpal bones that I came across a drawing in a book titled “Osteology of the White-Tailed Deer” that I remembered about the photos. Turns out it wasn’t a spine, but it’s close. These bones are located on the other end of the ribcage, the underside, helping to shape the barrel chest we imagine when we see a deer in our minds. These bones make up the sternum, or the breastbone of the deer.

To be totally honest, my research on the sternum has not provided much results. It is not covered in the literature as much as other bones in the deer body. I am unsure why, but this is what I have found. This disregard is also reflected in the naming of the bones which make up the sternum. The sternum of a deer has seven bones, but only two are considered important for use as landmarks on the body. The bone at the cranial (front end, closer to the head) end is called the manubrium (the bone labelled B in Fig. 2 below). This bone is missing from the specimen I found.

Fig 2. Sketch of an entire sternum (dorsal view) of a White-tailed Deer (Odocoileus virignianus) from Osteology of the White-Tailed Deer by Paul Frederick Rumph, 1975. Image is rotated horizontally to orient the same as the photos I took. B is the crainial end (closest to the head) and D is the caudal end (closest to the tail).

The bone labelled C in the Fig 2 above is called the Xiphoid process. This bone was included in the sternum I found. Is is located positioned caudally (towards the tail). The parts of the sternum are named after old words that depict parts of a sword: the Latin manubrium means “handle”. The prefix man- usually denotes something to do with the hand (manual, manufacture, manumut, etc). Xiphoid comes from xiphos which is Greek for ‘sword’ and, as many of my regular readers remember, the suffix -oid means ‘looks like’ or ‘is similar to’ (like the word humanoid meaning ‘looks human’).

Fig 3. Still from video by Olivia Garas (@Oliviasoddartisticcreations) shows orientation of deer sternum in relation to the skeletal structure as a whole.

Fig 4. Vertebrae, ribs, sternum. Image edited from Osteology of the White-tailed Deer, Rumph, 1975. The part labelled A is the ossified or bony part of the ribs. B is the cartilage. C is the ninth rib which is the first to indirectly articulate with the sternum.

The other segments of the sternum are called sternebrae and they are held together by cartilage when the deer is young, but as they get older this cartilage is replaced by bone. Looking at how fused the bones are of the sternum I found, I would think that this would be from an older deer. I have not yet found any information on how long it takes for the bone to develop and if it is possible to age a deer based on this.

If you can imagine the thoracic skeletal region of a White-tailed Deer being comprised of the thoracic vertebrae, ribs (costae) and the sternum. There are 13 thoracic vertebrae and 13 pairs of ribs which articulate with the vertebrae. The 8 ribs which are closest to the head of the deer articulate with the sternum through direct connection via cartilage attachment. These are called “true ribs”. The remaining 5 ribs to the posterior connect indirectly with the sternum and instead connection to other bands of cartilage which then articulate with the sternum. In some mammals the 13 rib doesn’t articulate at all with the sternum, and is then sometimes called a “floating rib”, a term I had heard before but never knew what it meant. When describing the interior of the cage-like area created by the vertebrae, ribs and sternum you can use the phrase “thoracic cavity” or “thoracic cage”. The ribs and sternum help protect the vital organs within the cavity such as the heart and lungs.

So far this is all I know about the sternum of White-tailed Deer. I want to learn more, and have found a couple of new textbooks on physiology and anatomy for vet techs, but they don’t have much on deer, and the books I do have on deer don’t have anything really about the sternum. I’ll have to keep looking for resources. Until I can suggest anything else, see the “To Learn More” section below.

To Learn More :
Osteology of the White-Tailed Deer (with link to download the pdf)
The Deer of North America by Leonard Lee Rue. The Lyons Press, 1997.
Dictionary of Word Origins by John Ayto. Bloomsbury, 1990.
Clinical Anatomy and Physiology for Veterinary Technicians by Thomas Colville and Joanna M. Bassert. Mosby Elsevier, 2008.
Anatomy & Physiology by Kevin Patton and Gary Thibodeau. Mosby Elsevier, 2013.
Check out Olivia Garas work on instagram @the_darling_dead or on youtube at @Oliviasoddartisticcreations