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The three skeleton mystery


This week's post is about age indicators on bones. Telling an animal's age can be easy when they are young, because there are lots of indicators with teeth (like my one), bone size and unfused bones, but the exact age of adult skeletons can be very difficult to say exactly. Even for deer, where I have lots of comparative skulls, teeth wear at different rates whether the deer eats grass or heather, so generally you can only say whether a deer is "very young", "juvenile", "adult" or "old" rather than a precise age.

But this week's post is a mystery too, and I'm hoping some experts have some ideas. If you're in a rush, just go down to the very end of the post, where I put the question I would like ideas about !

The mature adult skeleton

Here's the teeth, which are often a good way to age a skull, and the bottom jaw usually shows wear better than the top teeth. All six adult teeth are through, which happens in red deer at about 30 months.  The very back cusp (spike) on the back molar (just out of shot on the right) is worn as well, which put it at over four years. When seen from above the brown inner part of the tooth is wider than the white, making it quite worn, and an older deer. The brown part is more than twice as wide as the white enamel, which The Deer Initiative [PDF link] say is a sign that this deer is in older middle age, or between 9 and 12 years old.

This is the underside of the braincase. The first crack isn't visible, because it has fused, which shows this is an adult skull. The second crack hasn't yet fused, which shows it is not yet in old age.

The front nose bones are still loose from the skull, which again shows it isn't in old age.

I have noticed on my skulls that the pedicles, (which are the small tower of bone that the antlers grow on) get shorter and fatter in older deer. This pedicle is wider than any other deer I have.

The antlers are hardened, but broken. The left antler has no spikes (tines), and the right antler has the brow tine (forward facing bottom spike) and the bey tine (above it, the yellow arrow points to it). The main branch of both antlers has broken, which is odd. It might have happened "in velvet" when the antler was growing, or maybe the antler was malformed. The deer died when the antlers had hardened, though.

The bey tine means this was an older red deer. I have never seen this on a red deer stag until it has at least ten tines in total, which usually means an age of at least five or six.

The scapula are massive, with a big spine for muscle attachments. Male red deer are bigger all round than females, and this scapula shows it is from an adult red stag.

The femurs are also about 3-4cm or so longer than from a female adult red deer:

These rough parts at the knee end of the femurs are I think muscle attachments, and the rougher this part, the stronger the animal. 

All of the vertebrae are really big, and this thoracic vertebra had a massive 'spine' for muscle attachment over the shoulders. This was a mature, strong red deer.

The coccyx sacrum (thanks for the correction, Lena !) the bottom of the main spine, where the pelvis attaches) is well fused, again showing a mature adult. There is no sign of arthritis or any other extra bone growth from old age.

Again, the pelvis is in two parts, suggesting an adult, but not an old one, and the coccyx is not fused to the pelvis, which happens in older deer (I know the coccyx is the wrong way round !).

The bottoms of the metacarpals are fused, which means an adult as well. The brown part is a fat stain.

So if I had to take a guess at the age of this skeleton, I would say between five and ten years old, and probably at the top end of that, but certainly not younger or older.

The juvenile skeleton

The easiest way to identify bones from a juvenile (still growing) mammal is that the ends of the bones are not yet properly fused on. For example, on this neck vertebra, the plates on each end aren't properly fused on yet:

The same is true of this humerus, which is large, and the head is fused on, but the crack is still visible.

Here's a comparison between a femur from an adult female red deer, and that from this skeleton. The end is fused on, but the crack on this one is still visible, and the top edge, just by the knee, isn't yet smoothed.

Again, here are two thoracic vertebrae where the ends are not just not fused, but are missing entirely:

Clearly, this skeleton is from a deer where the bones were still growing, so the age of the deer would probably be under about 3.5 years old.

The elderly red deer skeleton

In older mammals, the bones begin to fuse together. This radius and ulna aren't yet fused completely, but you can see lots of bone growth along the contact points:

The back edge of the jaw also has extra bone growth along the edge, probably for muscle attachments. Again, this isn't something I have seen in younger skulls.

This is the pelvis, seen from underneath. It is still in two halves, but inbetween a Y-shaped piece of extra bone has formed, showing it is from an older animal. This means it is getting on a bit (like my dad !).

This isn't necessarily a sign of old age, but it looks like bone damage, probably from trauma or hitting something hard while running. It's from the metatarsal, which is a lower leg bone in deer, just above the ground. 

So clearly, this skeleton was entering into old age.

So what's the mystery ?

There are three skeletons. The first shows signs of being an old adult (maybe 5-9 years old), the second shows signs of being under four years old, and the third is the oldest, maybe at ten years or older.

But here's the mystery: there is only one skeleton ! The bones are all from the same red deer, Roger, who I discovered as an intact skeleton four years ago. I took Roger out for some TV filming recently, which is what made me think of it again.

So here's my question: how can one animal show signs of being an old juvenile, being an adult, and being an older animal all at once ? What could have caused this ?

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Lena said...

Excellent ageing analysis, taking into account tooth eruption, tooth wear, bone fusion and antler development. I think the main thing you're not thinking about (or at least not mention in the post) is that bones fuse at different ages.

Now, I'm not at work so this will be all from memory (and admittedly, most of my articles relate to cattle and sheep rather than deer). The metacarpals fuse earlier than the upper humerus, but after the lower end of the humerus. The upper and lower end of the femur fuse at approximately the same age give or take a couple of months (incidentally the same age as the upper humerus and the distal radius) - in cattle this is at about 3.5-4 years. The endplates on the spine are among the last to fuse. But also these fuse at different times. One animal can have both unfused, fusing and fused endplates. I think the Y-shaped epiphysis between the pelvis halves also fuse rather late, but it's not a bone that is used much for ageing.

As you can see from your skeleton the lower radius and femur are fused, and the upper femur and humerus are fusing. Now, bone fusion can be delayed if the animal is castrated, but as this would be a wild deer it is less likely. The fusing bones will therefore be a good indicator of the deer's age.

I'll check my notes on tooth wear when I'm in the office - there is a guide to tooth wear stages on fallow deer, and I'm sure red deer teeth wear similarly.

And a slight nitpick: the coccyx is the human tailbone - what your photo shows is the sacrum (also found in humans).

rob said...

do yo collect taxidermy because i do and where can i find skulls in surret

Jake said...

Ha, thanks for the correction, have changed it above !
I know different bones fuse at different ages, but I think this red deer is in the 6-10yo age group from the teeth and antlers, so all bones should have fused long ago (certainly by about the age of 3 or 4, the last adult tooth, M3, comes up at age of 2.5).

sedruff said...

What in David Duchovny's name could be going on here? (Yay! I got an excuse to say "What in David Duchovny's name") But that is really, really peculiar. Sorry, I've got nothing productive to add aside from, "Hmmmmm."

Matt Williams said...

Hmmm very strange, cant help you much there!

Anastasia said...

Nice work and very good observations and questions! You must become a scientist when you finish school :)

Lena's reply pretty much covers it. I am a human osteologist, so animals are not my expertise, but even in humans, we don't become adults overnight! We go through puberty, and even until our mid-20s, fusions and changes keep happening to our bones and teeth. Also, as you correctly suspected about one bone, when you see something unusual, which is definitely different from the other skeletons in your reference collection, you must think if it is injury (trauma) or disease (pathology) related. Keep up the good work!

PS. Your dad takes great photos.

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