June 6, 2011

Osteochronology and the Berenstain Bears

My daughter Chickpea's current obsession is the Berenstain Bears, the titular characters of those ubiquitous and flimsy books that littered my bedroom as a preschooler in the early 80s.  As she is coming up on her second birthday next week, and yet has no real idea what "birthday" means, she has made me read The Berenstain Bears and Too Much Birthday (1986) dozens of times in the past week while she exhorts Sister Bear to "blow out birthday candles!"

I used to look back fondly on the Berenstain Bears along with other remnants of my childhood - that is, until I had to sit down and read them to Chickpea.  The anthropologist in me is quite irritated by the rather strict gender lines drawn in the series, especially between Brother and Sister Bear, as well as the depiction of Papa Bear in the bumbling oaf trope that is at least as old as Archie Bunker and yet as recent as Homer Simpson.  Normally, I'll just take a book out of regular rotation if it doesn't meet my expectations, but sometimes Chickpea has already fallen in love with it - then I have to make up a different narrative on the spot or attempt to explain the problematic parts.  Too Much Birthday has some issues with it, but there is one I can't let pass by as an osteologist - unfortunately, it's one I can't explain to Chickpea yet.

"Too Much Birthday" (1986)
At the beginning of the book, Papa Bear has taken Sister and Brother into the forest to fell a tree.  The cubs notice that there are rings on the stump:
“Those are annual rings,” explained Papa. “They tell us how old the tree is.” [...]
“Do we have annual rings, Papa?” Sister asked.
“No,” said Papa, giving her a little hug. “We have something better – birthdays!”

We all know that most trees in temperate climates make one new growth ring per year.  The width of the ring varies based especially on the amount of moisture (rainfall) the tree enjoyed.  All the same tree species in the same general area and climate will have the same width of rings.  This fact is especially salient in archaeology, as dendrochronology can tell us not only how old a tree is but how old particular timbers are, which allows an archaeologist to figure out when a house was constructed, for example.  We are, of course, humans and not trees.  But I disagree with Papa Bear's dismissal of Sister's question.  Our bodies actually do have annual rings - of a sort.

Cross-section of a long bone
Our skeletons are made up of two different kinds of bone - compact and spongy.  The compact (or cortical) bone forms the outer shell of most of our skeleton: it is hard, strong, and stiff.  Spongy (sometimes called cancellous or trabecular) bone is softer and weaker, typically found at the ends of the long bones and in the middle of the vertebrae - areas of the body that need shock absorption.  This spongy bone is not very dense and looks, well, spongy because it's highly vascular and contains red bone marrow for the production of blood cells.  Compact bone, on the other hand, has a special structure because it is so dense.  The fundamental unit of compact bone is called the osteon (sometimes known as the Haversian system).

System of osteons

A schematic of the osteon in cross-section can be seen to the right (and, of course, in the background of this blog).  Each osteon consists of concentric layers of cortical bone that surround the Haversian canal, which supplies blood and houses the bone's nerves.  Although our bones don't grow longer after puberty, they do remodel - constantly.  Cells called osteoclasts remove old bone tissue, and new bone is laid down through the work of cells called osteoblasts.  Eventually, the tunneling work of osteoclasts and the deposition work of osteoblasts form the concentric rings of the osteon.  This formation of new osteons continues throughout our lives, so we develop more and more osteons as we age.  With an increase in the number of osteons, the probability that one osteon will cut into another osteon increases.  This overlapping of osteons is illustrated in the diagram below.  You can see a complete osteon (a), an old osteon that's been cut into by a newer one (b), and layers of cortical bone with no osteons in them (c).

Section of cortical bone showing osteons, from Ubelaker 1989 (p. 92, fig. 104)

Estimating a person's age-at-death from his adult skeleton can be difficult.  Using macroscopic methods - a visual assessment - accuracy isn't great for people over the age of about 50.  New and better methods are always being invented, but the best way to estimate age from a skeleton still seems to be microscopic or histological analysis.  Good reviews of these methods can be found in Crowder (2005) and Robling and Stout (2008), but the technique that started it all is a count of the osteons in a long bone.

Based on original research by Ellis Kerley (1965), which was notably refined over the years with the help of Doug Ubelaker (Kerley and Ubelaker 1978), it is possible to estimate the age of an individual by counting the number of osteons and osteon fragments and estimating the percentage of cortical bone remaining.  Using a variety of regression equations (e.g., the femoral intact osteon formula for people under 50 or the fibula osteon fragment formula for people over 60), an age-at-death estimate can be refined to within perhaps 8 years.  Kerley's method is best applied to North American populations, as it seems there are population-specific differences in number and density of osteons.  It's also best applied to adults; kids have been subject to a completely different method that looks at the microscopic growth of the enamel of their teeth (Fitzgerald and Rose 2008).

Is Papa Bear right that telling a person's age isn't the same as telling a tree's age?  Yes and no.  Our bones do grow and change, creating what often look like rings at the microscopic level.  We can use our knowledge of how these rings develop to estimate the age at which a person died.  But osteoclastic and osteoblastic activity does not take place on an annual cycle like tree ring growth does.  So one of these days, long after Chickpea has forgotten about Too Much Birthday, I'll teach her about the development and remodelling of bone on the cellular level and explain why Papa shouldn't have dismissed Sister's question so quickly.





References:

Crowder, C.M. 2005. Evaluating the Use of Quantitative Bone Histology to Estimate Adult Age at Death. PhD dissertation, University of Toronto.

Fitzgerald, C.M. and J.C. Rose. 2008.  Reading between the lines: dental development and subadult age assessment using the microstructural growth markers of teeth.  In: Biological Anthropology of the Human Skeleton, M.A. Katzenberg and S.R. Saunders, eds. Chapter 8.

E. Kerley (1965). The microscopic determination of age in human bone American Journal of Physical Anthropology, 23 (2), 149-163 DOI: 10.1002/ajpa.1330230215

Kerley ER, & Ubelaker DH (1978). Revisions in the microscopic method of estimating age at death in human cortical bone. American journal of physical anthropology, 49 (4), 545-6 PMID: 216268

Robling, A.G. and S.D. Stout.  2008.  Histomorphometry of human cortical bone: applications to age estimation.  In: Biological Anthropology of the Human Skeleton, M.A. Katzenberg and S.R. Saunders, eds. Chapter 5.

Ubelaker, D.H.  1989.  Human Skeletal Remains: Excavation, Analysis, Interpretation.  2nd ed.  Taraxacum, Washington D.C.

    5 comments:

    petoskystone said...

    mama bear always wearing a dress is what irritates me the most. i steered grandchild #1 away from bb. & towards 'scary godmother'. most interesting post, as usual.

    Anonymous said...

    I do not really understand this idea of not overgendering children. I understand breaking down restrictions that say this job is for boys, this one is for girls, pink is a girl color, blue is a boy color, etc., is a good idea, but I do not understand this idea that people should be raised as genderless as possible.

    Bone Girl said...

    I don't understand the idea of the "genderless" child either. What baby Storm's parents are doing (as told by Storm's mom, in her own words, here: http://www.edmontonjournal.com/news/Baby+Storm+mother+speaks+gender+parenting+media/4857577/story.html) is going too far. But I agree with you that breaking down today's strict gender lines and encoded behaviors (pink/blue, nurture/aggression, passive/active) is important. My daughter loves pink, along with other colors. She has a Snow White dress-up outfit, along with a Roman gladiator one. I think it's about balance, and the Berenstain Bears don't have enough balance for my taste.

    Fins said...

    I think the issue is the aspects of behavior we choose to gender-polarize (interests, hobbies, likes and dislikes) and how early an age we start the process at.

    Stephan said...

    Over on Ohdeedoh, which I usually like (aside from some of the classism and the overgendering of children that usually pops up on shopping/decorating blogs), there's a post about how someone threw their kids a "teepee party," complete with a cake decorated with plastic models of poorly stereotyped Native Americans.

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