Minimum Number of Sexless Individuals
October 16, 2010 10:52 PM   Subscribe

Say I have two femurs (okay, I do have two femurs, like most people), but say I have them lying on the table in front of me (I don't, but go with me), and I know one of them is from a woman and one is from a man, is there a way an archaelogist/paleontologist/someone-smarter-than-I could tell which is which?

I know, for example, the pelvis is one way of indentifying if a whole skeleton is male or female. But are there any other bones in the human skeleton that are typically used to indentify male or female persons from the bones alone?

And even if I don't know the two femurs on my table are from one male and one female (determining which is which might be easier if working with that knowledge, ie. comparison might yield a "this one is more likely male" result), is it plausible that the sex of the person could be determined just from the femur alone?
posted by crossoverman to Science & Nature (11 answers total) 4 users marked this as a favorite
Yes, yes it is. The femur is one of the go to bones for sex determination, because many of its characteristics are defined by the pelvis - which is the prime go to for sex determination.

You can do femoral head measurement (with smaller heads being female, larger being male, with a fuzzy section in between). You can look at the angle of the femoral neck, but that is a bit hit or miss.

You can also look at a few other markers on the bone, but those are the big two that I remember. If someone else doesn't answer this better by the time I'm awake tomorrow, I'll fish out my old bio-anth texts and find the rest of the sex measurements and landmarks for the femur.
posted by strixus at 10:59 PM on October 16, 2010

Best answer: Ok, so never mind, the book I wanted was easily at hand.

"Like bones of the arm, bones of the leg differ between males and females in terms of size and robusticity"

"For each of the bones, sexual dimorphism is relatively small" but in most cases the determination can be made unless it is a very fragmentary or borderline case.

Femoral head size is, as I remembered, the most accurate method of sexing a femur (males are rarely if ever smaller than 45mm, females are rarely larger than 46mm, which gives a very narrow margin for gray cases).

Midshaft circumference and anterior-posterior diameter at midshaft can both be used as well, but are slightly less accurate (82% accurate in some tests).

There are a few more complicated methods, but I was generally told that if I had a whole femur, and every part of the bone I looked at agreed, you were generally right 9 out of 10 times for most adults in the US.

Juvenile cases are harder, but that is generally true with any bone in terms of trying to sex juvenile skeletons.
posted by strixus at 11:07 PM on October 16, 2010 [1 favorite]

Oh, and the classic trick my bio-anth prof always played on us during exams was to make sure you didn't have two rights or two lefts of a bone when trying to sex. Because then you KNEW you had two individuals, and had to sex each one independently.

If you want to read more about this, the text I grabbed is The Anatomy and Biology of the Human Skeleton, Steele and Bramblett. It is a really accessible text, and very easy to learn from thanks to some great diagrams and very clear pictures.
posted by strixus at 11:10 PM on October 16, 2010

Response by poster: Are there any bones that are particularly difficult to sex determine? Can you easily sex determine a skull or a humerus?
posted by crossoverman at 11:21 PM on October 16, 2010

Best answer: No.

All of the sex-determining characteristics of bones only work on a relative scale. Yeah, the femoral head on a woman's femur is generally going to be smaller. But smaller than...what? Smaller than her brother's femoral head? Probably. Smaller than the femoral head of a woman of Nordic descent if she's, say, Japanese? I don't know.

Sex determination really only works if you have a large sample group (like if you were to do a dig in a cemetery in England that dates to the 1400s), but is pretty useless if you just find a femur. You can make educated guesses, but no self-respecting anthropologist would sex a bone with certainty.

I took an osteoarchaeology class several years ago, and every member of the class had a skeleton to work with. All of the skeletons were from Indian people (I have no idea how or why the anatomy department acquired them). It was pretty easy on most of them to make a good guess as to their sex, relative to each other, but when the professor brought out a skeleton of a generic European dude it dwarfed all of our bones. Male and female alike. So you really can't say. You run a better chance if you have the pelvis, but even then it's not 100%.

On our tests my professor would ask us to make a "best estimate and why" when sexing the bones (wow, that sounds dirty), and cautioned us never to say just "male" or "female". I always laugh when I'm watching Bones and Emily Deschanel says something doofy like "judging by the radius we're looking at a woman in her mid forties." Bullshit.

Age determination of skeletons is a lot more accurate. And a lot more interesting (in my opinion).
posted by phunniemee at 11:32 PM on October 16, 2010 [6 favorites]

Are there any bones that are particularly difficult to sex determine?

Yes. All of them. I think the pelvis and the mandible are your best bets.
posted by phunniemee at 11:33 PM on October 16, 2010

Having an idea of race helps, too, but I am biased despite my osteology coursework...being one of the grey cases (at least on estimate, as no one has actually removed my femur to check).

I liked the Human Bone Manual as an overview--it's a condensed version of White's larger osteology text. However, I think that's a 'different schools; different texts' thing, rather than a judgment in quality w/r/t Steele and Bramblett.
posted by Uniformitarianism Now! at 11:43 PM on October 16, 2010 [1 favorite]

I should say that yes, a great deal of it is dependent on populations, race, nutrition, etc. However, while we might not be able to say with 100% certainty in every case that a femur is male or female, chances are pretty good you can get it right if you know what to look for.

It really does help having a complete skeleton, and no, sex determination, even with a pelvis, is never going to be as good as age verification, but you can have a pretty high degree of accuracy.

So, allow me to rephrase what I said above - you can, to a very good degree of certainty, say that a femur is male or female, assuming you have a whole femur, and know what population the individual came out of, and know some metrics about average gender markers of weight, height, and what not are for that population.

I assumed the poster was asking about in a known population, because it is very uncommon that you do not have a general know population to be working out of, even if it is as broad as "average joe white American".

But even if you did have a mystery femur, assuming a whole femur in good shape, you can compare its numbers to a number of groups and populations and say well, if this is an African-American, here are the number ranges we'd look for, if it is Okinawan, we use a different set. And generally, since humans are skeletally more alike than different over our constructed population groups, they would, in most cases, trend towards being able to say that the femur might be male with x degree of certainty, or female for y degree.

The general rule I was given was always that the smaller a bone was, the more difficult it was to sex. Individual bones out of the wrist, feet, and spine are VERY hard if not impossible. I wouldn't want to do a determination based on even a whole hand, or a mix of partial bones from all over. Skulls are hard, but I know someone who did research on sex determination by eye orbit angles, so it can be done.
posted by strixus at 12:03 AM on October 17, 2010

I guess the point both phunniemee, Uniformitarianism Now!, and myself are trying it make is that yes, it is possible, but most experts worth their meager pay will always put it in terms of degrees of accuracy - you will never be 100% sure of sex unless you have some other way of linking the individual to their bones. But you can be fairly sure, really really fairly sure. But then you can also be totally clueless.

I mean really, when confronted with a drawer full of femurs, and told sort by pairs then by sex, I felt pretty ok with nearly every pair or single I put into a drawer, but I wouldn't have wanted to put a man in the electric chair based on my sorting, either.
posted by strixus at 12:07 AM on October 17, 2010

Best answer: Depending hugely on the condition and age of the bone, it may be possible to examine cells from it to look for sex chromosomes. For a whole femur I would think that the marrow would be your best opportunity for that.

This isn't my field, but I bet you could use PCR and then look for Y-chromosome markers.

In principle this would be the same for any bone.
posted by Chocolate Pickle at 12:36 AM on October 17, 2010

If you have a bone, and you know the population it comes from, then you can make a guess.

One of my anth professors did a lot of physical anthro and now focuses on gender in a cultural anth context. So during our Anthropology of Sex and Gender class it came up that some of my classmates think you can always (always!) tell if someone is a man or a woman just by looking at, and if you really have to, talking to them.

I was pissed until my dear professor took the time to explain again (she probably explined this once a week for the whole semester):

there is more variation among females as a group, and more variation among males as a group, than there is between the two groups 'male' and 'female'.

And of course we spent a great deal of time discussing chromosomal variation (please don't call it abnormality) involving the x and/or y. How would you label a Klinefelters bone? Or a xxx bone (oh, stop snickering). Does your text assert that especially tall and agressive males may be XYY? If so, it's outdated, because that theory was based on some pretty crap research.

So you know what? Take that femur, get some marrow out of it, and check the DNA, because physically...knowing and guessing are different.

Not to be a total downer, are we talking about a cis woman and man who might each have self identified as an opposite gender? Because, well. That's a sticky wicket. Trying to id a murder victim? Or brainstorm murder motives? Or make assumptions about how the deceased may have lived?

And now that I'm challenging those assumptions, check into Ann Fausto-Sterling. She asserts there are up to 7 genders, with some fluidity in the system that is very fluid compared to our binary system. (I'm not being judgy with that, binary makes much more sense when first meeting an interlocutor).

Oh. And I do find the idea that 'average American joe' could be a reasonable population guide to be more than a skosh ethnocentric. There's is nowhere near a sufficiently consistent phenotype for 'average American white' as this notion suggests. And my god, if you knew the skeleton was from a white person, and the gender was gruesome to have found a buried leg so well preserved or so close to time of death.
posted by bilabial at 5:37 AM on October 17, 2010 [3 favorites]

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