25 Days of Skeletal Facts: Day 5 – The Bony Pelvis

Parts of the Bony Pelvis. Source: Teach Me Anatomy

The bony pelvis consists of the right and left hip bones, the os coxae, articulated with the sacrum and the coccyx as you can see in the image to the right. The os coxa begins as three separate bones, the ilium (orange in the image below), the ischium (purple), and the pubis (red) that fuse together at the acetabulum (hip socket joint) at puberty. Bones are usually named for their shape but the os coxae is also known as the innominate, which is Latin for “not named” because anatomists could not think of an object this bone resembled. The sacrum and the coccyx are essentially the ending segments of your spinal column that have fused together. In animals like monkeys, individual coccygeal vertebrae continue posteriorly to form the tail.

If you’ve even seen an episode of Bones, you have probably witnessed Dr. Brennan proclaim something along the lines of “female, age 25” after quickly perusing a set of human remains. We won’t get into adult age-at-death estimates now (they are a bit more complicated) but for forensic anthropology and bioarchaeological purposes we look to the bony pelvis first to estimate biological sex of an individual. (Please note that gender expression is different from biological sex, and even sex is not the binary we have come to think of it as – see commentary on Sex, Gender, and Sexuality here and an article on transgender and intersex considerations in forensic anthropology here for more reading).

Complete bony pelvis (above), R os coxa (below)

In general when we look at the pelvis for sex estimation we are looking at multiple features and scoring them on different scales that are more female characteristics or ones that are more male for that feature. For example, the arch of each pubic bone (see above) form a subpubic angle, which is usually greater in females (approaching 120º) and narrower in males (closer to 90º). Typically areas of the female pelvis are wider to facilitate childbirth, should it occur. When traits are considered all together, sex estimates can place individuals on a scale from female -> probable female -> indeterminate -> probable male -> male. Everything is a spectrum. Estimating sex is an important starting point in forensic cases and it also helps us in bioarchaeology when we want to consider trends like age-at-death and stress/health patterns by sex at the population level in the past.

Now you know a little bit more about how Dr. Temperance Brennan operates in the field (although really you need to assess all the traits individually and it takes way more time to have a truly confident sex estimation). Tomorrow we’ll begin talking about the vertebrae, starting with your lumbar region, giving you a peak into lower back support.


The Pelvic Girdle. Teach Me Anatomy Series. 2020

Estimating Sex from the Pelvis. Future Learn. Durham University. 2020

Evaluating macroscopic sex estimation methods using genetically sexed archaeological material: The medieval skeletal collection from St John’s Divinity School, Cambridge. Inskip et al. American Journal of Physical Anthropology. 2018.

The Human Bone Manual. White & Folkens. 2005

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