Knowledge Feature – Determining MNI (Minimum Number of Individuals)

Pic: These are all walrus bones. The question is, how from how many walruses?

At work today I found myself presented with a great, quick teaching opportunity. I’m in the midst of cataloging the artifacts collected from the first part of a large field project we conducted during October/November 2017 (with part 2 coming up this spring/summer). My work is kind of split into two parts per bag of artifacts – identifying what’s in the bag and then counting them (over the last 6 or so weeks I’ve counted over 15,000, with many more still to come). It was while I was busy counting bird bones today that I found myself inspired to impart some wisdom.

In bio/osteoarchaeological work, determining the MNI is part of every single project. Even if it’s a neat and tidy, undisturbed burial (which, to be honest, it often is not). MNI stands for the “minimum number of individuals”. It refers to the fewest possible number of people (or in the case of zooarchaeology, animals) in the osteological assemblage you’re working with. It’s especially important for forensic cases (including mass disasters), when forensic specialists need to know how many victims are present.

First and foremost: in the case of human skeletal remains, hire a bio/osteoarchaeologist. In the case of faunal (animal) remains, hire a zooarchaeologist. Determining the MNI of an assemblage is not difficult, but a major part of it requires specialized knowledge in being able to identify and side (figure out if it’s left or right) bones. Now that I’ve said that, let’s get on with it. Like I’ve already mentioned, figuring out the MNI isn’t all that difficult. I’m going to use a large assemblage of bird bones to illustrate the steps for you. Disclaimer: this is the method I use, based on what I was taught. There are a couple of slightly different methods which I’ll briefly mention later, but because I’ve never used them I’m not going into great detail on them.

Step 1. NISP, then sub-NISP

NISPing and assemblage is easy. NISP stands for “number of identified specimens”. When talking to zooarchs, they sometimes use NISP for “number of identified species”. That second definition is what I refer to as sub-NISPing (what I, a non-zooarchaeologist, have been taught). The NISP is easy to determine – just count every single piece of bone you have, intact and fragments. That’s your NISP. In the case of faunal remains (or if you suspect you have fauna co-mingled with human remains, or if you want to know the differences between adults and juveniles, etc.), you’ll want to sub-NISP, or divide the bones according to specific class. This is where specialized training comes into play. You need to be able to identify different classes of fauna (mammal, bird, fish, reptile, amphibian), and depending on what level of detail your project requires you might also have to further divide them according to species. My current project luckily does not require species (yet), and for the purpose of this example I’m sticking with simple class divisions. My bird NISP was 226 bones.

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My NISP for this bird assemblage is 226. Different sizes of the same bones (i.e. femurs) tell me there are definitely multiple species present.
Step 2. MNE 

The next step is figuring out the MNE – “minimum number of elements” (we really like counting things in many different ways). The MNE refers to the minimum number of bones (elements) that can be positively identified as to what they are (i.e. this is a humerus, this is a tibia, this is a phalange, etc.). Again, this is where specialized knowledge is helpful – you need to be able to identify what each bone is. Or at least try to, fragments are really difficult to ID. Always keep a good reference source nearby (whether a book or website). By the time you’re done you’ll have several MNE’s listed (one for clavicles, one for humeri, one for tibiae, etc.).

Step 3. MNI

The final step is determining the actual MNI. By this point you have your assemblage divided according to specific identifiable elements. My personal preference is to first focus my attention on single-occurrence bones (bones we only have one of). It’s simple if these bones are relatively intact – you count how many are there. If these bones are fragmented, it becomes a little trickier. First, pick a specific feature of the bone to look for. From there you’ll have to identify sides (left or right) of the feature you’ve chosen and pick one side to count. That count is your MNI. For example: we only have one mandible, which has a left side and a right side. The feature I’ve selected to count is the mandibular condyle (the rounded part of the mandible that hooks into your skull). If I have two condyles and one is a left side and the other is a right side, my MNI is one – every individual has a right condyle and a left condyle. If I count two right condyles in the fragments, my MNI becomes two. Your MNI will always be the larger of the two numbers (for the two sides) you count. If there are three condyles and two are right and one is left, my MNI is two (that left condyle might be paired up with one of the rights). Four condyles and three are right and one is left my MNI is three. Etc, etc.

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Each bird has only one synsacrum. I counted 5 synsacrums, therefore my MNI is 5.
   Counting the MNI from sided bones (bones we have pairs of, a left and right) is easier. Count all the left sided bones, then count all the right sided bones, and the larger of the two numbers becomes your MNI. We have a left humerus and a right humerus. If you count 7 left humeri and 5 right humeri, your MNI is 7. We have a left femur and a right femur. If you count 3 left femurs and 6 right femurs, your MNI is 6.

 

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For the purpose of this example I first separated out all the identifiable bird femurs.
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I then counted the femurs according to sides. I had 8 right femurs and 11 left femurs (pictured here). My MNI therefore is 11.
There you have it. The easy way to determine the MNI of an assemblage. There are two additional, slightly different methods you could use. One is called zonation, where bones are physically divided (on paper in a handy chart) into different zones. You then count your assemblage into sides (letf/right) and from there the zones. Your MNI is the largest number of your zone counts. The second method, similar to zonation, is called landmark counting. Again, you first begin by identifying your bones according to sides. From there you look for specifically identifiable features, or landmarks, (again, as determined by a handy paper chart). I’ve personally never used either of those methods so I’m not comfortable going into much more detail on them. You also want to take into account size differences (which can result from different species, sex, and age). Each method, including the one I described above using the bird example, has its pros and cons and are designed to give you only a most likely number of individuals. They’re designed to point you in a direction to continue taking your work.

**Take away point** The MNI is the largest number of all the different counts of bone you have (you’ll have a lot). Returning to my bird example, I counted 5 synsacrums, 8 right femurs, and 11 left femurs. My MNI is 11.

Suggested Readings:

Lambachr, N., K. Gerdau-Radonic, E. Bonthorne, and F.J. Valle de Tarazaga Montero. 2016. Evaluating three methods to estimate the number of individuals from a commingled context. Journal of Archaeological Science: Reports (10): 674-683.

 

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