Classical MileEnd Alpacas
The International Alpaca Reference Library

Articles by Alpaca World Magazine:

How Dense Are You?

Dr Stephen Mulholland

Dr Stephen Mulholland trained as a laboratory scientist and holds a PhD is Biochemistry and Molecular Biophysics. In 2003 he moved to New Zealand, bought a small farm and started raising alpacas. In 2005 Stephen began, with the assistance of the AANZ and the NZLA to run health surveys of the llama and alpaca populations of New Zealand and to date has collected 15,000 animal years data on their morbidity, mortality and management. He led the team that produced the final draft of the Code of Welfare: Llamas and Alpacas that was launched in April 2013 whilst in June 2013 founded a camelid health and welfare charitable trust with other interested alpaca and llama owners: www.camelidhealth.org
How Dense Are You?
Generating a fair assessment of alpaca fleece density
Alpacas have the capacity to produce truly wonderful fleece - soft, lustrous, and in a panoply of colours. To maximise the profitability of alpacas, people are naturally pursuing breeding goals to maximise the output of fleece. In this pursuit the trait called 'density' is seen as a worthy objective.
But what is density, how do we measure it, and how do we fool ourselves? And perhaps most importantly, how can we make better informed decisions when it comes to breeding programmes seeking to increase density.
The scientific definition of density, in regard to alpaca fibre, is a measure of how many hairs grow per unit of skin, usually measured in follicles per square millimetre. This can be measured directly by taking a skin biopsy and studying it under a microscope to count the number of hair follicles. This also gives a measure of the number of primary fibres compared to secondary fibres.
There are a couple of practical problems with this direct biopsy measurement. Most obviously this is not something that you can do on your own, unless you happen to be a veterinarian with access to a good histology setup. So there is a cost to such sampling. A single biopsy also gives you one 'data point', it is a measure from a single area on the blanket region of the fleece.
When you are looking at large numbers of animals for sale (or for use as a stud), it is quite difficult (expensive) to get that many samples collected and analysed and some owners may not want skin punches taken from their animals by a possible tyre kicker. The single point in time measure of density is also problematic on young, growing animals that have not yet come into their adult fleece, which could be higher or lower density than their cria fleece.
One of the common techniques I see people applying is what I term 'a good grope'. While examining the fleece to look for colour, staple, crimp, they will at some point give it a good squeeze to see 'how dense it is'. I do this myself. But just what are we measuring when we give the fleece a squeeze? Density? Nope. What we are measuring is the resistance to mechanical compression. Now the resistance to mechanical compression has a relationship, correlation, to fleece density, but it is driven by many variables, is more complex than you might first think, and is very susceptible to incorrect assessment of fleece density.
Yes, more fleece is harder to compress than less fleece, but that is only one aspect that affects your perception.
1. Thick fibres are stiffer than thin fibres. An everyday example is tree limbs. The little branches bend out of your way when you apply pressure, but the same amount of wood in a thick branch would resist that pressure. Make a bit of wood a bit thicker makes it much stiffer. What this means is that a bunch of 25 micron fibres will be much better at resisting the compressive load than 15 micron fibres.
2. Bundled staples let fibre support one another. Like coiled bed springs, an 'organised' fleece where the fibres are bundled together will be much stiffer and thus resist your grasp.
3. Longer fibres bend more easily. The tactile response to hair varies with length. Short hairs can feel stiff, think of rubbing the head of a freshly crew cut friend, while the same hair a few centimetres long feels completely different. The same applies to alpaca.
4.You can end up confusing the amount of hair mass per unit area from the number of hair follicles. If all else was equal, and it's generally not, as the points above show, one 30 micron fibre has as much hair mass as four 15 micron fibres.
.Environmental factors can change the feel of a fleece, from humidity to the nature of the soil that the animal dust bathes in.
It is possible to account for all these factors and make a good estimate of density. You would need to estimate the micron, look at the fleece structure and examine the staple lengths. Then you juggle all these variables together in your head and make the final calculation. It takes time and lots of practice.
Of course, there is a difference between thinking you know how to estimate by feel and actually being able to do so. Just because you've had lots of practice, it doesn't mean you are doing it right.
The way to measure your effectiveness is a 'blind' test. This doesn't mean you work with your eyes closed. Rather, you are presented with a group of animals with known density measurements, either through skin samples or by using the maths I'll get into shortly. But the key here is that you don't know how dense they are. You study each animal and make your results known. Then the true values are revealed, and you see how close, or not, your predictions were.
Training in this process can be helpful - but only if the person training you knows what they are doing. A wrong method can be dutifully passed on from teacher to student just as readily as a correct method.
Of course, when you are assessing the density of animals for use (studs) or purchase, getting out to get a feel for the fleece may not be possible. Maybe they were just shorn. Maybe they are thousands of kilometres away and you can't justify the cost just to look at one or two animals. Or maybe you are surfing the web and have assembled a list of 200 animals across dozens of different farms you're interested in, you'd never have the time to go see them all.
I'm going to show you a technique called 'normalisation', this is how you can convert numbers so that when you compare two different alpacas you are, in a sense, comparing apples with apples. It does not provide a definitive answer as to the exact fleece density of an animal, rather it allows a large pool of animals to be compared to one another to determine which are better and which are worse.
The first and most important factor to correct for is the mean micron. It takes four times as many 15 micron fibres to make a kilogram of fleece as it does 30 micron fibres. So if animal A had one kilo of 15 micron fleece and animal B had one kilo of 30 micron fleece, then animal A would have four times the fleece density.
The maths behind the calculation is quite simple. I'm just taking the cross sectional area of a round fibre -π r2. The table below lets you compare the relative weights of different finenesses.
1 kg of Is equal to this many kg of fleece of a different micron µ

14 µ 16µ 18µ 20µ 22µ 24µ 26µ 30µ 35µ
14µ 1.0kg 1.15kg 1.65kg 2.05kg 2.47kg 2.94kg 3.45kg 4.6kg 6.24kg
16µ 0.76kg 1.0kg 1.27kg 1.56kg 1.89kg 2.25kg 2.64kg 3.52kg 4.78kg
18µ 0.6kg 0.79kg 1.0kg 1.23kg 1.49kg 1.78kg 2.08kg 2.78kg 3.78kg
20µ 0.49kg 0.64kg 0.81kg 1.0kg 1.21kg 1.44kg 1.69kg 2.25kg 3.06kg
22µ 0.40kg 0.53kg 0.67kg 0.82kg 1.0kg 1.19kg 1.40kg 1.86kg 2.53kg
24µ 0.34kg 0.44kg o.56kg 0.69kg 0.84kg 1.0kg 1.17kg 1.56kg 2.13kg
26µ 0.28kg 0.38kg 0.48kg 0.59kg 0.72kg 0.85kg 1.0kg 1.56kg 1.81kg
30µ 0.22kg 0.28kg 0.36kg 0.44kg 0.54kg 0.64kg 0.75kg 1.0kg 1.36kg
35µ 0.16kg 0.21kg 0.26kg 0.33kg 0.40kg 0.47kg 0.55kg 0.73kg 1.0kg

Example: After years of use, it is time to replace your stud 'Super Duper'. You want a boy who is a bit finer, but has at least the same density. Super Duper clips 2.5 kg of 22 µ blanket fleece. So all else being equal, we'll get to staple length and body size later, how much blanket fleece would you need on a finer animal to be the same density.
For a 14 µ stud you'd need a fleece weight of at least 2.5kg* 0.40=1.00kg
For a 16 µ stud you'd need a fleece weight of at least 2.5kg* 0.53=1.33kg
For a 18 µ stud you'd need a fleece weight of at least 2.5kg* 0.67=1.68kg
For a 20 µ stud you'd need a fleece weight of at least 2.5kg* 0.82=2.05kg
So if you find a 16µstud, he only needs a 1.33kg blanket fleece to equal the density of 22µ Super Duper.
Next we need to consider staple length. Obviously a longer fleece weighs more. This is great for increasing total fleece yield, but can confuse matters when considering density. Simple example, if you had two animals, A and B, with the same micron, and the same fleece weight, but A had a 12cm staple and B had a 6cm staple, then B has twice the follicle density.
Don't necessarily discount the density of low fleece weight animals. Their density may be fine, they may just need to be bred to an animals that will improve staple length.
Staple length consideration is especially important when considering first fleeces, which can be 15cm or more, to the fleece of an older animal that is only cutting 6cm a year. Once you account for staple length you'll be surprised just who is the densest.
This of course leads to body size. When you've just shorn a 2 month old, 20kg, cria how can you fairly compare its density to a fully adult stud male?
Micron and staple length play a major role, but you can correct for body size by measuring back length, from the bottom of the neck to the top of the tail. This defines the area that is clipped as the blanket. I know it is a linear approximation for what is a changing two dimensional shape which is the region we call the blanket but it will produce at least a ball park figure.
Adult animals generally have similar back lengths, so this correction factor can often be ignored. If you have an especially long backed animal, it is probably worth measuring.
It is also worthwhile to calculate the density of each animal each year as it changes over time.
A cria is not born with all its adult hair follicles, they physically couldn't fit. Instead the skin of a cria has thousands and thousands of 'placodes'; these are biochemical markers which say 'a hair can grow here'. That doesn't mean a hair will grow there, it is simply a limit on the maximum potential fibre density. A cria that is sick, injured, or underfed may never reach its full genetic potential and not every placode will turn into a hair follicle.
Likewise older animals can suffer from declining density. We all know what it is like when you get older and your hair starts getting a bit thin. But if you measure the density, correcting for micron and staple, you can then spot those animals that maintain good density in later life, a useful and probably heritable trait you can use to add value.
Finally a quick warning about breeding for any trait. Any trait, bred to an extreme, can be detrimental. It is possible that super high fleece density could lead to other health problems. Balance your goal of breeding for improved density with the overall health and welfare of your alpacas. More doesn't always mean better.
We would like to thank the Australian Alpaca Association for allowing us to reprint the text of Dr Mulholland's address at their 2014 Alpaca Excellence Conference.