Sunday, July 22, 2012

So, what is Fur Anyway?

Pet owners are usually well aware of how abundant fur can be
Human hair
The short version is that fur is a subset of hair, but that's a bit of a major oversimplification. To begin, let's look at the anatomy of hair in general.

As we all know, hair is something that only mammals have. It serves some of the same purposes as scales and feathers, but is definitely unique. The main function of hair is insulation and thus temperature regulation, which is vital for endotherms to maintain homeostasis. Interestingly enough, scientists haven't been able to pinpoint when it evolved, including whether endothermy or hair were first on the scene.

Anatomy of a hair follicle
Hair is a complicated product of the outermost layer of the skin, known as the epidermis. The hair follicles form there, then grow downward into the lower dermis. A bulb forms, and this bulb is where the live cells are and where hair growth happens. The part you actually will see, the hair shaft, is comprised entirely of dead cells. The hair root is surrounded by nerves which are sensitive to touch. This is why a hair cut doesn't hurt but pulling hair out does. Hair shafts will also have muscles and these muscles make your hair stand on end, which in humans will result in what we call goose bumps. Sebaceous glands will also be associated with hair follicles and are the source of the oil that can be so often found on and around hair.

Hair structure
Hair structure is also fairly complex. The innermost portion is call the medulla and the density of cells can vary a lot here. There is usually a lot of empty space, but in some species cells are entirely lacking, making the hair hollow. Most of the hair shaft is the cortex, which is made up of very densely packed cells. Lastly, there is a transparent layer known as the cuticle. This part is scaly in appearance and the scale pattern can vary quite a bit, to the point that examining hair is not a bad method for identifying a species. This is particularly true of species that may otherwise look very similar and prove difficult to identify using other characteristics.

Now, let's move on to types of hair. There are more than you might have originally thought. There are two basic types of hair with more specific types fitting into one of the two. Angora hair is hair that never stops growing and which may or may not shed. This would be what humans have on their heads, what horses have in their mane, and what certain breeds of dog have all over their bodies. In contrast, there's definitive hair, which will only grow so long and which shed and re-grow regularly. This would be what's on the body of a cat and most dogs. Within angora and definitive hair, there are a number of other relevant hair categories, and this is where it starts to get complicated.

Vibrissae on rats, along with pelage
Within the definitive hair type, you see vibrissae and pelage. Vibrissae are stiff and used for sensory purposes, with the best known type being whiskers. Pelage is what most people would describe as fur and which consists of guard hairs and underfur. Within this, there are a few different types of guard hair. Awns are the most common type, but spines, as in what you find on hedgehogs and porcupines, are also a kind of guard hair. As for angora hair, bristles are an important type, being used for visual display as in horses and lions. Wool and velli are also specialized types of hair. Wool is long and soft. Velli is the "downy" hair that you see on newborn animals.

Horses have numerous types of hair: vibrissae, pelage (both guard hair and undefur), and bristles
As a final note, in this blog I have used the terms fur and hair before in perhaps an unusual way. In general, I refer to angora hair as hair and definitive hair as fur. Though this is a gross oversimplification, it's at least a simple shorthand that breaks the two into appropriately separate categories and can be useful when talking about such things as dog coats. It's easier to explain to someone that a Shih Tzu has hair like humans have on their head rather than saying it's angora hair and thus exhibits a continuous growth pattern.

Source is Mammalogy: Adaptation, Diversity, Ecology. Images are from Wikimedia Commons and are under Creative Commons licenses or are copyright free: one, two, three, four, five, six.

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