NS: There isn¿t. Hair and fur are the same thing.
SA: Why is it then that, for example, my dog¿s fur is three inches long and it never seems to grow longer, while my own hair keeps growing and growing?
NS: Actually, a lot of types of human hair won¿t keep growing and growing. The normal length of the hair is an individual and species specific trait. So across the breadth of mammals, there are many norms for hair length, or fur length.
What¿s really different is the pattern of where it grows. Your dog or cat is basically covered with hair, whereas humans tend to grow hair in a few selected places. And that¿s one of the things that have changed through evolution in a number of mammal groups. Whales, for instance, are mammals, but they are nearly hairless. We lack hair over a lot of our bodies.
SA: Is hair a defining characteristic of mammals?
NS: It¿s one of them. Other features that define mammals include producing milk to nourish the offspring.
SA: When does hair appear to have arisen?
NS: We don¿t know, because the evolutionary lineage leading to mammals includes many fossil forms going way back in time, and hair, as a rule, doesn¿t fossilize. So we can¿t know whether many of these relatives of mammals from the age of dinosaurs and earlier had hair or not.
SA: Are there any impressions of hair in the fossil record?
NS: There are very few fossils where there are impressions of anything in terms of soft tissue.
SA: How did hair evolve?
NS: I think most evolutionary biologists believe that the evolution of hair is correlated with the evolution of endothermy, or warm¿bloodedness¿the ability to produce internal body heat¿and hair is a very good insulator. If you¿re going to spend a lot of metabolic energy heating your body, it¿s more efficient to hold on to that heat and not to lose it to the environment around you. So having hair as a means of insulation is one of the ideas about why we have hair. Of course, there is no way for us to tell whether hair evolved first and then endothermy evolved, or whether endothermy evolved and then somehow hair evolved. We really don¿t know anything about these things.
SA: Humans evolved in Africa, along with a lot of primates that are covered with fur. Why did humans lose most of theirs?
NS: We don¿t know. There¿s a lot of variation in how much of the body is covered with fur in various primate groups. Some are incredibly hairy, and some have considerably less fur on the face and the chest and so on. Primates tend to rely on facial expressions for social communication, and of course the better you can see the face, perhaps the better that social communication works. That doesn¿t mean you have to get rid of the hair to see the face. That just happens to be what happened in apes. But that could be one of the reasons why we don¿t have hair on our faces.
SA: Is a whisker a special kind of hair?
NS: Yes, it is. There are many different kinds of modified hairs to which we give different names. A porcupine¿s quills are greatly enlarged hairs. Whiskers are hairs that work as sensory receptors. There¿s a strange animal from the Old World called a pangolin, which has these scaly plates that cover most of its body¿those are modified hairs.
SA: So this is all the same material?
NS: This is all the same material.
SA: How does a whisker work as a sensory receptor?
NS: It has to do with its size, and whiskers have special nervous connections that make them highly sensitive to movement. Those nerves are directly connected to a part of the brain that keeps track of that information and allows the animal to interpret it as sensory information in conjunction with the other information it¿s getting from adjacent whiskers.
SA: When you see something that looks like a whisker on a catfish, for example, what is that structure?
NS: Well, it¿s a similar structure in the sense that it is a long, skinny thing that sticks out from the body and is used to help sense what¿s going on in the environment. But it¿s not homologous; it¿s independently evolved. It¿s not made of the same material, and it wasn¿t inherited from a common ancestor. It¿s a completely different structure that may serve something of the same purpose, but completely independently.
We may think about human hair¿curly versus straight versus whatever¿as being really different from what animals have, but if you think of the breadth of mammals out there you can find equivalents in many other groups for long hair versus short hair versus tightly curled hair and all that. You can actually find all of that in dogs, without even having to look to other species.