NS: It has to do with its size, and whiskers have special nervous connections that make them highly sensitive to movement. SA: How does a whisker work as a sensory receptor? Theres a strange animal from the Old World called a pangolin, which has these scaly plates that cover most of its bodythose are modified hairs. Whiskers are hairs that work as sensory receptors. A porcupines quills are greatly enlarged hairs. There are many different kinds of modified hairs to which we give different names. But that could be one of the reasons why we dont have hair on our faces. That just happens to be what happened in apes. That doesnt mean you have to get rid of the hair to see the face. 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. Some are incredibly hairy, and some have considerably less fur on the face and the chest and so on. Theres a lot of variation in how much of the body is covered with fur in various primate groups. SA: Humans evolved in Africa, along with a lot of primates that are covered with fur. We really dont know anything about these things. 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. So having hair as a means of insulation is one of the ideas about why we have hair. If youre going to spend a lot of metabolic energy heating your body, its more efficient to hold on to that heat and not to lose it to the environment around you. NS: I think most evolutionary biologists believe that the evolution of hair is correlated with the evolution of endothermy, or warmbloodednessthe ability to produce internal body heatand hair is a very good insulator. NS: There are very few fossils where there are impressions of anything in terms of soft tissue. SA: Are there any impressions of hair in the fossil record? So we cant know whether many of these relatives of mammals from the age of dinosaurs and earlier had hair or not. NS: We dont know, because the evolutionary lineage leading to mammals includes many fossil forms going way back in time, and hair, as a rule, doesnt fossilize. SA: When does hair appear to have arisen? Other features that define mammals include producing milk to nourish the offspring. SA: Is hair a defining characteristic of mammals? Whales, for instance, are mammals, but they are nearly hairless. And thats one of the things that have changed through evolution in a number of mammal groups. Your dog or cat is basically covered with hair, whereas humans tend to grow hair in a few selected places. Whats really different is the pattern of where it grows. So across the breadth of mammals, there are many norms for hair length, or fur length. The normal length of the hair is an individual and species specific trait. NS: Actually, a lot of types of human hair wont keep growing and growing. SA: Why is it then that, for example, my dogs fur is three inches long and it never seems to grow longer, while my own hair keeps growing and growing? SA: Is there a difference between hair and fur?
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