It’s cute, of course (check out the valentine patch), but today we’re paying attention to its fur. If you have a cat, or simply observe animals, you know what’s going on off screen. The kitten is freaked out: either it’s been scared or, more likely, has seen another threatening beast: another cat, a dog, or a vacuum cleaner. The rush of adrenaline associated with fear or threat causes it to erect its hair. This makes the cat look bigger than it really is, and in this case probably serves to ward off predators or competitors. (Note too how the kitten hunches its back and stands up on its toes—another way to exaggerate your size.)
What makes the hairs stand up on end? Attached to the base of each hair follicle is a tiny muscle, collectively called the arrector pili. When the muscle contracts, it pulls the hair upright. Here’s a diagram:
The muscles contract under several circumstances. When a mammal is cold, it raises its fur for insulation, trapping a layer of warm air next to its skin. If you have a cat, you’ve probably seen it bush out its fur in winter.
And, as noted, the muscles are used in a situation of defense, to exaggerate apparent size. But you also want to exaggerate your size during offense, when you’re trying to achieve dominance, get food, or intimidate another beast of the same species. Here’s a brown hyena (Hyaena brunnea) trying to dominate a confrère:
Our closest relatives, the chimpanzees, also use their arrector pili during encounters that show dominance. Dominant individuals sometimes approach others walking upright, with fur on end:
And like our closest relatives, we have arrector pili too—and they act in a similar way. When they contract, our hair stands up and we see the phenomenon of goose bumps—so called because our skin resembles that of a plucked goose.
When does our hair stand on end? In two situations: when we’re cold and when we’re freaked out (“that horror movie made my hair stand on end”; we “bristle” with anger or fear). These are precisely the occasions when the hairs stand up in other mammals. But in our species, erect body hair has no function in keeping us warm—certainly not in Africa where the bulk of our early evolution took place. As naked apes, we’re simply furred too sparsely. And our thin body hair can hardly act to threaten or intimidate other humans.
In humans, the arrector pili—and their function when we’re cold or watching a scary movie—is a vestigial trait. It serves no purpose in our own species, but is there as an evolutionary remnant, for it has an adaptive function in our relatives, and did so in our ancestors. It’s evidence of our evolution from fur-raising ancestors, and I discuss it on p. 62 of Why Evolution is True.
Why did it evolve in ancestral mammals? I’m not sure about the evolutionary origin of these muscles, or whether the selection pressures that produced them involved cold, threat, or both. As far as I know, they’re present in all terrestrial mammals* (somebody check echidnas and platypuses, please!), and mammals evolved from reptiles. The fur was undoubtedly an adaptation for thermoregulation, and if you could get warmer by erecting that fur, so much the better. And early mammals were largely nocturnal, so a hair-erecting threat display may have been of little use.
My best guess is that the muscles originally evolved to keep the mammals warmer, and were secondarily co-opted for threat displays. In other words, the hair-erect threat display would be what Steve Gould called an exaptation: a trait evolved for one purpose that subsequently comes to serve another. Now, of course, there’s been additional evolution, so that hair erection is automatically triggered by an adrenaline surge. That situational behavior can be seen as an adaptation—just like the “wings” of penguins, which were exaptations whose precise “finlike” form evolved, as an adaptation, through natural selection.
*Pinnipeds (sea mammals like seals and walruses) and sea otters lack arrector pili, almost certainly reflecting a secondary loss after invasion of the water. Why they’ve lost them, and we haven’t, is a mystery. Perhaps they’re more deleterious if you’re living in water.