A vestigial trait in humans: the arrector pili

February 22, 2011 • 6:02 am

Yesterday I came across a photograph of this kitten:

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:

And a photograph, showing the muscle fibers:

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.

52 thoughts on “A vestigial trait in humans: the arrector pili

        1. He says – “Adaptations of the mammalian integument towards an aquatic existence are well reviewed by
          Sokolov [1962 Adaptations of the mammalian skin to the aquatic mode of life. Nature, 195: 464- 466] and Harrison and Kooyman (1968)”
          If you are really interested – & have the time! –
          HARRISON, R. J., and G. L. KOOYMAN. 1968. General physiology of the Pinnipedia. In R. J. Harrison, R. C. Hubbard, R. S. Peterson, C. E. Rice, and R. J. Schusterman (eds.), The Behavior and Physiology of Pinnipeds, p. 211-296. Appleton-Century-Crofts, New York.

          1. Swimmers shave (or wear slick swimsuits) to move through the water more quickly, perhaps the loss of arrector pili serves a similar function to help move through the water more easily. That could certainly be a survival trait.

            1. I mean to say that the hair sticking up when cold or threatened would impede swimming speeds and maybe cause them to freeze to death.

  1. If I recall, sea otters (at least) trap a lot of air in their coats to stay warm. So messing with the way their fur lays while in the water might not be as warm as it would be on the land, since it would let water in, which is better at carrying heat away than air.

    (Speculating here — that on humans, it’s kind of useless, but on marine mammals, it might just make them colder.)

  2. Do elephants, hippos and rhinos still have this vestige? They’re hairless. Also, what makes their skin grey and ours…not?

    1. The animals you list are not hairless. They’re covered in small hairs like us. Google “rhino hair” for an example. I assume the gray skin color is melanin pigment to protect against the sun.

      1. Way to nitpick my comment! I did mean “hairless (like us)” as in…their Arrector Pili are useless, so I was wondering about their arrector pili.

        And then why is Rhino melanin grey, and human melanin brown?

        1. My guess is that their skin is thicker than ours – like the soles of our feet tend to be more whitish – so the colour of the actual melanin is less prominent. They are kind of brownish-grey, if you look.

  3. The Angwantibo – (Arctocebus calabarensis) does not have the arrector pili either – according to this abstract from Montagna et al American Journal of Physical Anthropology Volume 25, Issue 3, pages 277–290, November 1966

    To me the poor little tings look pretty terrified all the time!


  4. I wonder if the arrectores pilorum are truly useless in humans. “Goose bumps” do allow us to tell, at least in close quarters, when someone else is cold or frightened. I can imagine some social functions for not being able to hide that response.

  5. Interestingly enough, I have a condition that is similar to Tourette’s, I just don’t have the symptoms as often as someone with Tourette’s, and when I have one of my uncontrolled reactions I also get goosebumps. Most people don’t believe I can’t control the action until I show them the goosebumps.

    1. Most intriguing! And sorry you have to deal with the poor judgement of other people on top of however else this affects you.

      1. I barely notice anymore, I have had it all my life. When I am having a bad spell I get it about once or twice a week, so it is not very often at all. I can’t imagine what a person with Tourette’s goes through. they have to deal with it every day multiple times through out the day and deal with all the misconceptions.

  6. A seal that erected its fur would allow water into its coat, get cold and heavy, and die.

    Probably a bad plan, all things considered.

    1. As long as we are speculating: they may pay quadruple penalty in threat situations.

      Allowing water in makes them
      1. effectively heavier as you say (have to move more dragged along water)
      2. but also increase drag (area of boundary layer).

      And if hair modulates flow,
      3. positive control disappears
      4. possibly to be replaced with negative effect.

  7. Birds are also able to ruffle their feathers, and some reptiles change their skin shape.

    I wonder if the gene responsible is far more ancient than mammals. I may be way off, but I can imagine (and according to Anselm, this means I’m right) that this muscle first appears in a common ancestor with cephalopods, perhaps to regulate nutrient and waste flow across the skin. Cephalopods run with it, but our line allows the gene to go dormant. But then throughout our lineage, this gene is called upon in varying ways. Fish use it to flex spines, or perhaps (lacking fossil evidence, and I know of no modern examples) changing scale patterns to reflect light in odd ways. The first amphibious pioneers used the gene to shift their body shape, or skin shape, to optimize for land or water. Reptiles find the gene in their genetic attic and flare frills or make smooth skin into spiny skin on command. Birds then find this nifty CGAT heirloom and use it in mating dances and threat displays, to keep warm, and to aid in grooming. Indeed, if I am correct that this is the same gene, birds use it more than cephalopods (don’t tell PZ).

    But of course, before we have birds, we have mammals, coming off ancient reptiles. My guess (my Texas theory) is that the small, tasty reptiles we evolved from had long since dusted off this gene and slapped in onto their posterior regions to lift spines of some sort. As the spines refined into hair, the gene for having them controlled by individual muscles proves very useful.

    This is only a guess, but if the good Professor Coyne is looking for a grad student for the fall of 2012 (assuming the world hasn’t ended (no, not Mayan prophesies, Tea Party prophesies [Palin 2012])), I would be happy to get on the lit review and research proposal.

    1. I may be way off

      that’s putting it mildly, dude.
      Wow, you’re way off.

      Just, for example, the very first thing: the common ancestor of mammals and cephalopods was a probably microscopic worm-like thing with nothing–nothing!–like mammalian skin or mollusk mantles.

      1. “Just, for example, the very first thing: the common ancestor of mammals and cephalopods was a probably microscopic worm-like thing with nothing–nothing!–like mammalian skin or mollusk mantles.”

        I may still be way off, but not so far as you I am afraid.

        The common ancestry of mollusks and mammals, or invertebrates and vertebrates, probably occurred when the ancestor was larger than microscopic. We diverged no sooner than 500 million years after multicellular organisms evolved, after all, 1.5 billion years after the first macroscopic single cells probably emerged. You may be correct. It is possible that a surprisingly tiny representative of the animal kingdom was the sire of giant squid and elephants, but you must admit to your rather open minded and wild conjecture on that note, unless you’ve isolated the genes to prove your case, in which case I profoundly apologize for my candor.

        But you claim that this ancestor had skin nothing, and then you emphasize “nothing!” like the skin of the modern progeny. This I find to be beyond conjecture and a statement that either must be heavily supported with evidence or dismissed as a poor grasp of biology, and a poorer grasp of evolution.

        Mammals and birds have skin that is very, very different, as did whatever our common ancestor was; a reptile of some sort, covered in scales. Scales! Not fur, not feathers, but scales! Yes, Sven, we know that this is amazing. But we are not surprised nor confused by this.

        The flatworm (the most likely candidate for the mollusk/mammal LCA) did have skin. That skin did evolve into cephalopod skin. That skin did evolve into mammal skin. Any one of the intricate traits of the ancestor skin can still persist in both lineages today. In fact, many, if not most, of the traits do. Flatworms can alter the shape of their skin. If the flatworm that spawned man and oyster did this by arrector pili, or by an ancestral version of arrector pili, and that same mechanism evolved into the chromatophore muscles of cephalopods, then I am correct, not some extreme version of “way off.” Do flatworms use muscles to change shape? http://www.rzuser.uni-heidelberg.de/~bu6/Introduction02.html I would say step one of proving my case is complete.

        If you think I am grossly mishandling your argument because of my failure to understand, please respond with something less vague and factually handicapped. If you see a problem with my thinking, I do hope you’ll share. I just find this first attack (the good kind of attack, I must thank you) to be lacking. I am an ecology student, not an evolutionary biology student, and freely admit gaping ignorance in comparison to many of my fellow WEIT readers.

  8. I also get goosebumps when listening to music, particularly classical. My question is one of cause and effect: Do I get goosebumps because the music moves me, or do I find the music moving because it’s able to elicit a goosebump response?

  9. Do I get goosebumps because the music moves me, or do I find the music moving because it’s able to elicit a goosebump response?

    Neither causes the other; rather, both have a common cause in the ability of music to evoke emotion. Whyever the hell it can do that.

    1. Whyever the hell it can do that.

      Hrm, I guess I was asking something related. I’m wondering whether I perceive emotion before or after I get goosebumps. Wild speculation here, but what if music mimics a stimulus that used to be coupled with something that was genuinely fear-inducing (I’m thinking particularly of some type of predatory animal call)? Can the music elicit goosebumps first, which then triggers an emotional response?

      I ask this because, for me, goosebumps from music seem to precede a swelling of emotion. And I get emotional about many other things that don’t give me goosebumps, so the cause-and-effect isn’t clear to me. Maybe I’m weird.

      1. If you’re weird, we’re both weird. I have the same exact response (goosebumps before a swell in music.)

        I’d suggest it has something to do with anticipating a swell in the music. We’re “getting ready” for the feeling we expect to follow.

        If I can speculate more wildly, I’d suspect that the subconscious feeling we’re preparing for in the musical build is being “in sync.” Creatures being ready to respond in a predictable way together seems like an adaptation that is spread all across the animal kingdom, so I suspect it is very deeply wired, and probably enhanced in social organisms.

        I think the relationship between music, dance, and spoken language is wrapped up in some kind of deep subconscious wiring to anticipate and participate in collective, cooperative behaviors.

  10. showing the striation

    Pedantic niggle: “striation” is a technical term in muscle physiology; these are smooth muscle tissue; no striations.

  11. I don’t know why, but I like “erector pili” better. Both “arrector pili” and “erector pili” are correct, I think. Can anyone confirm?

  12. My arrectores pilorum are invariably stimulated by great music. Certainly this can’t be explained as part of the reaction to temperature. Would it somehow be explicable as part of the alternative cause Dr. Coyne offers – the “fight or flight” response?

  13. When I urinate I sometimes have a short bout of shivering. I think this happens more often if the environment is cold.

    I am guessing this reflex is due to the heat lost from my system. Does anyone know ?

    P.S. But no shivering results as far as I can recall when defecating


    1. I should add that the shiver is accompanied by a pleasurable orgasm-type sensation of around 4-out-of-10 strength !

      1. Look for the article on “post-micturition convulsion syndrome” on Straight Dope. It’s a bit old, but I haven’t found anything more recent that is as detailed.

    2. I commented on a thread here about a year ago that other scientists (I’m too busy on other things) need to get cracking and figure out what causes the piss shivers. (because the question bugs me and therefore should take top priority)

      As Sal Bro mentioned, Uncle Cecil made a stab at it, and has remarked that it was one of the few things he had tackled that did not admit any coherent answers (item #11).

      1. Sal & Sasq ~ thanks for the suggestions. Yes research grant needed pronto

        I suppose that the piss shivers somehow relates to the close proximity of the ‘wiring’ in the genital area for two different types of ‘relief’ 🙂

        1. Getting it while in need rather than during/after urination is a surprise to me

          Are you male ? Depending on where I look on the www the PS is exclusively a male experience or rarely a female experience

          1. I’m female, and I get them before and/or after when I really need to go. My impression is that it’s much more common in females than most people realize, probably in part because women usually pee in private and so don’t have any idea about what is common.

  14. “the absence of arrector pili muscle, and the absence of alkaline phosphatase in the hair follicle nerve end-organs are distinctive features of angwantibos” – American Journal of Physical Anthropology Volume 25, Issue 3, pages 277–290, November 1966
    Angwantibos are a type of loris. To me they look perpetually surprised with their big staring eyes!

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