Can you see that monkey up there?

by Greg Mayer

Among the first phenomena to be interpreted in a Darwinian manner after the publication of the Origin of Species was adaptive coloration, most famously Batesian mimicry (wherein a palatable organism mimics a noxious organism); Jerry has recently posted  on mimicry in insects and in birds. Matthew has brought to our attention a paper by J.M. Kamila and B.J. Bradley, in press in the Journal of Zoology, on another aspect of adaptive coloration: obliterative, or countershading, and in particular how it applies to primates. The Capuchin below is not countershaded.

Capuchin monkey, Guanacaste, Costa Rica, by David M. Jensen, from Wikipedia

Countershading, which is familiar to fishermen and military planespotters, consists of having the illuminated surface of an object darkened, and the unilluminated surface lightened, so as to “counteract the effect of shade and light”, producing “upon a rounded surface the illusionary appearance of flatness” (Cott, 1957:36). As such, it is one of the chief methods by which animals (as well as war planes, at least old ones) achieve concealment, and is very common. Kamila and Bradley, in their paper, ask: If primates spend a lot of time standing up on two legs (like we do), are they less likely to exhibit dorso-ventral countershading? Intuitively, it seems entirely plausible, and, after measuring the reflectance of the front and back of skins of 113 species of primates, they find that, indeed, the more bipedal a primate is, the less strongly it is countershaded.  So now we know why our backs and chests/bellies are about the same color– we’re too bipedal!

My experience is that monkeys in trees are hard to see, regardless of whether they are countershaded. The three common Costa Rican species shown in the pictures here, all of which I know in the wild, are hard to spot, even though they are not countershaded. The large white scrotum of the male mantled howler, below, known as “huevos”, do make the males somewhat more conspicuous, but this is almost certainly a sexually selected feature.

Mantled howler, Prov. Alajuela, Costa Rica, by Tim Ross, from Wikipedia.

The fourth Costa Rican monkey species (not pictured here), the squirrel monkey, is countershaded.

[Jerry’s note: I’ve added the picture below, which shows the countershading of a squirrel monkey: it’s darker on the illuminated dorsal (back) side and lighter on the ventral (belly) side:]

Of about 30 species of monkeys in the Guianas, Venezuela, and Colombia, many of which are very strikingly patterned, only a handful might be considered countershaded (Eisenberg, 1989).  Perhaps not surprisingly, Kamila and Bradley found that the effect of bipedal tendencies, while significant, was small. They did find that body size made a difference (bigger, less predation-prone primates are less countershaded), but that group size does not (although it was almost significant). Overall, the factors they considered explained only 14% of the variation in countershading in primates.

Spider monkey, Golfo Dulce, Costa Rica, by Steven G. Johnson, from Wikipedia.

Somewhat surprisingly, adaptive coloration was very controversial (critics considering resemblances of mimics and models, and the concealing effects of color patterns, to be coincidental) in Darwin’s time, and continued to be so for decades afterwards. It was not until 1940, that Hugh Cott, one of the 20th century’s most influential herpetologists, put the controversy to rest in his classic Adaptive Coloration in Animals. We’ll conclude with some video, taken by my wife, of monkeys leaping from tree to tree near Tortuguero, Costa Rica. It was a typical lowland Costa Rican day, quite warm, which enables me to label this video as “hot monkey action” (let’s see how many hits that phrase brings in!)

________________________________________________________________

Cott, H.B. 1940. Adaptive Coloration in Animals. Methuen, London.

Eisenberg, J.F. 1989. Mammals of the Neotropics. Vol. 1. The Northern Neotropics. University of Chicago Press, Chicago.

Kamilar, J.M. and B.J. Bradley. 2011. Countershading is related to positional behavior in primates. Journal of Zoology 283:227-233.

26 Comments

  1. TheBlackCat
    Posted March 12, 2011 at 3:27 pm | Permalink

    This doesn’t seem that surprising. There are several issues:

    1. They tend to live in areas with lots of foliage, and thus less light and less consistent light. This reduces the benefit of countershading compared to the sky or sea, since the difference in light and dark has less to do with what direction you are looking and more to do with the pattern of leaves and branches in the way.

    2. They aren’t oriented that consistently. Even if they are not bipedal, when in the trees they could be going up, down, hanging from their arms, hanging from their legs, or brachiating, none of which would benefit from countershading.

    For such situations I can see several patterns that would likely be more effective:

    1. A mottled pattern that mimics the pattern produced by leaves. Jungle snakes and leopards have this.

    2. A pattern with large areas of light and dark to break up the animals silhouette, such as thick stripes or different-colored extremities and central regions. You can see this in the second and fourth example, with the fourth example particularly showing the benefit of this approach. This is also used by modern fighter planes

    3. A pattern with frequently-exposed areas, like the front, being light while frequently-concealed areas, like the back being dark. The first picture has this, and the behavior it is exhibiting shows the benefit of this pattern.

    • Diane G.
      Posted March 12, 2011 at 3:44 pm | Permalink

      Your 3 hypotheses make great sense, as do your observations of the species fitting #2 and #3.

      Some sloths, esp. the well-algae-ed ones, approach the mottled appearance of #1!

      (Yes, I know sloths aren’t monkeys!)

    • whyevolutionistrue
      Posted March 13, 2011 at 7:28 am | Permalink

      Yes, most or all the alternatives you mention are discussed by Cott. His book is still quite worth a read.

      GCM

  2. Insightful Ape
    Posted March 12, 2011 at 3:32 pm | Permalink

    Huevos=eggs.

    • Leslie
      Posted March 12, 2011 at 5:17 pm | Permalink

      Colloquially, huevos refer to testicles. Which brings me to a cultural note – someplace in Latin America, huevos also refer to mormon missionaries. The image of huevos wearing ties and riding bikes is rather amusing . . .

      • Torbjörn Larsson, OM
        Posted March 13, 2011 at 4:47 am | Permalink

        Well, you know, they only see action when they see action, if you know what I mean? [*hint, hint, nudge, nudge*]

        Surely even the most well-trained huevos would eventually tire of hanging around and being cool!

        Not sure about the ties though, that fashion statement strikes me as a bit … restricted.

  3. Ichthyic
    Posted March 12, 2011 at 3:47 pm | Permalink

    Aside from the great points Black Cat made, the selective value of countershading, or any type of camouflage for that matter, entirely depends on who is doing the selecting.

    IOW, countershading to reduce predation by eagles would not be the same as that for a jaguar.

    Moreover, the position the predator commonly hunts from is one factor, another is how that specific predator actually sees its surroundings.

    for example, in many fish species that have been studied by John Endler, there are vast differences in pigment and pattern detection.

    this has to factor into what we define as any kind of camouflage coloration as well. Camouflage is only as good as the predator hunting is at working past it.

    after all, humans aren’t always the primary predators on other primates, so how we see them while walking through the forest might not be terribly relevant.

    • Ichthyic
      Posted March 12, 2011 at 3:51 pm | Permalink

      IOW, countershading to reduce predation by eagles would not be the same as that for a jaguar.

      I just realized how that looks!

      to explain better, I mean not above/below, but flying vs stalking.

      😛

  4. Michael Fisher
    Posted March 12, 2011 at 6:10 pm | Permalink

    WWII: disguising California

  5. Helen Wise
    Posted March 12, 2011 at 6:47 pm | Permalink

    “The large white scrotum of the male mantled howler, below, known as “huevos”, do make the males somewhat more conspicuous”.

    Yoicks. Ya think?

    Say, let me ask you something. If humans evolved from monkeys, how come monkeys are still around?

    Sorry. I should get out more.

  6. Michael Fisher
    Posted March 12, 2011 at 7:13 pm | Permalink

    I don’t think that military aviation camouflage was intended to ‘flatten’ the image as it says in the above post, because the line between the light underbelly & the dark upper works is usually very distinct on vintage ‘planes. If the purpose was to flatten the image to an observer flying at the same altitude, then the dark-painted top would have to fade into the light bottom on the rounded cross-section fuselage of the craft.

    It makes more sense to me that colours & brightness/darkness was applied to best match the likely background ~ so light/sky underneath & dark/earth on top

    BTW ~ I was wondering about human colour blindness…

    Colour-blind people are supposedly above average at certain tasks ~ such as identifying things hidden against similarly coloured backgrounds because they perceive objects more by shape & ‘texture’ and thus get less confused by camouflage. Therefore would it be profitable for a tribe of hunters to have some colour-blind males ?

  7. Torbjörn Larsson, OM
    Posted March 13, 2011 at 6:14 am | Permalink

    Michael Fisher’s question triggered an OT evolutionary question of my own. If you aren’t tickled by it or my naive understanding and formulation is too difficult, I understand. Unfortunately my curiosity is easily tickled.

    No doubt I should read WEIT first [/hangs head in shame], but as I haven’t:

    Genetic variation, whether selected for or not, seems to be rather low in absolute terms. Or rather its outcome is, i.e. not as the frequency of individuals that varies from a genetic norm, but as the functional variation from the norm.

    How do I compare to make such a tentative claim?

    – When working at a biochemical equipment company (detection of minute amounts of various interesting “traces”) I learned that chemical variation of deposition of wet bulk processes can be on the order of a few percent to tens of percents, as I understood it. This chemically “uncontrolled” variation is the high end.

    – When researching and later developing low pressure surface chemical and/or deposition or etching processes I learned that chemical variation of dry surface processes can be on the order of a few parts of percents to percents. This “controlled” (gas phase diffusion rate limited) variation is the low end.

    Genetic processes like height has a variation on the order of percents, indicating to me that these assumedly wet bulk processes are controlled by a network of control functions.

    Indeed we are told that height is a complex outcome of many genes (and developmental factors), so the fact of a complex network isn’t surprising.

    [Albeit I can’t see any hinder of a few genes latching additional control onto an already complex developmental and/or metabolic network. Assumedly there is no simple one-one relation between the two functional complexities, genetic vs phenotypic.]

    Are there cases of enhanced variation, where one can see that the functional variation itself is oomped up to advantage? Maybe I am asking about “hard core” evolvability issues, I don’t exactly know.

    [I assume that the network both is already at so small variation as it needs to be (see the developmental plasticity mentioned in my reply to Michael) and likely would be difficult to improve on anyway due to complexity of feedback and whatnot.]

    • Torbjörn Larsson, OM
      Posted March 13, 2011 at 6:23 am | Permalink

      “Genetic processes like height”

      Rather, outcome of genetic processes, like height.

    • whyevolutionistrue
      Posted March 13, 2011 at 11:23 am | Permalink

      Do read WEIT. There’s lots of genetic variation.

      GCM

  8. Michael Fisher
    Posted March 13, 2011 at 6:55 am | Permalink

    Beyond parody: Tacky, redneck camouflage-theme weddings (including camo-cake)

    Yee hah !

  9. Dominic
    Posted March 14, 2011 at 5:03 am | Permalink

    Hot monkey action – you old wag!

    “adaptive coloration was very controversial” – that is itself very interesting. Thanks!


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  1. Alisha Yoakum…

    Thank you for your own article.Many thanks again. Wonderful….

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