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.
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.
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.
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.