[JAC: There was some discussion this morning about why so many mammals have light bellies. Greg answered in the comments, but I’d also direct you to this article on countershading (yes, it’s from Wikipedia, but it’s the best I could find). Greg happens to be our resident expert on animal coloration, and decided to add a short post based on a picture he saw in the local paper.]
by Greg Mayer
As the picture below shows, countershading doesn’t always work– sometimes the hawk does spot the chipmunk.
I saw this just today in my local paper. As was discussed in the comments on the latest set of readers’ wildlife photos, chipmunks being dark above and light below gives them a “flattened” aspect and makes them harder to see, but no protective coloration is perfect. There have been years when hawks nested in trees on my block quite close to my house, and mangled chipmunk remains would appear frequently below the nest. This year, I haven’t seen any hawks near the house, and chipmunks seem more common than usual.
I’m sure it must be something of an arms race. As the prey gets harder to see, the predators get better at seeing….
b&
It’s not just mammals. Most fish (nearly all, I’d guess) have dark backs and white bellies.
Yes! Predators looking down into the ocean depths will have a harder time to see such dorsal colouring while predators looking up will also have a similar difficulty when prey have light ventral coloration.
My whole body is the colour of a fish belly. 😉
Sub
Dawkins has several pages on countershading in “The Ancestor’s Tale”(pp 327-329 in my edition).
Among other things, he introduces us to the upside down catfish, which has a dark belly and a whiteish back.
I love upside-down catfish–great aquarium residents.
Perhaps I’m a bit slow, but I don’t see how having a dark back and a light belly makes an animal more conspicuous from above than having a dark back and a dark belly.
Does the light belly adsorb light from the ground, making for less visible shadow as seen from above?
The countershading makes them less conspicuous from above, because they lose their 3-dimensionality (they are “flattened’).Have a look at the Wikipedia article Jerry linked to above, and you’ll see it in action.
GCM