Countershading:
What does an eagle soaring in the sky, a shark dwelling
beneath the surface, and a deer out on the fields all have in common? They
obviously live in different environments, and they all are shaped
very different from each other, and they don't eat the same food...
But what they all have, is wish of not being spotted. So why do I
bring this up? I have already talked about camouflage. What I want to
talk about know is a very special form of camouflage you can find
everywhere, it is called countershading, and it is found among many
sorts of animals.
If you look at an animal from the sea,
like a shark for example, you will notice its colors are dark on its
back, and light underneath. A lot of animals share this pattern of
gradual color change from dorsal side to ventral side. It might be
more abundant in the sea, where penguins, fish, sharks, dolphins and
wales display this, but lot of terrestrial species also show this
pattern, like birds, ungulates, and a range of other animals.
It is very common, and therefor it must
also matter in a way, it must be advantageous somehow. In nature
almost nothing is random, and neither is this. There is several
reasons for why this has developed among animals at such a high
degree, but everything comes down to being spotted.
A lot of animals, as well as humans,
rely to some degree of 3D vision. Shadows and different colors and
shades, create an image that is easy for us to spot-, we easily sort
it out from the background, even if the individual is staying still.
Light from above will reach an animal, and create a lighter looking
surface area where the sunlight hits, usually at the dorsal side. The
lateral sides, where the light don't reaches, will look darker, and
a shadow might appear towards the ventral side or at the ground where
the animal is standing. This is if you look at an animal from the
side(lateral side towards you). Light will make it easier to discover
an animal, because of this lively 3D created image. Of course, for an
animal which don't want to be spotted, this is a major disadvantage,
and nature has encountered this problem. By making the dorsal side
darker, and the ventral side lighter, the animal counteract the
affect of light. They cancel out the shadings that are created by the
sun, and make it look less three dimensional and more flat, and
thereby they get harder to detect.
This is to show how it works:
picture from
http://en.wikipedia.org/wiki/File:Countershading.jpg viewed 18 march 2014
Another good reason for
having light colors ventrally and dark colors dorsally, is because
you might be spotted from to different angles, and depending on where
the light enters from, the environment looks very different, and
therefor the colors also must be different. If a shark close to the
surface is spotted from above surface, it will have to look like the
dark blue sea beneath it so it won't be spotted. Animals further down
in the sea beneath the shark, will look up towards the surface with a
light background, and the shark must therefor also display a light
color to be less visble. This is in general very common in the sea,
regardless if you look at a predator or a herbivore. Also a lot of
birds have a light ventral side, so that animals below them don't
spot them easily with the light sky as a background, while from above
they look dark as the ground under them. Predators like eagles use
this so that their prey won't spot them, this makes it easier for
them to get close. Their prey on the other hand is safer with this
coloration since they are looking like the environment no matter if
the predator is flying above , or looking up towards the sky.
Under is a couple of
pictures showing different animals with countershading, the metod is
the same, although the animals look very different.
Picture:< http://rfotd.tumblr.com/post/2728583939/open-water-sharks-have-a-color-pattern-called > viewed 26 March 2014
Picture:< http://gurneyjourney.blogspot.com.au/2013/01/countershading.html > viewed 26 March 2014
Picture:< http://www.nhptv.org/natureworks/nwep2a.htm > viewed 26 March 2014
Bibliography:
Breed, MD &
Moore, J 2011, Animal Behaviour, Elsevier Science Publishing
Co Inc, San Diego, United States.
Stevens,
M & Merilaita, S 2011, Animal
Camouflage : Mechanisms and Function ,Cambridge
University Press.
Science
and the Sea; Countershading 2011,
The
University of Texas Marine Science Institute,
Port
Aransas, Texas.
Great examples! It’s great that you included marine and terrestrial examples, as well as different taxonomic examples. Do insects also use counter-shading as a mechanism to avoid predation? Would counter-shading only be effective if the predator/prey is viewed from above or below? What about if the animal is viewed directly from the side? Although the animal looks 2D, wouldn’t it still be easily spotted? Great blog!
SvarSlettThanks:) I'm not sure if insects use counter shading as explaned in the text, but some insects do have a shade differences when it comes to dorsol and ventral side, but I don't know if that is related to what I talk about here in the text. From what I know insects more often depend on different kinds of camouflage and mimicry. What I do know(also written in the text), is that counter shading helps when you are spotted from the side, this is because the counter shading removes the 3D effect, and lots of animals use depth to distinguish between objects, and the animal will in this case appear flat, and is not as easily spotted. This is explained in the first picture:)
SlettHi! Wow this is really cool! and I love your examples :)
SvarSlettI was just wondering how the patterning of zebra-stripes would be described and explained? I have heard some arguments that it is to do with camouflage of the individual amongst the herd. looking forward to seeing more! :)
Hi and thanks for commenting:) I have not red much about the zebra stripes, but I am actually planning on writing about it, it is as far as I know, in a category unlike what I have talked about so far, something called motion dazzle, which basically as you say, makes it harder to seperate one individual from another one when they move in large herds. Hopefully I will be writing about this later, because it is really interesting, but I haven't done to much research about it yet, unfortunately.
Slett