Wednesday, May 9, 2012

Too Dark, Too Light, or Just Right?

As I turned on the tv this evening, I saw a Nature episode on PBS chronicling the lives of two white white lion cubs in Africa. Individuals such as these indeed lead a difficult life; their inability to produce dark pigments leaves them not only highly visible to predators, but often not very attractive to potential mates. It's a rough life being different, something I think we can all identify with. However albinism isn't the only condition that can affect an animal's appearance. There are several ways an animal's coloration can vary, from not enough pigment, to too much, to poor diet.

Leucism, as seen in the lion above, is very similar to albinism, causing animals to be white or at least lighter in coloration. This is due to a lack of melanin, the pigment mostly (more on that later) responsible for an animal's color. The difference between albinism and leucism is that in albinism, the body cannot produce melanin and in leucism, melanin can be produced, but just isn't deposited normally on the skin, fur, feathers, scales, etc. Sometimes leucism just occurs in patches, resulting in what is referred to as a pied or piebald color morph.
Hey, who forgot to finish painting this snake? Nah, it's just a piebald ball python.

To tell an albino from a leucistic animal as in these American alligators, just look at the color of the eyes. Albinos have red eyes due to the lack of pigment--the red you see is due to the blood vessels in the eyes. Animals that are leucistic will still have colored eyes, but will be lighter than usual; commonly blue. 

Melanism is the opposite of albinism and leucism, meaning an animal has an over production of melanin, resulting in a color that is much darker than usual. Whereas animals that are lighter than usual often have a much lower survival rate because they stick out like a sore thumb in their environment, animals that are darker than usual may be at an advantage. One classic case of this is known as "industrial melanism", as seen in the peppered moth. 

Before the industrial revolution, peppered moths looked like this:
Yes, I promise there are two moths in this picture.

Their light coloration made for a spot-on match to lichen covered tree bark. Those moths that were too dark, ie melanistic, were quickly picked off by predators. However, once the industrial revolution began, pollution entered the air, killing off the sensitive lichens. This resulted in a role reversal: the light moths were now picked off the bare tree bark and the dark moths proliferated.
Dark moth: Yeah, who's laughing now?

Melanism became an adaptation, an advantage, in this situation. Other situations similar to these also occur, independent of human causes, for example a melanistic nocturnal animal better suited to hide in the dark night.

Remember how I said that melanin is the pigment mostly responsible for an animal's coloration? Well if you don't, I'm beginning to question just how closely you really read my blog. There are other pigments responsible for color, such as carotenoids. Carotenoids are produced in plants and are responsible for reds, oranges, and yellows in animals, mostly in birds and crustaceans. The only problem is that animals cannot produce their own carotenoids, so they must get it through their diet. The classic example that most people have heard about is the flamingo, which derive their pink from algae and invertebrates that they consume.

This good little ibis has been
eating all of his pellets.
Flamingos and other birds, such as the scarlet ibis, do not have the same foods available to them in captivity as they would in the wild. So how come they are still a vibrant pink? Dietary supplements! At the zoo I used to work at, we could always tell which scarlet ibises were good little birdies and ate their ibis food, and which preferred to steal from the diets of other species: the offenders were always a lighter shade of pink.

Many of our more familiar backyard birds rely on carotenoids for their brilliant colors: northern cardinals, goldfinches, and house finches, for example. 
The house finch is a common visitor to backyard feeders.

For house finches living in Hawaii, their diet has come to be quite different than their relatives living on the mainland, and as a result, their feathers look like this:
Aloha! Oh look, it's the NAI International Conference!

Whether it's albinism, leucism, melanism, or carotenoids, there are many ways in which an animal's appearance may vary. Some may be a disadvantage, while others may provide an advantage. Another fun factoid for you: black feathers are actually stronger than feathers of other colors! This is why if you look at birds that spend a lot of time soaring, such as gulls, white-pelicans, and white ibises, you'll see many of them have wings with black tips. Regardless of the reason for their colors, as humans we are always fascinated with any deviations that we see in an animal's color--an instance when being different is truly celebrated.

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