Biology Question #165
Ryan Dretke, a 21 year old male from the Internet asks on April 19, 2000,
Why are a duck's feet coloured orange?
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A famous evolutionary biologist once said of a particular bird species with bright red males and green females that he would be ready to die once he understood completely why this was so. Of course this means that the answers to these questions are not fully understood - and scientists really want to know.
The first thing to note is that not all ducks have orange feet. However in many species of ducks and other birds the feet are bright orange, red, or yellow. The physiological reason for the coloration is the skin contains carotene and related compounds. The question is: why carotene, making the skin orange, rather than another compound that would make the skin a different color?
The answer seems to have to do with the attraction that these traits hold for members of the opposite sex. In some species orange (or red, blue or yellow) feet (or heads, wings, beaks, etc.) perform this function, while in others it is wattles, combs, inflatable throat pouches, loud noises, long tails, special feathers, metallic-colored spots, extravagant displays, and smells. These traits seems to have evolved because they have power to attract mates. Lots of work has amply demonstrated this to be true in many species in a variety of interesting ways.
Soometimes it is clear WHY potential mates find these traits attractive. In crickets, for example, females have to locate males and do so by the loud songs males make. But why would a female like orange feet more than say flesh-colored feet?
One idea is that the traits are arbitrary; they evolved because (for example) females in a population happened to develop a preference for brighter wattles. If females had such a preference, brighter wattles would confer a mating advantage to males that had them, even if the reason for the preference was completely arbitrary. The idea works in theory, at least.
And it is known that males of some species (e.g. zebra finches) can be made really sexy by fitting them with colored leg bands or even bizarre colored head ornaments - features which seem pretty arbitrary!
If females like a particular male trait NO MATTER WHY OR HOW, males with that trait are selected for. But, could the preference for the trait have some basis? For example, could the trait reveal individual quality or ability in some way (e.g. parasite resistance, or high suitability for the local environs)? Male ducks with bright orange feet may be showing that they are better able to resist the parasites and diseases found in a region, and that if the females want ducklings with the best genes for parasite resistance, he's the guy. Alternately, the bright orange feet may reveal that the male is good at foraging (carotene comes in the diet), or better able to elude predators, or better able to care for offspring.
It is proving pretty difficult to disentangle all the factors that may operate in natural populations, and ecologists don't know for sure which (if any) of these ideas are correct. A big picture of the puzzle still seems to be missing. But work is continuing, and lots is published on this question in the scientific literature each year.
In response to some follow up questions by Andrew Foster, Dr. Ydenberg replies:
Question: Could the feet be orange to distract a predator while the rest of the duck was flying away, as in lizards with detatchable tails?
After I thought about it for a bit, I realized that this is not as strange an idea as I first thought. The literature on predator-prey interactions has several hypotheses like this one that are used to explain strange color patterns, such as the black tips of weasels tails, and others. Some even have experimental evidence in support. I think it is unlikely in the case of duck feet, but I know of no evidence that bears on the question.
Question: Is orange easier to see under water, or not, or against a sky background of blue or grey?
I'm not sure if the question is orange easier to see underwater than another color, or whether orange is easier to see in air than in water. In any case, it's a pretty complicated question because the answer depends a lot of hue, intensity, light conditions and so forth, and of course it also depends on 'visible to whom?'. Animals' visual systems vary a lot and so even if the color is easier to see for humans that doesn't provide any good information about whether a predator such as a fish would see orange feet in the same way.
Question: Is there a connection of the colour with the restricted vascularity in the feet, of necessity due to constant exposure to very cold water and risk of hypothermia?
I don't think so: the orange in orange feet is a pigment in the tissue that as far as I know is not affected by blood flow.
Question: In a given population of ducks, which color of duck feet gives a longer life?
I don't know about duck feet per se, but there is an idea currently being looked at by several labs that the melanin used to create black pigments in feathers and skin also contributes to structural strength.
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